A facile one-pot synthesis of 3-amino-[1,2,4]-triazolo[4,3-a]pyridines from thiosemicarbazides through anion mediated synthesis is reported. Thiosemicarbazides derived from 2-hydrazino pyridine, 5-chloro 2-hydrazino pyridine, and 2-hydrazine quinoline were formed in situ as anion receptors in the presence of TBAF. Under microwave heating, thiosemicarbazides furnished the triazolo pyridines in good to moderate yields. The formation of the thiosemicarbazides hydrogen bonding anion receptors was critical in cascading the reaction toward the formation of the triazolo pyridines.

White-light-emitting materials have attracted significant interest in recent years due to their potential applications in solid-state lighting and flat-panel displays. Design of such materials is challenging and often relies on the use of multiple fluorophores despite the fact that single component systems yield materials with enhanced stability and reproducibility. Herein, we have developed a white-light-emitting system based on the formation of discrete lanthanide-based self-assembled complexes using a newly-designed ligand. We demonstrate that fine tuning of the lanthanide ions molar ratio in the self-assemblies combined with the intrinsic blue fluorescence of the ligand allows for the successful emission of pure white light with CIE coordinates of (0.33, 0.34).

A V-shaped 4-amino-1,8-naphthalimide derived dipyridyl ligand comprising the Tröger's base structural motif has been synthesised and subsequently used in the formation of two new supramolecular coordination polymers.

A heterotopic naphthalimide ligand N-(4-picolyl)-4-(4′-carboxyphenoxy)-1,8-naphthalimide HL is utilised for the formation of self-assembled soft materials. In the presence of K+ ions, L− forms a robust photoluminescent hydrogel 1 which is reversible under thermal, mechanical or chemical stimuli.

A V-Shaped 4-amino-1,8-napthalimide derived tetracarboxylic acid linker (L; bis-[N-(1,3-benzenedicarboxylic acid)]-9,18-methano-1,8-naphthalimide-[b,f][1,5]diazocine) comprising the Tröger's base (TB) structural motif was rationally designed and synthesised to access a nitrogen-rich fluorescent supramolecular coordination polymer. By adopting the straight forward precipitation method, a new luminescent nanoscale Zn(II) coordination polymer (TB-Zn-CP) was synthesized in quantitative yield using Zn(OAc)2·2H2O and tetraacid linker L (1 : 0.5) in DMF at room temperature. The phase-purity of as-synthesised TB-Zn-CP was confirmed by X-ray powder diffraction analysis, infra-red spectroscopy, and elemental analysis. Thermogravimetric analysis suggests that TB-Zn-CP is thermally stable up to 330 °C and the morphological features of TB-Zn-CP was analysed by SEM and AFM techniques. The N2 adsorption isotherm of thermally activated TB-Zn-CP at 77 K revealed a type-II reversible adsorption isotherm and the calculated Brunauer–Emmett–Teller (BET) surface area was found to be 72 m2 g−1. Furthermore, TB-Zn-CP displayed an excellent CO2 uptake capacity of 76 mg g−1 at 273 K and good adsorption selectivity for CO2 over N2 and H2. The aqueous suspension of as-synthesized TB-Zn-CP showed strong green fluorescence (λmax = 520 nm) characteristics due to the internal-charge transfer (ICT) transition and was used as a fluorescent sensor for the discriminative sensing of nitroaromatic explosives. The aqueous suspension of TB-Zn-CP showed the largest quenching responses with high selectivity for phenolic-nitroaromatics (4-NP, 2,4-DNP and PA) even in the concurrent presence of other potentially competing nitroaromatic analytes. The fluorescence titration studies also provide evidence that TB-Zn-CP detects picric acid as low as the parts per billion (26.3 ppb) range. Furthermore, the observed fluorescence quenching responses of TB-Zn-CP towards picric acid were highly reversible. The highly selective fluorescence quenching responses including the reversible detection efficiency make the nanoscale coordination polymer TB-Zn-CP a potential material for the discriminative fluorescent sensing of nitroaromatic explosives.

Ruthenium(II) [Ru(II)] polypyridyl complexes have been the focus of intense investigations since work began exploring their supramolecular interactions with DNA. In recent years, there have been considerable efforts to translate this solution-based research into a biological environment with the intention of developing new classes of probes, luminescent imaging agents, therapeutics and theranostics. In only 10 years the field has expanded with diverse applications for these complexes as imaging agents and promising candidates for therapeutics. In light of these efforts this review exclusively focuses on the developments of these complexes in biological systems, both in cells and in vivo, and hopes to communicate to readers the diversity of applications within which these complexes have found use, as well as new insights gained along the way and challenges that researchers in this field still face.

Fluorescent chemosensors for ions and neutral analytes have been widely applied in many diverse fields such as biology, physiology, pharmacology, and environmental sciences. The field of fluorescent chemosensors has been in existence for about 150 years. In this time, a large range of fluorescent chemosensors have been established for the detection of biologically and/or environmentally important species. Despite the progress made in this field, several problems and challenges still exist. This tutorial review introduces the history and provides a general overview of the development in the research of fluorescent sensors, often referred to as chemosensors. This will be achieved by highlighting some pioneering and representative works from about 40 groups in the world that have made substantial contributions to this field. The basic principles involved in the design of chemosensors for specific analytes, problems and challenges in the field as well as possible future research directions are covered. The application of chemosensors in various established and emerging biotechnologies, is very bright.

A family of five benzene-1,3,5-tricarboxamide (BTA) compounds with varied side arm functionality [alkyl: n = 3, 4 and 5 chains; and ester and carboyxic acids] is reported. Investigations into their self-assembly behaviour revealed the extent to which minor changes in the side arm functionality can affect the structures formed. The carboxylic acid derivatives have a tendency to form gels, while the ester derivatives instead form crystalline phases with associated thermotropic phase transitions. The influence of the side chain length and terminal functional group is rationalised by a combination of single crystal X-ray diffraction studies and examinations of the bulk material properties.

A Tröger's base-functionalised covalent organic polymer (TB-COP) was synthesised and used as an adsorbent for the efficient removal of picric acid (PA) from water through the use of weak and reversible supramolecular interactions such as hydrogen bonding and π–π interactions. TB-COP was readily synthesised in quantitative yield using a one-pot metal-free polymerisation reaction strategy between a semi-flexible aromatic triamine [L; benzene-1,3-5-tricarboxylic acid-tris-(4-amino-phenyl-amide)] and dimethoxymethane. The molecular structure, physicochemical and morphological characteristics of TB-COP were analysed by using various spectroscopic and imaging techniques. Thermogravimetric analysis showed TB-COP to be thermally stable up to 380 °C; while the calculated Brunauer–Emmett–Teller (BET) surface area was found to be 34 m2 g−1 at 273 K. The picric acid adsorption study of the activated TB-COP showed an excellent adsorption capacity of ca. 90% within 60 minutes of contact time at 298 K (Langmuir isotherm model: KL = 0.0541 ± 6 L mg−1, R2 = 0.9962); the adsorption efficiency being shown to improve with increasing temperature. The extraction of PA was also clearly visible to the naked eye, where the yellow colored PA solution became transparent upon addition of TB-COP. Other interfering phenolic organic pollutants showed poor to moderate adsorption efficiency. Importantly, TB-COP could be used to store PA over a long period of time in a safe manner, without any leakage or any significant loss in extraction efficiency. Moreover, the polymer could be reused for several adsorption cycles, as PA could be released back into the solution by simply changing the pH of the aqueous media. This makes TB-COP an extremely promising material for the selective and efficient removal of picric acid from water, and TB-COP can be considered as being a ‘fast’ and naked eye colorimetric indicator for such analytes.

We report here a novel one-pot synthetic strategy for the synthesis of a family of N-alkyl-1,8-naphthalimide based Tröger's bases via a nucleophilic substitution reaction of a common ‘precursor’ (or a ‘synthon’) N-aryl-1,8-naphthalimide Tröger's base heated at 80 °C in neat aliphatic primary amine, in overall yield of 65–96%. This methodology provides an efficient and one-step facile route to design 1,8-naphthalimide derived Tröger's base structures in analytically pure form without the use of column chromatography purification, that can be used in medicinal chemistry and as supramolecular scaffolds. We also report the formation of the corresponding anhydride, and the crystallographic analysis of two of the resulting products, that of the N-phenyl-4-amino-1,8-naphthalimide and the anhydride derived Tröger's bases.

Many breakthrough developments in the past few decades have been achieved through multidisciplinary research. Supramolecular chemistry has played an extensive role in bridging chemical, physical, and biological sciences together. These interdisciplinary studies benefit our understanding of individual and hybrid systems. The supramolecular approach has emerged as a toolkit with remarkable capability for the assembly of rationally designed building blocks, which precisely organize in three-dimensional space to form intricate molecular assemblies as well as bulk material with functional properties. Because of such potential, supramolecular chemistry has emerged as a platform for the discovery of new systems and/or novel applications for existing ones. This field also holds great promise for advancement in the discovery of a vast array of systems that can be utilized in material chemistry, nanomaterials, soft materials, light-emitting devices, delivery agents, sensors, etc.Self-assembly has proven to be a powerful tool for the construction of complex superstructures. The assembly of monomers into supramolecular architectures via non-covalent interactions is chiefly directed by the molecular structures, their functional groups, and environmental conditions. The principal advantage of non-covalent interactions is reversibility, which allows the assembly of monomers into supramolecular structures in situ depending on the local conditions. In addition, the supramolecular approach provides a degree of control over self-assembly at the molecular level, thereby influencing the macroscopic level and facilitating tuning of the bulk material properties. This review discusses the meritorious examples of supramolecular materials constructed through the molecular assembly process, guided by the classical principles of supramolecular chemistry. Furthermore, this year (2017) marks the 50th anniversary of supramolecular chemistry in honor of the first example of supramolecular structure reported by Charles J. Pedersen and the achievements in the area of supramolecular chemistry ever since.

Luminescent sensors for ions and molecules have found numerous applications in areas such as biology, medicine, security, and environmental monitoring. However, designing portable sensors for differentiating similar chemical species, such as D2O in H2O, still remains a challenge. In this issue of Chem, Humphrey and co-workers elegantly demonstrate the use of phosphorescent MOFs to achieve this.

We have prepared a series of N-picolyl-1,8-naphthalimide derivatives and explored their coordination chemistry with d-block metal ions in the solid and solution states, and exploited the structural and chemical properties of these ligands to generate a family of robust metallogels. The 2-picolyl substituted ligands N-(2-picolyl)-1,8-naphthalimide L1 and N-(2-picolyl)-4-nitro-1,8-naphthalimide L2 showed interesting coordination behaviour in Mn, Co and Zn complexes 1–5, chelating through the pyridyl nitrogen atom and naphthalimide oxygen atom with subsequent bond localisation in the imide ring, the first example of such coordination behaviour to be structurally characterised. The 3-picolyl substituted ligands N-(3-picolyl)-1,8-naphthalimide L3 and N-(3-picolyl)-4-nitro-1,8-naphthalimide L4 vary in their coordination behaviour; the 4-unsubstituted ligand L3 acts as a simple monodentate donor giving crystalline assemblies, while the 4-nitro analogue L4 instead forms robust metallogels on reaction with MnII or CoII. The gels were found to be thermally and chemically reversible under various stimuli, and similar materials could be generated by installing a piperidinyl substituent at the naphthalimide 4-position, giving intensely coloured gels. These results describe previously unknown coordination chemistry and physical properties of the widely-used 1,8-naphthalimide backbone, and outline the important role played by subtle geometric variations in the physical characteristics of supramolecular metallogels.

A flexible ditopic ligand 1 containing two N,N,O-tridentate (1,2,3-triazol-4-yl)-picolinamide chelating pockets is reported and the formation of multimetallic architectures is explored in the solid and the solution phase. The self-assembled ZnII complex [Zn4(1)4](ClO4)8 exhibited a folded [2 × 2] square grid supramolecular architecture that selectively assembled in MeCN solution as shown using various spectroscopic techniques. The closely related FeII complex shows equivalent behaviour in the solid state, while a discrete dinuclear species [Cu2(NO3)41]·5MeCN was the sole product observed in the solid state from the reaction between 1 and CuII under similar conditions.

Herein some examples of the use of lanthanide ions (f-metal ions) to direct the synthesis of luminescent self-assembly systems (architectures) will be discussed. This area of lanthanide supramolecular chemistry is fast growing, thanks to the unique physical (magnetic and luminescent) and coordination properties of the lanthanides, which are often transferred to the resulting supermolecule. The emphasis herein will be on systems that are luminescent, and hence, generated by using either visibly emitting ions (such as EuIII, TbIII and SmIII) or near infrared emitting ions (like NdIII, YbIII and ErIII), formed through the use of templating chemistry, by employing structurally simple ligands, possessing oxygen and nitrogen coordinating moieties. As the lanthanides have high coordination requirements, their use often allows for the formation of coordination compounds and supramolecular systems such as bundles, grids, helicates and interlocked molecules that are not synthetically accessible through the use of other commonly used templating ions such as transition metal ions. Hence, the use of the rare-earth metal ions can lead to the formation of unique and stable species in both solution and in the solid state, as well as functional and responsive structures.

Herein we present a supramolecular (delayed luminescent) Eu(III)-based pH-responsive probe/sensor with the ability to detect the urease-mediated hydrolysis of urea in aqueous solution. A series of photophysical titrations show this Eu(III) chelate behaves as an “on–off” luminescent switching probe, with its luminescence being quenched upon urea being enzymatically converted into ammonia and carbon dioxide. Calculation of the rate constant (k) and activation energy (Ea) for this hydrolysis reaction are detailed; the results demonstrate a direct observation of enzymatic activity in solution by the sensor. The potential application of this probe in detecting the onset of catheter-associated urinary tract infections (CAUTIs) is also demonstrated by incorporating 1.Eu into water-permeable hydrogels that can be utilized as an alternative coating for catheters.

The development of Ru(II) functionalized gold nanoparticles 1–3·AuNP is described. These systems were found to be mono-disperse with a hydrodynamic radius of ca. 15 nm in water but gave rise to the formation of higher order structures in buffered solution. The interaction of 1–3·AuNP with DNA was also studied by spectroscopic and microscopic methods and suggested the formation of large self-assembly structures in solution. The uptake of 1–3·AuNP by cancer cells was studied using both confocal fluorescence as well as transmission electron microscopy (TEM), with the aim of investigating their potential as tools for cellular biology. These systems displaying a non-toxic profile with favourable photophysical properties may have application across various biological fields including diagnostics and therapeutics.

Two new luminescent ditopic naphthalimide-derived ligands, N-(4-cyanophenylmethylene)-4-(4-cyanophenoxy)-1,8-naphthalimide (L3) and N-(4-carboxyphenylmethylene)-4-(4-carboxyphenoxy)-1,8-naphthalimide (H2L4), have been prepared, and their coordination chemistry has been explored in the synthesis of three new coordination polymer materials. Complex poly-[Ag(L3)2]BF4·4.5H2O·0.5THF (1) is a 3-fold 2D → 2D parallel interpenetrated coordination polymer in which three interwoven sheets define inter- and intralayer channels containing anions and solvent molecules. Molecules of L3 interact in 1 through dominant head-to-head π–π stacking interactions, in an opposite aggregation mode to that observed in the free ligand in the crystalline phase. Complexes poly-[Cu(L4)(OH2)]·2DMF·0.5H2O (2) and poly-[Cd2(L4)2(OH2)2]·1.5DMF·3H2O (3) are related noninterpenetrated two-dimensional coordination polymers defined by one-dimensional metal–carboxylate chains, forming layers that interdigitate with adjacent networks through naphthalimide π–π interactions. Both materials undergo structural rearrangements on solvent exchange with acetonitrile; in the case of 3, this transformation can be followed by single-crystal X-ray diffraction, revealing the structure of the acetonitrile solvate poly-[Cd2(OH2)2(L4)2]·2MeCN (4), which shows a significant compression of the primary channels to accommodate the solvent guest molecules. Both materials display modest CO2 adsorption after complete evacuation, and the original expanded phases can be regenerated by reimmersion in DMF. The photophysical properties of each ligand and complex were also explored, which revealed variations in emission wavelength, based on solid-state interactions, including a notable shift in the fluorescence emission band of 3 upon structural rearrangement to 4.

A new divergent and self-complementary halogen bond donor–acceptor molecule 2,6-bis(iodoethynyl)pyridine L has been prepared and structurally characterized, and used to generate a series of extended halogen-bonded adducts with tetrabutylammonium halide salts. The reaction between L and anions such as either bromide or chloride gives the one-dimensional polymeric species {L·TBABr} or {L·TBACl}, respectively, which contains helical strands linked by C–I···X– halogen bonds which partially encapsulate the associated organic cations. Varying the reaction solvent from ethyl acetate to 5:95 methanol/ethyl acetate gives rise to another polymeric phase on combining L with tetrabutylammonium chloride, the one-dimensional looped chain structure {L2·TBACl} in which each chloride ion acts as a four-connected square planar node for halogen bonding interactions originating from L. Similar solvent effects are observed in the discrete macrocyclic species {L·TBAI}-α and {L·TBAI}-β, both consisting of cyclic (L2I2)2– species which vary in their extended structures to adopt different packing modes, resembling those observed in the chloride and bromide adducts. Reaction of L with tetrabutylammonium fluoride in the presence of methanol gives a one-dimensional polymeric assembly {L2·TBAF·MeOH} containing a unique MeOH···F– node in which the strongly hydrogen-bonded anionic species is supported solely by four halogen bonding interactions. Excluding hydrogen bond donors from the reaction mixture affords {L3TBAF2}, an overall two-dimensional polymeric assembly consisting of a mixture of one- and two-dimensional networks based on iodine-fluoride halogen bonds. In all cases, the strong interactions between the iodoethynyl groups and halide anions overcome the tendency for L to form the self-complementary halogen-bonded network, showing that synthetically accessible heterocyclic iodoethynyl compounds present exciting new directions in the structural chemistry field.

The synthesis of the ‘click’ derived 1,4-di(2-pyridyl)-1,2,3-triazole (dpt) chelator/ligand from 2-azidopyridine and 2-ethynylpyridine using Cu(I) and TBTA by microwave assisted synthesis is presented. The complexes of the subtly unsymmetric dpt ligand with Cu(I), Pt(II), Co(II), and Ag(I) were structurally characterised by using conventional methods, as well as using single crystal and powder diffraction analysis. The results of the studies showed formation of discrete molecules displaying preferential binding of the d-metal cations through the pyridyl nitrogen N1 and the proximal triazolyl nitrogen N2 i.e. the 2-(1H-1,2,3-triazol-4-yl)pyridine or ‘regular' chelate moiety despite the presence of a second potential binding pocket i.e. the 2-(1H-1,2,3-triazol-1-yl)pyridine chelate or ‘inverse’ moiety. This binding selectivity was corroborated through the study of the self-assembly of dpt with Cu(I) and Ag(I) using 1H NMR titration in CD3CN solution, as well as using UV-Vis absorption titrations; the former showing a broadening of the proton peaks associated with that chelate pocket.

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A graphical abstract is available for this content

Herein we present the use of lanthanide directed self-assembly formation (Ln(III) = Eu(III), Tb(III)) in the generation of luminescent supramolecular polymers, that when swelled with methanol give rise to self-healing supramolecular gels. These were analyzed by using luminescent and 1H NMR titrations studies, allowing for the identification of the various species involved in the subsequent Ln(III)-gel formation. These highly luminescent gels could be mixed to give a variety of luminescent colors depending on their Eu(III):Tb(III) stoichiometric ratios. Imaging and rheological studies showed that these gels prepared using only Eu(III) or only Tb(III) have different morphological and rheological properties, that are also different from those determined upon forming gels by mixing of Eu(III) and Tb(III) gels. Hence, our results demonstrate for the first time the crucial role the lanthanide ions play in the supramolecular polymerization process, which is in principle a host–guest interaction, and consequently in the self-healing properties of the corresponding gels, which are dictated by the same host–guest interactions.

The binding of asymmetrical and optically pure tridentate ligands (L = 1(S) and 1(R)) containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions (M = La(III) and Eu(III)) was studied in CH3CN, showing the successive formation of M:L, M:L2 and M:L3 stoichiometric species in solution. The europium complexes EuL3 were also synthesised, structurally characterised and their photophysical properties probed in CH3OH and CH3CN. The changes in the chiroptical properties of both 1(S) and 1(R) were used (by circular dichroism (CD) spectroscopy) to monitor the formation of these chiral self-assemblies in solution. While circularly polarised luminescence (CPL) showed the formation of Eu(1(S))3 and Eu(1(R))3 as enantiomers, with high luminescence dissymmetry factors (glum), fitting the CD changes allowed for binding constants to be determined that were comparable to those seen in the analyses of absorbance and luminescence changes.

The emergence of synthetic glycoconjugates as chemical probes for the detection of glycosidase enzymes has resulted in the development of a range of useful chemical tools with applications in glycobiology, biotechnology, medical and industrial research. Critical to the function of these probes is the preparation of substrates containing a glycosidic linkage that when activated by a specific enzyme or group of enzymes, irreversibly releases a reporter molecule that can be detected. Starting from the earliest examples of colourimetric probes, increasingly sensitive and sophisticated substrates have been reported. In this review we present an overview of the recent advances in this field, covering an array of strategies including chromogenic and fluorogenic substrates, lanthanide complexes, gels and nanoparticles. The applications of these substrates for the detection of various glycosidases and the scope and limitations for each approach are discussed.

The synthesis of five new 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligands is described: the self-assembly behaviour of the tri-methyl ester, 1, with Eu(III) showed the formation of a luminescent 1:3 Eu:btp complex, Eu13, which was studied in solution and in the solid state; while the tri-carboxylic acid, 2, formed a hydrogel and its corresponding complex Eu23, gave rise to a strongly red luminescent healable metallogel.

The non-covalent incorporation of responsive luminescent lanthanide, Ln(III), complexes with orthogonal outputs from Eu(III) and Tb(III) in a gel matrix allows for in situ logic operation with colorimetric outputs. Herein, we report an exemplar system with two inputs ([H+] and [F−]) within a p(HEMA-co-MMA) polymer organogel acting as a dual-responsive device and identify future potential for such systems.

Ruthenium polypyridyl complexes show great promise as new photodynamic therapy (PDT) agents. However, a lack of detailed understanding of their mode of action in cells poses a challenge to their development. We have designed a new Ru(II) PDT candidate that efficiently enters cells by incorporation of the lipophilic aromatic pdppz ([2,3-h]dipyrido[3,2-a:2′,3′-c]phenazine) ligand and exhibits photoactivity through incorporation of 1,4,5,8-tetraazaphenanthrene ancillary ligands. Its photoreactivity toward biomolecules was studied in vitro, where light activation caused DNA cleavage. Cellular internalization occurred via an energy dependent mechanism. Confocal and transmission electron microscopy revealed that the complex localizes in various organelles, including the mitochondria. The complex is nontoxic in the dark, with cellular clearance within 96 h; however, upon visible light activation it induces caspase-dependent and reactive-oxygen-species-dependent apoptosis, with low micromolar IC50 values. This investigation greatly increases our understanding of such systems in cellulo, aiding development and realization of their application in cancer therapy.

Ligands containing the [2,6-bis(1,2,3-triazol-4-yl)pyridine] (btp) motif have recently shown promise in coordination chemistry. The motif is synthesized via the Cu(I)-catalyzed “click” reaction and can be conveniently functionalized when compared to other terdentate chelating motifs. Ligand 1 was synthesized and shown to sensitize Eu(III) and Tb(III) excited states effectively. The use of these ions to synthesize self-assembly structures in solution was investigated by carrying out both 1H NMR and photophysical titrations. The latter were used to determine high binding constants from changes in the absorption, ligand emission (fluorescence), and lanthanide-centered emission. A small library of amino acid derivatives of 1, ligands 3, were prepared upon coupling reactions with Gly, Ala, Phe, and Trp methyl esters, with a view to introducing biologically relevant and chiral moieties into such ligands. All of these derivatives were shown to form stable, emissive Ln(III) self-assemblies, emitting in the millisecond time range, which were studied by means of probing their photophysical properties in organic solutions using lanthanide ion titrations. All the Tb(III) complexes, with the exception of Trp based derivatives, gave rise to highly luminescent and bright complexes, with quantum yields of Tb(III) emission of 46–70% in CH3CN solution. In contrast, the Eu(III) complexes gave rise to more modest quantum yields of 0.3–3%, reflecting better energy match for the Tb(III) complexes, and hence, more efficient sensitization, as demonstrated by using low temperature measurements to determine the triplet state of 1.

The tripodal terpyridine ligand, L, forms 1D helical supramolecular polymers/gels in H2O–CH3OH solution mediated through hydrogen bonding and π–π interactions. These gels further cross-link into 3D supramolecular metallogels with a range of metal ions (M) such as Fe(II), Ni(II), Cu(II), Zn(II), and Ru(III); the cross-linking resulting in the formation of colored or colorless gels. The fibrous morphology of these gels was confirmed using scanning electron microscopy (SEM); while the self-assembly processes between L and M were investigated by absorbance and emission spectroscopy from which their binding constants were determined by using a nonlinear regression analysis.

A series of tetra-substituted ‘pseudo’ dipeptide ligands of cyclen (1,4,7,10,-tetraazacyclododecane) and a tri-substituted 3′-pyridine ligand of cyclen, and the corresponding lanthanide(III) complexes were synthesised and characterised as metallo-ribonuclease mimics. All complexes were shown to promote hydrolysis of the phosphodiester bond of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP, τ1/2 = 5.87 × 103 h), a well known RNA mimic. The La(III) and Eu(III) tri-substituted 3′-pyridine lanthanide(III) complexes being the most efficient in promoting such hydrolysis at pH 7.4 and at 37 °C; with τ1/2 = 1.67 h for La(III) and 1.74 h for Eu(III). The series was developed to provide the opportunity to investigate the consequences of altering the lanthanide(III) ion, coordination ability and hydrophobicity of a metallo-cavity on the rate of hydrolysis using the model phosphodiester, HPNP, at 37 °C. To further provide information on the role that the logKa of the metal bound water plays in phosphodiester hydrolysis the protonation constants and the metal ion stability constants of both a tri and tetra-substituted 3′pyridine complex were determined. Our results highlighted several key features for the design of lanthanide(III) ribonucelase mimics; the presence of two metal bound water molecules are vital for pH dependent rate constants for Eu(III) complexes, optimal pH activity approximating physiological pH (∼7.4) may be achieved if the logKa values for both MLOH and ML(OH)2 species occur in this region, small changes to hydrophobicity within the metallo cavity influence the rate of hydrolysis greatly and an amide adjacent to the metal ion capable of forming hydrogen bonds with the substrate is required for achieving fast hydrolysis.

The synthesis, spectroscopic characterisation and biological evaluation of mono- and bis-1,8-naphthalimide-conjugated ruthenium(II)-polypyridyl complexes is presented. Spectroscopic DNA titrations, together with denaturation studies, show strong binding of both species to DNA through the naphthalimide arms. Linear and circular dichroism (LD and CD) spectroscopy reveal close association of the Ru(bpy)32+ core with DNA in the case of the mono-naphthalamide complex, [Ru(bpy)2(bpy-NAP)]2+. Significantly, binding by the second naphthalimide arm in the [Ru(bpy)2(bpy-NAP2)]2+ complex is found to displace the Ru(bpy)32+ centre from the DNA backbone. This ‘negative allosteric effect’ is found to have a dramatic influence on the photoinduced damage of plasmid DNA, and the viability of HeLa cancer cells upon photoactivation. Overall the study clearly maps and correlates the relationship between molecular structure, in vitro binding and activity, and in cellulo function.

Ligands containing the btp [2,6-bis(1,2,3-triazol-4-yl)pyridine] motif have appeared with increasing regularity over the last decade. This class of ligands, formed in a one pot ‘click’ reaction, has been studied for various purposes, such as for generating d and f metal coordination complexes and supramolecular self-assemblies, and in the formation of dendritic and polymeric networks, etc. This review article introduces btp as a novel and highly versatile terdentate building block with huge potential in inorganic supramolecular chemistry. We will focus on the coordination chemistry of btp ligands with a wide range of metals, and how it compares with other classical pyridyl and polypyridyl based ligands, and then present a selection of applications including use in catalysis, enzyme inhibition, photochemistry, molecular logic and materials, e.g. polymers, dendrimers and gels. The photovoltaic potential of triazolium derivatives of btp and its interactions with anions will also be discussed.

•We review the effort at Trinity College Dublin to develop lanthanide based self-assembly structures.•A substantial breadth of research is presented collating representative examples from the group published to date.•Cyclen-derived systems are reviewed as probes, as discrete complexes and AuNP conjugated assemblies, for various analytes.•Self-assembly structures are reviewed covering mono- and di- metallic bundles, helicates and leading to catenated systems.•Progress towards soft matter applications of lanthanide-based self-assembled gels are discussed.This review details the progress made in our laboratory in Dublin within the area of supramolecular lanthanide chemistry; where the main objective has been to develop functional lanthanide luminescent systems that emit within the visible or the near-infrared regions of the electromagnetic spectrum. The application of the lanthanide-centred assembly is two-fold, firstly to use it for sensing application of biologically and environmentally relevant anions, and secondly to use the emission to report on the formation of lanthanide directed self-assembly processes. This review begins with the progress made from the use of lanthanide cyclen (1,4,7,10-tetraazacyclododecane) complexes: to develop displacement assays for sensing or probing applications; for their incorporations onto gold nanoparticles where the lanthanide emission can be ‘switched on-off’ by external stimuli; and their application in the formation of mixed f–d metal ion based self-assemblies and sensors and imaging agents for DNA. The second part of this review focuses on the development of self-assemblies formed using 2,6-diamidopyridyl based ligands. A variety of ligands are presented that have been employed in the formation of chiral self-assembly bundles, dimetallic triple stranded helicates and Langmuir–Blodgett films that give rise to circularly polarised luminescence. The formation of such organised self-assembly monolayers of water–air interface brings us to the last topic of this review, along with the formation of novel soft-material such as gels, where the lanthanide emission is employed to report on the formation of three dimensional supramolecular structures, and their application as ‘nano-gardens’ in growing ionic structures with novel morphologies.

The formation of interlocked lanthanide-based catenanes using Eu(III)-directed synthesis is described (catenation being achieved via a ring-closing metathesis reaction); the self-assembly formation of the supramolecular structures was analysed by HRMS, NMR and luminescent spectroscopies.

The synthesis and characterisation of novel aryl-pyridyl ureas are described, which form self-assembly structures via extended hydrogen bonding and π–π interactions in the solid state, and in selected cases, form supramolecular gels with antimicrobial properties against Staphylococcus aureus and/or Escherichia coli.

The combination of lanthanides and nanoparticles to develop new hybrid nanomaterials has become a highly topical area of research in the field of sensing, biomedical imaging, drug delivery, etc. However, these novel nanomaterials have to be carefully designed to ensure that the unique properties conveyed by each component, i.e., lanthanide ions and nanoparticles, are maximized and not negatively affected by one another. In this Forum Article, the main advances in the design of lanthanide-based nanoparticles will be discussed, with the first part focusing on the design of gadolinium(III)-based nanoparticles and their use as magnetic resonance imaging agents. The second part will then describe the main and most recent designs of luminescent lanthanide-based nanoparticles and their applications as sensors or imaging agents, with a special emphasis on our contribution to this area.

The luminescent dimeric ternary lanthanide–cyclen complexes (2-(Ln.1)2; Ln = Tb/Eu) were designed and both their self-assembly formation and their ability to detect anions via displacement assays were investigated using spectrophotometric titrations in MeOH solution. The formation of 2-(Tb.1)2 and 2-(Eu.1)2 was investigated in solution, and determination of the binding constants and stoichiometry showed that the former was formed almost exclusively over the 1:1 complex 2-(Tb.1) after the addition of two equivalents of 2; while for 2-(Eu.1)2 a mixture of both stoichiometries existed even after the addition of four equivalents of 2. Of these two systems, 2-(Tb.1)2 was studied in details as a probe for anions, where significant changes where observed in the photophysical properties of the complex; with the characteristic Tb(III)-centred emission being fully switched off upon the sensing of phosphates and nitrate, giving rise to the formation of a H2PO4−:Tb.1 complex in a 1:2 stoichiometry upon sensing of H2PO4− by 2-(Tb.1)2, while NO3− gave 1:1 complex formation and two equivalents of NO3−·Tb.1.

The mono-dentate, pyridyl containing, nitro naphthalimide ligands N-(4-pyridyl)-4-nitro-1,8-naphthalimide (L1) and N-(3-pyridyl)-4-nitro-1,8-naphthalimide (L2) were prepared and complexed with a selection of copper salts [Cu(OAc)2, Cu(CF3SO3)2 and Cu(ClO4)2]. Crystallographic studies were undertaken and revealed that dinuclear acetate bridged complexes resulted from reactions with Cu(OAc)2, while mononuclear systems resulted from reactions with Cu(CF3SO3)2 and Cu(ClO4)2. Despite the differing coordination environments the naphthalimide based ligands provided a range of interesting π-based interactions in the form of π⋯π, anion⋯π, nitro⋯π, solvent⋯π and CO⋯π associations. Solid state EPR spectra were in agreement with the coordination environments observed from crystallography.

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) is a terdentate binding motif that is synthesised modularly via the CuAAC reaction. Herein, we present the synthesis of ligands 1 and 2 and the investigation of the coordination chemistry, photophysical behaviour and electrochemistry of complexes of these with a number of d-metal ions (e.g. Ru(II), Ir(III), Ni(II) and Pt(II)). The X-ray crystal structures of ligand 1 and the complexes [Ru·22](PF6)Cl, [Ni·12](PF6)Cl and [Ir·1Cl3] are also presented. All of the complexes displayed non-classical triazolyl C–H⋯Cl− hydrogen bonding. All but one complex showed no metal-based luminescence at room temperature, while all of the Pt(II) complexes displayed luminescence at 77 K. The electrochemistry of the Ru(II) complexes was also studied and these complexes were found to have higher oxidation potentials than analogous compounds. The redox behaviour of [RuL2]2+ complexes with both 1 and 2 was nearly identical, while [Ru·1Cl2(DMSO)] was oxidised at significantly lower potential. We also show that the Ru(II) complex of 2, [Ru·22](PF6)Cl, gave rise to the formation of a metallo-supramolecular gel, the morphology of which was studied using scanning electron and helium ion microscopy.

The synthesis and characterisation of five bis-1,8-naphthalimide containing Tröger's bases 1–5 formed from their corresponding 3-amino-1,8-naphthalimide precursors 6–10 is described. The photophysical investigations of 1–5 and 6–10 were carried out in several organic solvents as well as in water and as a function of pH using UV-Vis absorption and fluorescence spectroscopies. The DNA binding affinities of 1–5 in aqueous solution at pH 7.4 were also investigated using several UV-Vis absorption and fluorescence experiments by using calf thymus DNA (ct-DNA). These molecules exhibited significant DNA binding affinities; where large binding values (Kb) in the range of 106 M−1 were determined, even in competitive media (50 mM and 160 mM NaCl at pH 7.4). Thermal denaturation measurements also showed that 1–5 significantly stabilised the DNA helix. Using linear and circular dichroism we further demonstrated that the DNA binding interaction occurs both by intercalation and by groove binding. The Tröger's bases were further shown to be rapidly taken up into cells using confocal fluorescence spectroscopy; and cytotoxic studies in HeLa and MCF-7 cells showed that most of the Tröger's bases were effective cytotoxic agents with EC50 values of between 1.1–12 μM and that all the active compounds induced programmed cell death by apoptosis, where up to 70% cellular death was observed after 24 h of incubation for 4.

The synthesis and photophysical studies of two cationic Tröger’s base (TB)-derived bis-naphthalimides 1 and 2 and the TB derivative 6, characterized by X-ray crystallography, are presented. The enantiomers of 1 and 2 are separated by cation-exchange chromatography on Sephadex C25 using sodium (−)-dibenzoyl-l-tartarate as the chiral mobile phase. The binding of enantiomers with salmon testes (st)-DNA and synthetic polynucleotides are studied by a variety of spectroscopic methods including UV/vis absorbance, circular dichroism, linear dichroism, and ethidium bromide displacement assays, which demonstrated binding of these compounds to the DNA grooves with very high affinity (K ∼ 106 M–1) and preferential binding of (−)-enantiomer. In all cases, binding to DNA resulted in a significant stabilization of the double-helical structure of DNA against thermal denaturation. Compound (±)-2 and its enantiomers possessed significantly higher binding affinity for double-stranded DNA compared to 1, possibly due to the presence of the methyl group, which allows favorable hydrophobic and van der Waals interactions with DNA. The TB derivatives exhibited marked preference for AT rich sequences, where the binding affinities follow the order (−)-enantiomer > (±) > (+)-enantiomer. The compounds exhibited significant photocleavage of plasmid DNA upon visible light irradiation and are rapidly internalized into malignant cell lines.

The development of functional 1,8-naphthalimide derivatives as DNA targeting, anticancer and cellular imaging agents is a fast growing area and has resulted in several such derivatives entering into clinical trials. This review gives an overview of the many discoveries and the progression of the use of 1,8-naphthalimides as such agents and their applications to date; focusing mainly on mono-, bis-naphthalimide based structures, and their various derivatives (e.g. amines, polyamine conjugates, heterocyclic, oligonucleotide and peptide based, and those based on metal complexes). Their cytotoxicity, mode of action and cell-selectivity are discussed and compared. The rich photophysical properties of the naphthalimides (which are highly dependent on the nature and the substitution pattern of the aryl ring) make them prime candidates as probes as the changes in spectroscopic properties such as absorption, dichroism, and fluorescence can all be used to monitor their binding to biomolecules. This also makes them useful species for monitoring their uptake and location within cells without the use of co-staining. The photochemical properties of the compounds have also been exploited, for example, for photocleavage of nucleic acids and for the destruction of tumour cells.

The bi-functional complex [PtII(terpy)(py-naphth)](NO3)2(1), incorporating both 2,2′:6′,2′′-terpyridine and 4-N,N′-dimethylamino-1,8-naphthalimide moieties, displays a high binding affinity for DNA as well as displaying cytotoxicity towards and inducing apoptosis in malignant cell lines.

The synthesis of five pyridyl derived thiosemicarbazides, 1–5, is presented. All five were formed in a single step from 2-hydrazinopyridine with commercially available isothiocyanates using microwave assisted synthesis. Compounds 1–4 were structurally characterised by single crystal diffraction analysis, and showed extended supramolecular hydrogen bonding arrays. Furthermore, the Zn(II) complexes 6 and 7 have been prepared using ligands 1 and 5, and structurally characterised, again showing elegant assemblies of metal-supramolecular structures with solvent accessible channels, demonstrating that these building units are excellent candidates for use in supramolecular chemistry and crystal engineering.

The synthesis and characterisation of two cationic pyridinium based 4-amino-1,8-naphthalimide derivatives (2 and 3) are described and compared to those of compound 1. The photophysical properties of 2 and 3 are shown to vary greatly with the solvent polarity and H-bonding ability. The dimethylamino substitution in 3 results in a weak quantum yield of fluorescence emission due to faster non-radiative deactivation of the excited singlet state than that seen for 2. As with 1, the fluorescence of 2 was found to be enhanced in its 1:1 complex with 5′-adenosine-monophosphate (5′-AMP) while it was partially quenched in its complex with 5′-guanosine-monophosphate (5′-GMP). In contrast, the fluorescence of 3 was enhanced (‘switched on’) in the presence of both adenine and guanine rich sequences. Linear and circular dichroism studies showed that each of 1, 2 and 3 binds to double-stranded DNA by intercalation. However, 2 and 3 do not show the preference for AT-rich DNA observed for 1. Comparative fluorescence studies with double stranded DNA show that the emission of 3 was 16 times enhanced in its DNA bound form, suggesting potential use of this structure as a spectroscopic probe for studying nucleic acid structure.

The thiosemicarbazide receptors 1 and 2, containing three N–H donors (including a thiourea motif), have been synthesized and studied for their anion binding abilities. The recognition of 1 and 2 with various anions [AcO−, F−, H2PO4−, SO42− and OH− for 1; OAc− and F− for 2] as their tetrabutylammonium salts was studied spectroscopically in CH3CN and EtOH. Significant changes were observed in the UV–vis absorption spectra of 1 which was red-shifted in both solvents when titrated with these anions. A dramatic colour change from yellow to red was also clearly observed. In contrast, the absorption changes for 2 were smaller with a colour change from colourless to yellow. These changes were determined to be due to deprotonation of the central N–H moiety (belonging to the thiourea) and not due to hydrogen bonding of these anions to the receptors. This was confirmed using single crystal X-ray crystallography, as crystals of 1 grown with TBAAc were found to correspond to the deprotonated structure 3.

Thiourea-functionalized Tröger’s base receptors 1 and 2 have been synthesized and evaluated as novel for the recognition of anions. Receptor 2 gave rise to significant changes in the absorption spectrum upon titration with AcO– and H2PO4– and acted as a colorimetric sensor for F–, the interaction of which was also evaluated using 1H NMR spectroscopy.

In this article, we examine the phenomenon of single-crystal halide salt wire growth at the surface of porous materials. We report the use of a single-step casting technique with a supramolecular self-assembly gel matrix that upon drying leads to the growth of single-crystal halide (e.g., NaCl, KCl, and KI) nanowires with diameters ∼130–200 nm. We demonstrate their formation using electron microscopy and electron-dispersive X-ray spectroscopy, showing that the supramolecular gel stabilizes the growth of these wires by facilitating a diffusion-driven base growth mechanism. Critically, we show that standard non-supramolecular gels are unable to facilitate nanowire growth. We further show that these nanowires can be grown by seeding, forming nanocrystal gardens. This study helps understand the possible prefunctionalization of membranes to stimulate ion-specific filters or salt efflorescence suppressors, while also providing a novel route to nanomaterial growth.Keywords: chemical garden; nanowires; self-assembly; sodium chloride; supramolecular gel

The incorporation of chiral amphiphilic lanthanide-directed self-assembled Nd(III) complexes (Nd.13 and Nd.23) into stable Langmuir monolayers, and the subsequent Langmuir–Blodgett film formation of these, is described. The photophysical properties of the enantiomeric pair of ligands 1 and 2 in the presence of Nd(CF3SO3)3 were also investigated in CH3CN solutions using UV–vis, fluorescence, and lanthanide luminescence spectroscopies. Analysis of the resulting self-assembly processes revealed that two main species were formed in solution,1:1 and 1:3 Nd:L self-assembly complexes, with the latter being the dominant species upon the addition of 0.33 equivalents of Nd(III). Excited state lifetime measurements of Nd.13 and Nd.23 in CH3OH and CD3OD and CH3CN were also evaluated. The formation of the self-assembly in solution was also monitored by observing the changes in the circular dichroism (CD) spectra; and large differences were observed between the 1:3 and other stoichiometries in the spectra, allowing for correlation to be made with that seen in the emission studies of these systems. Surface pressure–area and surface pressure–time isotherms evidenced the formation of stable Langmuir monolayers of Nd.13 and Nd.23 at an air–water interface, and the deposition of these monolayers onto a quartz solid substrate (Langmuir–Blodgett films) gave rise to immobilized chiral monomolecular films which exhibited Nd(III) NIR luminescence upon excitation of the ligand chromophore, demonstrating efficient energy transfer to the Nd(III) excided state (sensitized) with concomitant emission centered at 800 and 1334 nm.

The synthesis of a novel Tb(III) luminescent probe for the detection of thiols is presented. The probe 1.Tb, possessing a maleimide moiety, as its sulfhydryl acceptor, was poorly emitting in aqueous pH 7 solution in the absence of a thiol. However, upon addition of thiols such as glutathione (GSH), large enhancements were observed, particularly within the physiological pH range. In contrast no enhancements were observed in the presence of the oxidized form of glutathione (GSSG), except in the presence of the enzyme glutathione reductase and NADPH which enabled 1.Tb to be used to observe the enzymatic reduction of GSSG to GSH in real time.

The synthesis and photophysical properties of 1 and 2, two Ru(II)-polypyridyl based-1,8-naphthalimide Tröger's bases, are described; these were found to stabilize double stranded DNA, undergo rapid cellular uptake, displaying good luminescence without affecting cell viability even after 24 hours of incubation.

A novel near-infrared (NIR) emissive lanthanide-based zinc sensor was designed, based on the self-assembly in aqueous solution between the nonemissive coordinatively unsaturated Yb(III) cyclen complex 2·Yb and the sulfonated 8-hydroxyquinoline (8-HQS) chromophore, which was employed as a sensitizing antenna. The resulting ternary complex, 2·Yb·8-HQS, displayed metal-centered emission in the NIR range upon excitation of the antenna with high quantum yield of Q = 0.23 ± 0.03% in pH 7.4 buffered aqueous solution; demonstrating efficient sensitization from 8-HQS. The addition of zinc led to quenching of the NIR emission as a result of the dissociation of the luminescent ternary 2·Yb·8-HQS complex, where the 8-HQS antenna was displaced from the Yb(III) center in favor of the formation of more stable chelates with Zn(II). These newly formed Zn(II) complexes were shown to exhibit strong green fluorescence; allowing for the simultaneous sensing of Zn(II) both within the visible and the NIR regions at physiological pH in competitive media. Furthermore, 2·Yb·8-HQS was shown to be able to detect Zn(II) with good selectivity and in a reversible manner, even in the presence of competitive group (I) and (II) metal ions as well as in the presence of several biologically important d-metal ions.

The new Ru(II) polypyridyl complex 1 was synthesised using microwave irradiation from the new polypyridyl ligand 2 ‘DipyTAP’, and its photophysical properties, and DNA binding abilities were investigated using various spectroscopic techniques; and 1 was shown to act as a ‘NIR molecular light switch’ for DNA with an emission window between 680 and 860 nm.

The investigation into the luminescence properties of a lanthanide-binding peptide, derived from the Ca-binding loop of the parvalbumin, and modified by incorporating a 1,8-naphthalimide (Naph) chromophore at the N-terminus is described. Here, the Naph is used as a sensitising antenna, which can be excited at lower energy than classical aromatic amino acids, such as tryptophan (the dodecapeptide of which was also synthesised and studied herein). The syntheses of the Naph antenna, its solid phase incorporation into the dodecapeptide, and the NMR investigation into the formation of the corresponding lanthanide complexes in solution is presented. We also show that this Naph antenna can be successfully employed to sensitize the excited states of both europium and terbium ions, the results of which was used to determined the stability constants of their formation complexes, and we demonstrated that our peptide ‘loop’ can selectively bind these lanthanide ions over Ca(II).

The design and synthesis of dinuclear-lanthanide complexes possessing triazole-based bridges, formed by using copper catalysed 1,3-cycloaddition reactions between heptadentate alkyne functionalised cyclen europium or terbium complexes and di-azides (CuAAC reactions), are described. While this click reaction worked well for the formation of the homo-Eu(III) and Tb(III) bis-tri-arm cyclen N,N-dimethyl acetamide complexes, 2Eu and 2Tb, and for the homo-Eu(III) chiral N-methylnaphthalene based complexes 3Eu (S,S,S) and 4Eu (R,R,R), the formation of the Eu(III) complex of the primary amide analogue of 2, namely 1Eu, was not successful, clearly demonstrating the effect that the nature of the pendant arms has on this reaction. Furthermore, the click reactions between the free alkyne cyclen bis-derivatives (5–8) and the di-azide were unsuccessful, most likely due to the high affinity of the cyclen macrocycles for Cu(II). The Eu(III) complexes of 2–4 and 2Tb all gave rise to sensitised metal ion centred emission upon excitation of the triazole or the naphthalene antennae in methanol solution, and their hydration states were determined, which showed that while the Eu(III) mono-nuclear complexes had q ∼ 2, the click products all had q ∼ 1. In the case of 3Eu (S,S,S) and 4Eu (R,R,R), the circular polarised emission (CPL) was also observed for both, demonstrating the chiral environment of the lanthanide centres.

The synthesis, characterisation and solid state crystal structure of a cationic 4-amino-1,8-naphthalimide derivative (1) are described. The photophysical properties of 1 are shown to vary with the solvent polarity and H-bonding ability. The fluorescence of 1 is enhanced and blue-shifted in its 1:1 complex with 5′-adenosine-monophosphate while it is partially quenched and red-shifted in its complex with 5′-guanosine-monophosphate. Linear and circular dichroism measurements show that 1 binds to double-stranded DNA by intercalation. Comparative UV-visible and fluorescence studies with double stranded synthetic polynucleotides poly(dA–dT)2 and poly(dG–dC)2 show that 1 binds much more strongly to the AT polymer; 1 also has a strong preference for A–T rich sequences in natural DNA. Thermal denaturation measurements also reveal a much greater stabilisation of the double-stranded poly(dA–dT)2 than of natural DNA.

The self-assembly between the amidothiourea ligands 1 and 2 and Tb(III) gave rise to the formation of 1:1 and 3:1 (ligand–Tb) supramolecular architectures that were shown to be luminescent in both MeOH and in water–DMSO solutions; preliminary investigations in the latter system also showed that their emission was modulated upon interacting with anions such as acetate and phosphate due to host–guest formation.

The inclusion of a urea functionality into the coordination sphere of a Ru(II)-polypyridyl complex (Ru·L1) resulted in a system that can function as an effective long wavelength emissive fluorescent anion sensor. The MLCT emission of Ru·L1 is sensitive to the binding of acetate, phosphate and pyrophosphate but not fluoride in organic solvent. In addition, Ru·L1 can distinguish between phosphate and pyrophosphate with an emission increase upon binding of H2PO4- (“turn on” sensor) and an emission decrease upon binding of HP2O73- (“turn off” sensor), which occurs via hydrogen bonding to the urea receptor moiety as demonstrated by carrying out NMR titrations as well as by employing [Ru(II)bpy3](PF6-)2 as a model compound that lacks the anion receptor moiety.Graphical abstractThe synthesis, solid state structure and photophysical analysis of a new type of Ru(II) polypyridyl based MLCT emissive sensor for anions is presented. The anion sensing in CH3CN is shown to be both selective for phosphate and pyrophosphate, both of which are recognized with high affinity.Highlights► We include a urea functionality into a Ru(II)-polypyridyl complex (Ru·L1). ► We examine changes in photophysical properties of Ru·L1 on addition of anions. ► Luminescence of Ru·L1 is modulated by acetate, phosphate and pyrophosphate. ► Ru·L1 can distinguish between phosphate and pyrophosphate.

In this paper, the synthesis and the spectroscopic investigation of new colorimetric receptors for anions 3–6, possessing a glycol chain at the 4-position of the pyridyl ring, and 1 and 2, which lack such a chain, and the X-ray crystal structure of 2 is presented. Structures 3–6 are able to bind to anions in competitive media, such as alcohol or in a mixture of methanol and water, where the anion recognition gives rise to changes in the absorption spectra, which is red-shifted, in 1:1 or 1:2 (sensor/anion) stoichiometry. The anion recognition for 1 and 2 was also investigated in organic solvents and in a 4:1 mixture of DMSO/H2O. The binding of 1 to anions such as acetate, phosphate, and fluoride was also evaluated using 1H NMR in DMSO-d6.

The synthesis and photophysical and biological investigation of Ru(II)-polypyridyl stabilized water-soluble, luminescent gold nanoparticles (AuNPs) are described. These structures bind to DNA and undergo rapid cellular uptake, being localized within the cell cytoplasm and nucleus within 4 h.

The quaternarized pdppz derivative 1 was shown to bind strongly to DNA with concomitant changes in its ground and excited state photophysical properties. Furthermore, the compound also showed rapid cellular uptake, and induced apoptosis upon light irradiation in various cancer cell lines after 24 hours of incubation.

1·Eu·BPS was developed as a luminescent lanthanide sensor for use in displacement assays for detection of d-metal ions by monitoring the changes in the europium emission, which was quenched for iron(II), with a detection limit of ∼10 pM (0.002 μg L−1) for Fe(II) in buffered pH 7.4 solution.

The formation of self-assembly complexes between the ligands 1 (SS) and 2 (RR) and terbium or europium was undertaken and shown (using various spectroscopic titrations) to give rise to the exclusive formation of 2:1 (L:Ln) stoichiometry and not the anticipated 3:1 stoichiometry.

The synthesis and binding investigations of first generation C3v-symmetrical hydrogen bonding urea-amide based tripodal receptors, 1–6, with various anions such as acetate, phosphate, sulfate and chloride in DMSO-d6 are presented. Analysis of the 1H NMR titrations of 1–6 showed on all occasions the selective formation of 1:1 stoichiometries.

The reaction of tetrasodium-4,4′,6,6′-tetracarboxy-2,2′-bipyridine (Na4L) with various lanthanide ions yields a family of isostructural supramolecular {Na2[Ln2L2]} complexes (1−4), where LnIII = Eu, Nd, Gd, and Tb. Strikingly, these complexes luminesce in buffered H2O or D2O solutions in either the visible or near-IR regions, despite their high hydration states.

As luminescent surface-functionalized gold nanoparticles emerged as potential powerful analytical tools in the biomedical fields, understanding the interaction of such systems with proteins has become crucial. In the present study, the interaction of luminescent water-soluble gold nanoparticles (AuNP-1·Eu-nta), obtained through the self-assembly of a naphthalene β-diketone antenna with a Eu(III) cyclen complex tethered to the gold surface via a C12 alkyl thiol spacer, with bovine serum albumin (BSA) was investigated. The changes in the UV–visible absorption and fluorescence spectra of both the antenna and protein, as well as in the time-resolved Eu(III)-centered emission, of the resulting self-assembly were monitored, at physiological pH, as a function of the BSA concentration. We demonstrate that the Eu(III) emission arising from the self-assembly on the AuNP surface is almost completely quenched upon addition of BSA. Binding constant determination clearly showed that the sensitizing antenna was not displaced and that the quenching was the result of the interaction between the antenna and BSA. Detailed spectroscopic studies performed on the nta–BSA system brought a better insight in the strength of such interaction as well as its effect on the protein secondary structure. Finally, the information gathered on each system resulted in applying AuNP-1·Eu-nta–BSA for the luminescent detection of drugs via the perturbation of the nta–BSA interaction. Competitive titrations using ibuprofen and warfarin showed that nta was located in the binding site II of BSA and that the presence of warfarin, a site I drug, did not interfere with the detection of site II ibuprofen.Keywords: bovine serum albumin; competitive displacement assay; Eu(III)-functionalized gold nanoparticles; luminescence quenching; luminescent ternary complex; protein−nanoparticle interaction

This critical review focuses on the development of anion sensors, being either fluorescent and/or colorimetric, based on the use of the 1,8-naphthalimide structure; a highly versatile building unit that absorbs and emits at long wavelengths. The review commences with a short description of the most commonly used design principles employed in chemosensors, followed by a discussion on the photophysical properties of the 4-amino-1,8-naphthalimide structure which has been most commonly employed in both cation and anion sensing to date. This is followed by a review of the current state of the art in naphthalimide-based anion sensing, where systems using ureas, thioureas and amides as hydrogen-bonding receptors, as well as charged receptors have been used for anion sensing in both organic and aqueous solutions, or within various polymeric networks, such as hydrogels. The review concludes with some current and future perspectives including the use of the naphthalimides for sensing small biomolecules, such as amino acids, as well as probes for incorporation and binding to proteins; and for the recognition/sensing of polyanions such as DNA, and their potential use as novel therapeutic and diagnostic agents (95 references).

The selective sensing of fluoride (F−; an important breakdown product of some chemical warfare agents such as sarin) was achieved by observing quenching in the metal to ligand charge transfer (MLCT) emission of the 1,8-naphthalimide–ruthenium conjugate Ru–Nap-NH2, which occurs at long wavelengths in CH3CN, using steady state fluorescence spectroscopy. The F− recognition was also visible to the naked eye, with a clear colour change from yellow to red. The sensing mechanism is most likely initially via hydrogen bonding, between the anion and the amine, which, at higher concentrations, gives rise to deprotonation of the acidic 4-amino-1,8-naphthalimide moiety. However, counter ion effects may also be contributing to the overall emission changes. Other anions such as acetate, phosphate and chloride also give rise to quenching in the fluorescence emission with only minor changes in the UV–vis absorption spectra of Ru–Nap-NH2. Moreover, phosphate also gave rise to shifts in the λmax in the emission spectra.

The design, synthesis and photophysical evaluation of 1.Tb.Na, a Tb(III)-cyclen-based sensor, possessing a phenyl iminodiacetate-based receptor, for the selective detection of Cu(II) and Hg(II) ions in water is demonstrated. Sensitisation of the Tb(III) 5D4 excited state was achieved by excitation of the phenyl receptor, which in water gave rise to a characteristic time-delayed and line-like Tb(III) emission. The Tb(III) emission was shown to be pH independent over the physiological pH window. The changes in the Tb(III) emission were monitored by carrying out metal titrations using various groups I, II and transition metal ions. Of these, only the titrations of Cu(II) and Hg(II) gave rise to modulations in the Tb(III) emission; resulting in quenching in the Tb(III) emission by ca. 65% and 40%, respectively.

The quinoxaline 1, possessing a 2,6-pyridyl-based amidothiourea moiety, with the view of forming a pre-organised molecular cleft, was developed as a fluorescent anion sensor. The sensing ability of 1 was evaluated in organic solution where both the ground and the excited state of 1 was affected upon recognition of anions such as acetate [as tetrabutylammonium salt (TBAAc) solution] at the amiodothiourea moieties in MeCN. The fluorescence of 1, with λmax at 477 nm, was, on all occasions quenched, upon anion recognition. Using TBAOH, we also show that the same anion-induced changes occurred; demonstrating that for this particular sensor, the anion-sensing takes place via a deprotonation mechanism. This anion-induced deprotonation event was further investigated by carrying out 1H NMR titrations on 1, using both AcO− and OH− in DMSO-d6.

The synthesis and photophysical evaluation of a new supramolecular lanthanide complex is described which was developed as a luminescent contrast agent for bone structure analysis. We show that the Eu(III) emission of this complex is not pH dependent within the physiological pH range and its steady state emission is not significantly modulated by a series of group I and II as well as d-metal ions and that this agent can be successfully employed to image mechanically formed cracks (scratches) in bone samples after 4 or 24 h, using confocal laser-scanning microscopy.

The synthesis and photophysical evaluation of two enatiomerially pure dimetallic lanthanide luminescent triple-stranded helicates is described. The two systems, formed from the chiral (R,R) ligand 1 and (S,S) ligand 2, were produced as single species in solution, where the excitation of either the naphthalene antennae or the pyridiyl units gave rise to Eu(III) emission in a variety of solvents. Excitation of the antennae also gave rise to circularly polarized Eu(III) luminescence emissions for Eu2:13 and Eu2:23 that were of equal intensity and opposite sign, confirming their enantiomeric nature in solution providing a basis upon which we were able to assign the absolute configurations of Eu2:13 and Eu2:23.

The formation of a self-assembly between a sensitising antenna and an Eu(III) functionalised cyclen complex 1·Eu, tethered to a gold surface via a C12 alkyl thiol spacer, is described where changes in the Eu(III) emission signal the formation, and dissociation, of a ternary complex.

A new family of mixed-lanthanide (YbIII and NdIII) transition-metal (f−d) cyclen-RuII(phen)3 (phen = 1,10-phenanthroline) complexes were synthesized as dual visible- and near-infrared (NIR)-emitting DNA probes/sensors. Significant changes were seen in both the RuII visible and the YbIII-centered NIR emission, which was switched off upon binding to DNA at pH 7.4. In contrast, no changes were seen in the NdIII emission of the analogue f−d conjugate.

The synthesis and photophysical properties of the Eu(III) complex 1.Eu, based on the use of 1,10-phenanthroline (phen) as a combined sensitizing antenna and a transition metal ion coordinating ligand, is described. The long-wavelength Eu(III) emission from this complex was found to be highly pH sensitive, giving rise to a ‘off-on-off’ pH profile with maximum emission occurring within the physiological pH range. This allowed for the use of 1.Eu as a luminescent sensor for transition metal ions, where the titration with ions such as Cu(II), Co(II) and Fe(II) gave rise to the formation of mixed f–d nuclear complexes, with concomitant changes in the photophysical properties of 1.Eu. Here, changes in both the ground and the singlet excited state properties of the phen antenna were observed, but the largest changes were observed for the delayed Eu emission, which was fully quenched upon titration with these ions in aqueous pH 7.4 buffered solutions. In comparison, no changes were observed in the Eu(III) emission upon titration with ions such as Zn(II) or group I and II ions. From these changes, we were able to demonstrate the binding stoichiometry and the binding constant for the formation of novel supramolecular complexes between 1.Eu and Cu(II), Co(II) and Fe(II), which showed that either two or three equivalents of 1.Eu complexed to each of these transition metal ions, giving rise to the formation either linear f–d–f or branched f3–d based mixed nuclear complexes in solution.

A cyclen based lanthanide luminescent sensor, Tb.1, has been developed by taking advantage of a combination of hydrogen bonding and f-metal ion coordination binding sites for anionic species. Analysis of the ground state, the emission from the singlet and the Tb(III) excited states clearly show the ability of the Tb(III) complex to signal the presence of anions in CH3CN, through multiple binding interactions consisting of hydrogen bonding and metal coordination. The delayed lanthanide luminescence from the Tb(III) diaryl–urea complex was found to be significantly enhanced only upon recognition of H2PO4−, at the same time as displaying good selectivity over other competitive anions, such as CH3COO−.

The thiourea based 4-amino-1,8-naphthalimide molecules 1–5 were designed as fluorescent anion sensors and their photophysical properties investigated upon recognition of biologically relevant anions such as acetate, dihydrogen phosphate and fluoride in DMSO. Synthesised in a single step from their respective aniline precursors, 6–9, these molecules were designed on the fluorophore–spacer–receptor principle, where in the case of sensors 1–3 the thiourea anion recognition moieties were connected to the fluorophore via the 4-amino moiety, while sensors 4 and 5 had the thiourea moieties connected to the ‘imide’via a CH2 spacer. The current study showed that 1–5 operated as photoinduced electron transfer (PET) sensors, as no significant changes were observed in their absorption spectra, while their fluorescence emissions were quenched upon recognition of ions such as AcO−, H2PO4− and F−, which demonstrates that bidirectional PET sensing occurs in such naphthalimide based anion sensors.

The design and synthesis of 1Tb, a Tb(III) quinoline–cyclen based complex, are described. The results from the photophysical investigation of 1Tb are compared with the results observed from its Eu(III) analogue, 1Eu. The Tb(III) emission of 1Tb was found to be highly pH dependent, being reversibly switched “off–on–off”, with maximum emission intensities observed within the physiological pH range, contrary to what was observed with the Eu(III) analogue. Furthermore, the Tb(III) emission was also found to be sensitive to molecular oxygen concentration. By using a stoichiometric amount of both complexes, the changes observed in the combined lanthanide Eu/Tb-emissions (described as output) as a function of pH and O2 (as ion/molecular inputs) can be employed in ratiometric sensing of these analytes. Furthermore, the outputs can also be described in molecular logic gate terms, where the emission at three different emission wavelengths can be described in terms of Boolean algebra, corresponding to the NAND, NOR and NOT logic gates functions.

The synthesis, NMR and luminescent analysis of a novel polypeptide possessing a sensitising naphthalimide antenna at the amino terminus, and a model compound with a Trp moiety, and their Eu(III) and Tb(III) complexes are described.

The design, synthesis and physical evaluation of 1, a visible colorimetric ‘naked eye’ pyridyl based bis-amidothiourea sensor for anions, is described. This charge neutral sensor gives rise to significant changes in the absorption spectra upon interactions with several important biological anions such as AMP and ADP in 4 : 1 DMSO–H2O solution, while ATP was not detected. These colorimetric changes are due to the formation, or the combination of hydrogen bonding complexes and/or deprotonation between these anions and 1.

Luminescent europium and terbium complexes and a mixed Eu(III)–Tb(III) complex were prepared, each with three macrocycles coordinating to a single lanthanide ion to form a trimetallic system, and can be used for the ratiometric sensing of anions in the case of the mixed complex.

The synthesis and the photophysical evaluation of a novel pH dependent lanthanide luminescent self-assembly in water between a cyclen based europium complex and a β-diketonate is described and its use as a luminescent sensor in displacement assays for anions such as acetate, bicarbonate and lactate, where the Eu(III) emission was quenched upon anion recognition.

The delayed lanthanide luminescence of the terbium [Tb(III)] diaryl-urea complex 1·Tb is significantly enhanced upon sensing of dihydrogenphosphate (H2PO4−) in CH3CN, which occurs through multiple anion binding through hydrogen bonding interactions and potential metal ion coordination to Tb(III).

The design and synthesis of two novel fluorescent sensors based on the photoinduced electron transfer (PET) concept, 1 and 2, for the detection of zinc under competitive media is described. These sensors are based on the 4-amino-1,8-naphthalimide fluorophore, which has an absorption band centred at 450 nm and emits in the green with λmax ∼550 nm. By functionalizing the chromophore with a simple benzyl or ethyl-aryl based iminodiacetate receptor at the 4-position, both high selectivity and sensitivity were achieved for the sensing of Zn(II) over other competitive transition and Group I and II metal ions. These sensors were also shown to be pH independent, with a pKa of 2.3 being determined for 1, which allows these to be used in highly competitive pH media. Upon sensing of Zn(II) the fluorescence emission spectrum is ‘switched on’ demonstrating the suppression of PET from the receptor to the fluorophore. For 1, the sensing of Zn(II) was achieved with Kd = 4 nM when measured in pH 7.4 buffered solution, in the presence of 1.1 mM of EGTA.

This article gives some highlights of the recent advances in the development of novel lanthanide based complexes, conjugates and self-assembly structures formed from the use of organic ligands and organo-metallic (transition metal) complexes, that are designed with the aim of capitalising on the high coordination requirement of the lanthanide ions. The examples shown, demonstrate the versatility of the lanthanide ions as luminescent probes and sensors that emit at long wavelength either in the visible or the near infrared (NIR) part of the electromagnetic spectrum.

The synthesis of several cyclen based lanthanide [Eu(III) and La(III)] complexes is described; these metallo ribonuclease mimics are based on the use of alkyl amines as pendent arms, which give rise to fast hydrolysis within the physiological pH range of HPNP (an RNA model compound) that is highly dependent on the length of the alkyl spacer.

The design and synthesis of several bis-macrocyclic cyclen (1,4,7,10-tetraazacyclododecane) ligands and their corresponding lanthanum or europium complexes is described; these dinuclear lanthanide systems were made by connecting two macrocyclic cyclen moieties through a rigid, covalent, p-xylylenediamide bridge or a flexible aliphatic hexane bridge. These ligands were subsequently functionalised with six acetamide pendant arms (CONR1R2: R1 = R2 = H or CH3, or R1 = H, R2 = CH3). The corresponding lanthanide bis-complexes were then formed by reaction with La(III) and Eu(III) triflates, yielding overall cationic (+VI charged) complexes.

The design and synthesis of several novel X-ray contrast agents 1–3, developed for targeting bone structures, and in particularly microcracks in bones, using CT (Computer Tomography) detection is described. These contrast agents are based on the use of the well known triiodobenzene platform, which was conjugated into one or more phenyliminodiacetate moieties, which can be used to ‘lock’ onto bone matrices. Compounds 1–3 were all tested for their ability to visualise cracks in bone structures (bovine bones) using μ-CT imaging.

The synthesis of the fluorescent photoinduced electron transfer (PET) chemosensors 1–3 for bis-anions such as bis-carboxylates and pyrophosphate in organic solvents is described herein. These sensors are based on the receptor–spacer–fluorophore–spacer–receptor motif where the receptors are charge neutral aromatic thiourea or urea receptors and the fluorophore is anthracene. The anion recognition was evaluated using 1H NMR as well as absorption and fluorescence spectroscopy in DMSO. For simple anions such as acetate or fluoride, the recognition was shown to be through hydrogen bonding of the corresponding anion to the receptors. This gave rise to only minor changes in the absorption spectra, but significant changes in the fluorescence emission spectra, which was substantially (70–95%) quenched. Analysis of these recognition events implied a 1 : 2 (sensor : anion) binding and ideal PET behaviour for ions such as AcO− and H2PO4−. For F−, the luminescent quenching indicated a 1 : 1 binding, but we deduced that this was due more to complete quenching of the excited state after the addition of one equivalent of the anion. For all of the anions, the quenching contributed to enhanced efficiency of PET from the receptors to the excited state of the fluorophore. In the case of the bis-anions (ambient), such as di-carboxylates, similar fluorescence quenching was observed. However, here either a 1 : 1 or a 1 : 2 binding was observed depending on the length of the spacer separating the two carboxylate moieties and the nature of the receptor. Whereas both pyrophosphate and malonate gave rise to a 1 : 1 binding, glutarate gave rise to ∼1 : 2 binding for the thiourea sensors 1 and 2. However, for the urea based sensor 3, the binding was found to be 1 : 1 for all the bis-anions. For such a 1 : 1 binding we propose that the anion most likely bridges the fluorophore moiety. This was also evident from the 1H NMR (DMSO-d6) spectrum where the anthracene resonances were significantly affected. By simply modifying the electronic structure of the receptor, the sensitivity of the recognition process could also be modified; e.g. compound 1, bearing the trifluoromethyl substituent, showed stronger binding to the bis-anions than 2, which possessed a simple phenyl moiety.

The synthesis of four bis-macrocyclic conjugates made from the coupling of either diaza-15-crown-5 ethers (1 and 3) and diaza-18-crown-6 ethers (2 and 4) to either amide or carboxylate functionalized cyclen (1,4,7,10-tetraazacyclododecane), and their corresponding cationic Tb(III) complexes, Tb·1, Tb·2, and neutral complexes Tb·3 and Tb·4 are described. The effect on the ground, singlet excited states and the Tb(III) emission, was investigated either as a function of pH or the concentration of several Group I and II cations, upon excitation at 300 nm. The ground state and singlet excited states of the Tb(III) complexes were found to be modulated by ions such as H+, Na+ or K+, signifying the recognition of these ions by the crown ether receptors. In acidic media, below pH 4, the Tb(III) emission was highly pH sensitive, gradually increasing with large orders of magnitude of luminescence enhancements. For Tb·1 and Tb·2 complexes, the Tb(III) emission was also ‘switched on’ in alkaline media above pH 8. At pH 7.4, the recognition of Na+ or K+ also gave rise to a significant change in the Tb(III) emission due to the modulation of the antenna-receptor moieties by these ions. For Tb·1 and Tb·3 the largest changes were seen for Na+, whereas for Tb·2 and Tb·4 the largest changes were seen for K+.

The cationic cyclen based Eu(III)–phen conjugated 1·Eu was synthesised as a chemosensor for Cu(II), where the recognition in water at pH 7.4 gave rise to quenching of the Eu(III) luminescence and the formation of tetranuclear polymetallic Cu(II)–Eu(III) macrocyclic complexes in solution where Cu(II) was bound by three 1·Eu conjugates.

The synthesis of four fluorescent photoinduced electron transfer (PET) chemosensors 1–4 for anions is described. These are all based on a simple design employing charge neutral aliphatic or aromatic thiourea anion receptors connected to an anthracene fluorophore via a methylene spacer. Here the anion recognition occurred through 1 : 1 hydrogen bonding between the thiourea protons and the anion, as demonstrated by observing the changes in the 1H NMR in DMSO-d6 where the two thiourea protons were shifted downfield upon addition of anions. Whereas 1–3 were designed for the detection of anions such as fluoride, acetate or phosphate, 4 was made for the recognition of N-protected amino acids. All the sensors showed ‘ideal’ behaviour where only the fluorescence emission was quenched upon anion recognition, due to enhanced efficiency of electron transfer quenching from the receptor to the excited state of the fluorophore. By simply varying the nature of the thiourea substituent it was possible to modulate, or tune, the acidity of the thiourea receptor moiety, altering the sensitivity of the anion recognition. For 1, the anion selectivity and the degree of the fluorescence quenching were in the order of F− > AcO− > H2PO4−, with Cl− or Br− not being detected.

The cyclen based aromatic diaza-15-crown-5 and 18-crown-6 ether conjugates 1Tb–4Tb were designed as luminescent switches for sodium and potassium where the delayed Tb(III) emission, occurring as line-like emission bands between 490–622 nm, was ‘switched on’ upon recognition of these ions in pH 7.4 buffered water solution.

The naphthalimide derivative 1 was designed as a fluorescence PET sensor for Zn(II); 1 showed excellent selectivity for Zn(II) at pH 7.4, even in the presence of other competitive cations, the emission, being pH independent above pH 3.5, was switched on upon Zn(II) recognition.

The coordinately unsaturated terbium complexes Tb.1 and Tb.2 possess two labile metal-bound water molecules that can be displaced upon metal chelation to aromatic carboxylic anions such as salicylic acid in water, which gives rise to large enhancements in the Tb(III) luminescence.

The lanthanide ion based macrocyclic complexes 1·Ln mimic the hydrophobic nature of ribonucleases, where the lanthanide ions induce the formation of a hydrophobic cavity for 1, giving rise to a large order of magnitude enhancement in the hydrolytic cleavage of HPNP.

The syntheses of the bis-aromatic 15-crown-5 and 18-crown-6-ether derivatives 1 and 2, for the selective recognition of intra- and extracellular concentrations of Na+ and K+, from o-anisidine are described. These compounds were made with the aim of incorporating them into luminescent sensors. To facilitate their incorporation into such sensors, the compounds were nitrated. Whereas the use of conventional nitration reagents such as HNO3, HNO3–H2SO4 or NO2BF4 only gave low yields of the desired products, both mono- and bis-nitration products were formed in good yield by one-pot synthesis using NaNO2 in a mixture of water and acetic acid (10 ∶ 1). The use of X-ray crystallography proved the presence of the nitro-substituted products. An investigation into this mild nitration method revealed that the substitution pattern on the aromatic ring governed the ability of the compounds to undergo nitration.

The azo-dye based colorimetric chemosensors 1 and 2 were designed for the detection of intracellular concentrations of Na+ and K+ respectively. The two sensors were synthesised from the o-anisidine based receptors 3 and 4 in a one step synthesis. Both displayed good selectivity and sensitivity for the relevant group I cations in the appropriate physiological conditions, with 1 displaying large changes in the absorption spectra upon Na+ recognition with λmax being shifted from 488 nm to 377 nm in the absorption spectra when measured in 50 ∶ 50 MeOH–H2O buffered pH 7.4 solutions and in the presence of high ionic strength. Similarly, 2 showed both a hypsochromic shift and hypochromic effect upon titration with K+. These changes were clearly visible; with red to yellow colour changes taking place upon ion recognition.

We demonstrate for the first time that the charge neutral anthracene based fluorescent sensors 1a–c, having an aromatic or aliphatic thiourea moiety as an anion receptor, show ideal PET sensor behaviour where the anthracene fluorescence emission is selectively quenched upon titration with AcO−, H2PO4− and F− but not by Cl− and Br− in DMSO.

The Tb(III) complex 3, is the first example of a molecular logic gate corresponding to a two-input INHIBIT function, A ⁁ B′ where the ‘output’, a sharp, line-like, terbium emission, is only observed with two chemical inputs (i) the presence of protons and (ii) the absence of molecular oxygen.

The cyclen-based ligand 1, possessing a [1,10]-phenanthroline moiety as a pendant arm, has been used as a ditopic ligand for the complexation of d- and near infrared (NIR) emitting (and) f-metal ions. This ligand forms kinetically stable complexes, 1·Ln, with lanthanide ions such as Ln = ytterbium, neodymium, and lutetium (formed as a non-IR emitting reference compound), the synthesis and photophysical properties of which are described herein in detail. These 1·Ln complexes were then used as building blocks for the formation of mixed d−f heteropolymetallic self-assemblies, where the phen moiety was used to complex a ruthenium (Ru(II)) ion, giving the d−f3 complexes Ru·Ln3 (Ln = Nd(III), Yb(III), Lu(III)). The formation of these supramolecular coordination conjugates was studied by using absorption and luminescence spectroscopy, while the solution structure of the Ru·Lu3 was elucidated by 1H NMR in D2O and H2O. Of these conjugates, both Ru·Nd3 and Ru·Yb3 displayed an intense NIR-emission in H2O at pH 7.4 (with QYbL = 0.073% and QNdL = 0.040%) and in D2O (with QYbL = 0.23% and QNdL = 0.10%). By comparison with their monometallic analogues Ln·1 (Ln = Nd(III), Yb(III)), we demonstrate that our new design possesses an enhanced sensitization efficiency for lanthanide metal centered sensitization upon using the [Ru(phen)3] moiety (d → f energy transfer) as a visibly exciting antenna, and we demonstrate that the intensity of the Ru(II)-based luminescence strictly correlates to the efficiency of the d → f energy transfer processes.

The new Ru(II) polypyridyl complex 1 was synthesised using microwave irradiation from the new polypyridyl ligand 2 ‘DipyTAP’, and its photophysical properties, and DNA binding abilities were investigated using various spectroscopic techniques; and 1 was shown to act as a ‘NIR molecular light switch’ for DNA with an emission window between 680 and 860 nm.

The synthesis, spectroscopic characterisation and biological evaluation of mono- and bis-1,8-naphthalimide-conjugated ruthenium(II)-polypyridyl complexes is presented. Spectroscopic DNA titrations, together with denaturation studies, show strong binding of both species to DNA through the naphthalimide arms. Linear and circular dichroism (LD and CD) spectroscopy reveal close association of the Ru(bpy)32+ core with DNA in the case of the mono-naphthalamide complex, [Ru(bpy)2(bpy-NAP)]2+. Significantly, binding by the second naphthalimide arm in the [Ru(bpy)2(bpy-NAP2)]2+ complex is found to displace the Ru(bpy)32+ centre from the DNA backbone. This ‘negative allosteric effect’ is found to have a dramatic influence on the photoinduced damage of plasmid DNA, and the viability of HeLa cancer cells upon photoactivation. Overall the study clearly maps and correlates the relationship between molecular structure, in vitro binding and activity, and in cellulo function.

The synthesis of the ‘click’ derived 1,4-di(2-pyridyl)-1,2,3-triazole (dpt) chelator/ligand from 2-azidopyridine and 2-ethynylpyridine using Cu(I) and TBTA by microwave assisted synthesis is presented. The complexes of the subtly unsymmetric dpt ligand with Cu(I), Pt(II), Co(II), and Ag(I) were structurally characterised by using conventional methods, as well as using single crystal and powder diffraction analysis. The results of the studies showed formation of discrete molecules displaying preferential binding of the d-metal cations through the pyridyl nitrogen N1 and the proximal triazolyl nitrogen N2 i.e. the 2-(1H-1,2,3-triazol-4-yl)pyridine or ‘regular' chelate moiety despite the presence of a second potential binding pocket i.e. the 2-(1H-1,2,3-triazol-1-yl)pyridine chelate or ‘inverse’ moiety. This binding selectivity was corroborated through the study of the self-assembly of dpt with Cu(I) and Ag(I) using 1H NMR titration in CD3CN solution, as well as using UV-Vis absorption titrations; the former showing a broadening of the proton peaks associated with that chelate pocket.

A heterotopic naphthalimide ligand N-(4-picolyl)-4-(4′-carboxyphenoxy)-1,8-naphthalimide HL is utilised for the formation of self-assembled soft materials. In the presence of K+ ions, L− forms a robust photoluminescent hydrogel 1 which is reversible under thermal, mechanical or chemical stimuli.

The synthesis of coordinatively unsaturated tetra-substituted 1,4,7,10-tetraazacyclododecane (cyclen) lanthanide complexes is described; these structures, possessing hydrophobic (C12-alkyl) tails and hydrophilic head groups, self-assemble into supramolecular micellar structures in aqueous solution, and hence can be utilised as novel contrast agents for MRI.

A V-Shaped 4-amino-1,8-napthalimide derived tetracarboxylic acid linker (L; bis-[N-(1,3-benzenedicarboxylic acid)]-9,18-methano-1,8-naphthalimide-[b,f][1,5]diazocine) comprising the Tröger's base (TB) structural motif was rationally designed and synthesised to access a nitrogen-rich fluorescent supramolecular coordination polymer. By adopting the straight forward precipitation method, a new luminescent nanoscale Zn(II) coordination polymer (TB-Zn-CP) was synthesized in quantitative yield using Zn(OAc)2·2H2O and tetraacid linker L (1:0.5) in DMF at room temperature. The phase-purity of as-synthesised TB-Zn-CP was confirmed by X-ray powder diffraction analysis, infra-red spectroscopy, and elemental analysis. Thermogravimetric analysis suggests that TB-Zn-CP is thermally stable up to 330 °C and the morphological features of TB-Zn-CP was analysed by SEM and AFM techniques. The N2 adsorption isotherm of thermally activated TB-Zn-CP at 77 K revealed a type-II reversible adsorption isotherm and the calculated Brunauer–Emmett–Teller (BET) surface area was found to be 72 m2 g−1. Furthermore, TB-Zn-CP displayed an excellent CO2 uptake capacity of 76 mg g−1 at 273 K and good adsorption selectivity for CO2 over N2 and H2. The aqueous suspension of as-synthesized TB-Zn-CP showed strong green fluorescence (λmax = 520 nm) characteristics due to the internal-charge transfer (ICT) transition and was used as a fluorescent sensor for the discriminative sensing of nitroaromatic explosives. The aqueous suspension of TB-Zn-CP showed the largest quenching responses with high selectivity for phenolic-nitroaromatics (4-NP, 2,4-DNP and PA) even in the concurrent presence of other potentially competing nitroaromatic analytes. The fluorescence titration studies also provide evidence that TB-Zn-CP detects picric acid as low as the parts per billion (26.3 ppb) range. Furthermore, the observed fluorescence quenching responses of TB-Zn-CP towards picric acid were highly reversible. The highly selective fluorescence quenching responses including the reversible detection efficiency make the nanoscale coordination polymer TB-Zn-CP a potential material for the discriminative fluorescent sensing of nitroaromatic explosives.

White-light-emitting materials have attracted significant interest in recent years due to their potential applications in solid-state lighting and flat-panel displays. Design of such materials is challenging and often relies on the use of multiple fluorophores despite the fact that single component systems yield materials with enhanced stability and reproducibility. Herein, we have developed a white-light-emitting system based on the formation of discrete lanthanide-based self-assembled complexes using a newly-designed ligand. We demonstrate that fine tuning of the lanthanide ions molar ratio in the self-assemblies combined with the intrinsic blue fluorescence of the ligand allows for the successful emission of pure white light with CIE coordinates of (0.33, 0.34).

A flexible ditopic ligand 1 containing two N,N,O-tridentate (1,2,3-triazol-4-yl)-picolinamide chelating pockets is reported and the formation of multimetallic architectures is explored in the solid and the solution phase. The self-assembled ZnII complex [Zn4(1)4](ClO4)8 exhibited a folded [2 × 2] square grid supramolecular architecture that selectively assembled in MeCN solution as shown using various spectroscopic techniques. The closely related FeII complex shows equivalent behaviour in the solid state, while a discrete dinuclear species [Cu2(NO3)41]·5MeCN was the sole product observed in the solid state from the reaction between 1 and CuII under similar conditions.

Four new Ru(II) polypyridyl complexes that contain an extended aromatic moiety derived from pyrazino[2,3-h]dipyrido[3,2-a:2′,3′-c]phenazine and either 1,10-phenanthroline (phen) or 1,4,5,8-tetraazaphenanthrene (TAP) have been synthesized, their solid state X-ray crystal structure determined and their photophysical and biological properties evaluated. Their interactions with DNA have been studied, and they have been tested for their potential as photodynamic therapeutic (PDT) agents in the treatment of cancer. A practical modification of a method by Carter, Rodriguez and Bard has been introduced and used to calculate binding parameters for the complexes which show a strong affinity for DNA with binding constants in the order of 107 M−1 (in 10 mM phosphate buffer). The complexes containing phen as an ancillary ligand become emissive upon binding to DNA (“light switch effect”), but do not show selective cytotoxicity upon light irradiation. On the other hand, the TAP complexes, which show an inverse “light switch effect” (emission quenched upon binding to DNA), are strongly photo-toxic suggesting their use in Photodynamic Therapy (PDT). In HeLa cells the best PDT agent shows an IC50 value (light) = 4 μM vs. IC50 value (dark) = 62 μM.

We have prepared a series of N-picolyl-1,8-naphthalimide derivatives and explored their coordination chemistry with d-block metal ions in the solid and solution states, and exploited the structural and chemical properties of these ligands to generate a family of robust metallogels. The 2-picolyl substituted ligands N-(2-picolyl)-1,8-naphthalimide L1 and N-(2-picolyl)-4-nitro-1,8-naphthalimide L2 showed interesting coordination behaviour in Mn, Co and Zn complexes 1–5, chelating through the pyridyl nitrogen atom and naphthalimide oxygen atom with subsequent bond localisation in the imide ring, the first example of such coordination behaviour to be structurally characterised. The 3-picolyl substituted ligands N-(3-picolyl)-1,8-naphthalimide L3 and N-(3-picolyl)-4-nitro-1,8-naphthalimide L4 vary in their coordination behaviour; the 4-unsubstituted ligand L3 acts as a simple monodentate donor giving crystalline assemblies, while the 4-nitro analogue L4 instead forms robust metallogels on reaction with MnII or CoII. The gels were found to be thermally and chemically reversible under various stimuli, and similar materials could be generated by installing a piperidinyl substituent at the naphthalimide 4-position, giving intensely coloured gels. These results describe previously unknown coordination chemistry and physical properties of the widely-used 1,8-naphthalimide backbone, and outline the important role played by subtle geometric variations in the physical characteristics of supramolecular metallogels.

The emergence of synthetic glycoconjugates as chemical probes for the detection of glycosidase enzymes has resulted in the development of a range of useful chemical tools with applications in glycobiology, biotechnology, medical and industrial research. Critical to the function of these probes is the preparation of substrates containing a glycosidic linkage that when activated by a specific enzyme or group of enzymes, irreversibly releases a reporter molecule that can be detected. Starting from the earliest examples of colourimetric probes, increasingly sensitive and sophisticated substrates have been reported. In this review we present an overview of the recent advances in this field, covering an array of strategies including chromogenic and fluorogenic substrates, lanthanide complexes, gels and nanoparticles. The applications of these substrates for the detection of various glycosidases and the scope and limitations for each approach are discussed.

The bi-functional complex [PtII(terpy)(py-naphth)](NO3)2(1), incorporating both 2,2′:6′,2′′-terpyridine and 4-N,N′-dimethylamino-1,8-naphthalimide moieties, displays a high binding affinity for DNA as well as displaying cytotoxicity towards and inducing apoptosis in malignant cell lines.

The synthesis of five new 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligands is described: the self-assembly behaviour of the tri-methyl ester, 1, with Eu(III) showed the formation of a luminescent 1:3 Eu:btp complex, Eu13, which was studied in solution and in the solid state; while the tri-carboxylic acid, 2, formed a hydrogel and its corresponding complex Eu23, gave rise to a strongly red luminescent healable metallogel.

The non-covalent incorporation of responsive luminescent lanthanide, Ln(III), complexes with orthogonal outputs from Eu(III) and Tb(III) in a gel matrix allows for in situ logic operation with colorimetric outputs. Herein, we report an exemplar system with two inputs ([H+] and [F−]) within a p(HEMA-co-MMA) polymer organogel acting as a dual-responsive device and identify future potential for such systems.

The formation of interlocked lanthanide-based catenanes using Eu(III)-directed synthesis is described (catenation being achieved via a ring-closing metathesis reaction); the self-assembly formation of the supramolecular structures was analysed by HRMS, NMR and luminescent spectroscopies.

The synthesis and characterisation of novel aryl-pyridyl ureas are described, which form self-assembly structures via extended hydrogen bonding and π–π interactions in the solid state, and in selected cases, form supramolecular gels with antimicrobial properties against Staphylococcus aureus and/or Escherichia coli.

The quaternarized pdppz derivative 1 was shown to bind strongly to DNA with concomitant changes in its ground and excited state photophysical properties. Furthermore, the compound also showed rapid cellular uptake, and induced apoptosis upon light irradiation in various cancer cell lines after 24 hours of incubation.

The synthesis and photophysical properties of 1 and 2, two Ru(II)-polypyridyl based-1,8-naphthalimide Tröger's bases, are described; these were found to stabilize double stranded DNA, undergo rapid cellular uptake, displaying good luminescence without affecting cell viability even after 24 hours of incubation.

The formation of a self-assembly between a sensitising antenna and an Eu(III) functionalised cyclen complex 1·Eu, tethered to a gold surface via a C12 alkyl thiol spacer, is described where changes in the Eu(III) emission signal the formation, and dissociation, of a ternary complex.

1·Eu·BPS was developed as a luminescent lanthanide sensor for use in displacement assays for detection of d-metal ions by monitoring the changes in the europium emission, which was quenched for iron(II), with a detection limit of ∼10 pM (0.002 μg L−1) for Fe(II) in buffered pH 7.4 solution.

The formation of self-assembly complexes between the ligands 1 (SS) and 2 (RR) and terbium or europium was undertaken and shown (using various spectroscopic titrations) to give rise to the exclusive formation of 2:1 (L:Ln) stoichiometry and not the anticipated 3:1 stoichiometry.

The synthesis and binding investigations of first generation C3v-symmetrical hydrogen bonding urea-amide based tripodal receptors, 1–6, with various anions such as acetate, phosphate, sulfate and chloride in DMSO-d6 are presented. Analysis of the 1H NMR titrations of 1–6 showed on all occasions the selective formation of 1:1 stoichiometries.

The synthesis, NMR and luminescent analysis of a novel polypeptide possessing a sensitising naphthalimide antenna at the amino terminus, and a model compound with a Trp moiety, and their Eu(III) and Tb(III) complexes are described.

The synthesis and the photophysical evaluation of a novel pH dependent lanthanide luminescent self-assembly in water between a cyclen based europium complex and a β-diketonate is described and its use as a luminescent sensor in displacement assays for anions such as acetate, bicarbonate and lactate, where the Eu(III) emission was quenched upon anion recognition.

The binding of asymmetrical and optically pure tridentate ligands (L = 1(S) and 1(R)) containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions (M = La(III) and Eu(III)) was studied in CH3CN, showing the successive formation of M:L, M:L2 and M:L3 stoichiometric species in solution. The europium complexes EuL3 were also synthesised, structurally characterised and their photophysical properties probed in CH3OH and CH3CN. The changes in the chiroptical properties of both 1(S) and 1(R) were used (by circular dichroism (CD) spectroscopy) to monitor the formation of these chiral self-assemblies in solution. While circularly polarised luminescence (CPL) showed the formation of Eu(1(S))3 and Eu(1(R))3 as enantiomers, with high luminescence dissymmetry factors (glum), fitting the CD changes allowed for binding constants to be determined that were comparable to those seen in the analyses of absorbance and luminescence changes.

The delayed lanthanide luminescence of the terbium [Tb(III)] diaryl-urea complex 1·Tb is significantly enhanced upon sensing of dihydrogenphosphate (H2PO4−) in CH3CN, which occurs through multiple anion binding through hydrogen bonding interactions and potential metal ion coordination to Tb(III).

Ligands containing the btp [2,6-bis(1,2,3-triazol-4-yl)pyridine] motif have appeared with increasing regularity over the last decade. This class of ligands, formed in a one pot ‘click’ reaction, has been studied for various purposes, such as for generating d and f metal coordination complexes and supramolecular self-assemblies, and in the formation of dendritic and polymeric networks, etc. This review article introduces btp as a novel and highly versatile terdentate building block with huge potential in inorganic supramolecular chemistry. We will focus on the coordination chemistry of btp ligands with a wide range of metals, and how it compares with other classical pyridyl and polypyridyl based ligands, and then present a selection of applications including use in catalysis, enzyme inhibition, photochemistry, molecular logic and materials, e.g. polymers, dendrimers and gels. The photovoltaic potential of triazolium derivatives of btp and its interactions with anions will also be discussed.

The development of functional 1,8-naphthalimide derivatives as DNA targeting, anticancer and cellular imaging agents is a fast growing area and has resulted in several such derivatives entering into clinical trials. This review gives an overview of the many discoveries and the progression of the use of 1,8-naphthalimides as such agents and their applications to date; focusing mainly on mono-, bis-naphthalimide based structures, and their various derivatives (e.g. amines, polyamine conjugates, heterocyclic, oligonucleotide and peptide based, and those based on metal complexes). Their cytotoxicity, mode of action and cell-selectivity are discussed and compared. The rich photophysical properties of the naphthalimides (which are highly dependent on the nature and the substitution pattern of the aryl ring) make them prime candidates as probes as the changes in spectroscopic properties such as absorption, dichroism, and fluorescence can all be used to monitor their binding to biomolecules. This also makes them useful species for monitoring their uptake and location within cells without the use of co-staining. The photochemical properties of the compounds have also been exploited, for example, for photocleavage of nucleic acids and for the destruction of tumour cells.

Herein some examples of the use of lanthanide ions (f-metal ions) to direct the synthesis of luminescent self-assembly systems (architectures) will be discussed. This area of lanthanide supramolecular chemistry is fast growing, thanks to the unique physical (magnetic and luminescent) and coordination properties of the lanthanides, which are often transferred to the resulting supermolecule. The emphasis herein will be on systems that are luminescent, and hence, generated by using either visibly emitting ions (such as EuIII, TbIII and SmIII) or near infrared emitting ions (like NdIII, YbIII and ErIII), formed through the use of templating chemistry, by employing structurally simple ligands, possessing oxygen and nitrogen coordinating moieties. As the lanthanides have high coordination requirements, their use often allows for the formation of coordination compounds and supramolecular systems such as bundles, grids, helicates and interlocked molecules that are not synthetically accessible through the use of other commonly used templating ions such as transition metal ions. Hence, the use of the rare-earth metal ions can lead to the formation of unique and stable species in both solution and in the solid state, as well as functional and responsive structures.

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This critical review focuses on the development of anion sensors, being either fluorescent and/or colorimetric, based on the use of the 1,8-naphthalimide structure; a highly versatile building unit that absorbs and emits at long wavelengths. The review commences with a short description of the most commonly used design principles employed in chemosensors, followed by a discussion on the photophysical properties of the 4-amino-1,8-naphthalimide structure which has been most commonly employed in both cation and anion sensing to date. This is followed by a review of the current state of the art in naphthalimide-based anion sensing, where systems using ureas, thioureas and amides as hydrogen-bonding receptors, as well as charged receptors have been used for anion sensing in both organic and aqueous solutions, or within various polymeric networks, such as hydrogels. The review concludes with some current and future perspectives including the use of the naphthalimides for sensing small biomolecules, such as amino acids, as well as probes for incorporation and binding to proteins; and for the recognition/sensing of polyanions such as DNA, and their potential use as novel therapeutic and diagnostic agents (95 references).

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) is a terdentate binding motif that is synthesised modularly via the CuAAC reaction. Herein, we present the synthesis of ligands 1 and 2 and the investigation of the coordination chemistry, photophysical behaviour and electrochemistry of complexes of these with a number of d-metal ions (e.g. Ru(II), Ir(III), Ni(II) and Pt(II)). The X-ray crystal structures of ligand 1 and the complexes [Ru·22](PF6)Cl, [Ni·12](PF6)Cl and [Ir·1Cl3] are also presented. All of the complexes displayed non-classical triazolyl C–H⋯Cl− hydrogen bonding. All but one complex showed no metal-based luminescence at room temperature, while all of the Pt(II) complexes displayed luminescence at 77 K. The electrochemistry of the Ru(II) complexes was also studied and these complexes were found to have higher oxidation potentials than analogous compounds. The redox behaviour of [RuL2]2+ complexes with both 1 and 2 was nearly identical, while [Ru·1Cl2(DMSO)] was oxidised at significantly lower potential. We also show that the Ru(II) complex of 2, [Ru·22](PF6)Cl, gave rise to the formation of a metallo-supramolecular gel, the morphology of which was studied using scanning electron and helium ion microscopy.

The mono-dentate, pyridyl containing, nitro naphthalimide ligands N-(4-pyridyl)-4-nitro-1,8-naphthalimide (L1) and N-(3-pyridyl)-4-nitro-1,8-naphthalimide (L2) were prepared and complexed with a selection of copper salts [Cu(OAc)2, Cu(CF3SO3)2 and Cu(ClO4)2]. Crystallographic studies were undertaken and revealed that dinuclear acetate bridged complexes resulted from reactions with Cu(OAc)2, while mononuclear systems resulted from reactions with Cu(CF3SO3)2 and Cu(ClO4)2. Despite the differing coordination environments the naphthalimide based ligands provided a range of interesting π-based interactions in the form of π⋯π, anion⋯π, nitro⋯π, solvent⋯π and CO⋯π associations. Solid state EPR spectra were in agreement with the coordination environments observed from crystallography.

The luminescent dimeric ternary lanthanide–cyclen complexes (2-(Ln.1)2; Ln = Tb/Eu) were designed and both their self-assembly formation and their ability to detect anions via displacement assays were investigated using spectrophotometric titrations in MeOH solution. The formation of 2-(Tb.1)2 and 2-(Eu.1)2 was investigated in solution, and determination of the binding constants and stoichiometry showed that the former was formed almost exclusively over the 1:1 complex 2-(Tb.1) after the addition of two equivalents of 2; while for 2-(Eu.1)2 a mixture of both stoichiometries existed even after the addition of four equivalents of 2. Of these two systems, 2-(Tb.1)2 was studied in details as a probe for anions, where significant changes where observed in the photophysical properties of the complex; with the characteristic Tb(III)-centred emission being fully switched off upon the sensing of phosphates and nitrate, giving rise to the formation of a H2PO4−:Tb.1 complex in a 1:2 stoichiometry upon sensing of H2PO4− by 2-(Tb.1)2, while NO3− gave 1:1 complex formation and two equivalents of NO3−·Tb.1.

The synthesis and photophysical properties of the Eu(III) complex 1.Eu, based on the use of 1,10-phenanthroline (phen) as a combined sensitizing antenna and a transition metal ion coordinating ligand, is described. The long-wavelength Eu(III) emission from this complex was found to be highly pH sensitive, giving rise to a ‘off-on-off’ pH profile with maximum emission occurring within the physiological pH range. This allowed for the use of 1.Eu as a luminescent sensor for transition metal ions, where the titration with ions such as Cu(II), Co(II) and Fe(II) gave rise to the formation of mixed f–d nuclear complexes, with concomitant changes in the photophysical properties of 1.Eu. Here, changes in both the ground and the singlet excited state properties of the phen antenna were observed, but the largest changes were observed for the delayed Eu emission, which was fully quenched upon titration with these ions in aqueous pH 7.4 buffered solutions. In comparison, no changes were observed in the Eu(III) emission upon titration with ions such as Zn(II) or group I and II ions. From these changes, we were able to demonstrate the binding stoichiometry and the binding constant for the formation of novel supramolecular complexes between 1.Eu and Cu(II), Co(II) and Fe(II), which showed that either two or three equivalents of 1.Eu complexed to each of these transition metal ions, giving rise to the formation either linear f–d–f or branched f3–d based mixed nuclear complexes in solution.

A cyclen based lanthanide luminescent sensor, Tb.1, has been developed by taking advantage of a combination of hydrogen bonding and f-metal ion coordination binding sites for anionic species. Analysis of the ground state, the emission from the singlet and the Tb(III) excited states clearly show the ability of the Tb(III) complex to signal the presence of anions in CH3CN, through multiple binding interactions consisting of hydrogen bonding and metal coordination. The delayed lanthanide luminescence from the Tb(III) diaryl–urea complex was found to be significantly enhanced only upon recognition of H2PO4−, at the same time as displaying good selectivity over other competitive anions, such as CH3COO−.

Luminescent europium and terbium complexes and a mixed Eu(III)–Tb(III) complex were prepared, each with three macrocycles coordinating to a single lanthanide ion to form a trimetallic system, and can be used for the ratiometric sensing of anions in the case of the mixed complex.

A series of tetra-substituted ‘pseudo’ dipeptide ligands of cyclen (1,4,7,10,-tetraazacyclododecane) and a tri-substituted 3′-pyridine ligand of cyclen, and the corresponding lanthanide(III) complexes were synthesised and characterised as metallo-ribonuclease mimics. All complexes were shown to promote hydrolysis of the phosphodiester bond of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP, τ1/2 = 5.87 × 103 h), a well known RNA mimic. The La(III) and Eu(III) tri-substituted 3′-pyridine lanthanide(III) complexes being the most efficient in promoting such hydrolysis at pH 7.4 and at 37 °C; with τ1/2 = 1.67 h for La(III) and 1.74 h for Eu(III). The series was developed to provide the opportunity to investigate the consequences of altering the lanthanide(III) ion, coordination ability and hydrophobicity of a metallo-cavity on the rate of hydrolysis using the model phosphodiester, HPNP, at 37 °C. To further provide information on the role that the logKa of the metal bound water plays in phosphodiester hydrolysis the protonation constants and the metal ion stability constants of both a tri and tetra-substituted 3′pyridine complex were determined. Our results highlighted several key features for the design of lanthanide(III) ribonucelase mimics; the presence of two metal bound water molecules are vital for pH dependent rate constants for Eu(III) complexes, optimal pH activity approximating physiological pH (∼7.4) may be achieved if the logKa values for both MLOH and ML(OH)2 species occur in this region, small changes to hydrophobicity within the metallo cavity influence the rate of hydrolysis greatly and an amide adjacent to the metal ion capable of forming hydrogen bonds with the substrate is required for achieving fast hydrolysis.

The synthesis and characterisation of two cationic pyridinium based 4-amino-1,8-naphthalimide derivatives (2 and 3) are described and compared to those of compound 1. The photophysical properties of 2 and 3 are shown to vary greatly with the solvent polarity and H-bonding ability. The dimethylamino substitution in 3 results in a weak quantum yield of fluorescence emission due to faster non-radiative deactivation of the excited singlet state than that seen for 2. As with 1, the fluorescence of 2 was found to be enhanced in its 1:1 complex with 5′-adenosine-monophosphate (5′-AMP) while it was partially quenched in its complex with 5′-guanosine-monophosphate (5′-GMP). In contrast, the fluorescence of 3 was enhanced (‘switched on’) in the presence of both adenine and guanine rich sequences. Linear and circular dichroism studies showed that each of 1, 2 and 3 binds to double-stranded DNA by intercalation. However, 2 and 3 do not show the preference for AT-rich DNA observed for 1. Comparative fluorescence studies with double stranded DNA show that the emission of 3 was 16 times enhanced in its DNA bound form, suggesting potential use of this structure as a spectroscopic probe for studying nucleic acid structure.

The self-assembly between the amidothiourea ligands 1 and 2 and Tb(III) gave rise to the formation of 1:1 and 3:1 (ligand–Tb) supramolecular architectures that were shown to be luminescent in both MeOH and in water–DMSO solutions; preliminary investigations in the latter system also showed that their emission was modulated upon interacting with anions such as acetate and phosphate due to host–guest formation.

The synthesis and characterisation of five bis-1,8-naphthalimide containing Tröger's bases 1–5 formed from their corresponding 3-amino-1,8-naphthalimide precursors 6–10 is described. The photophysical investigations of 1–5 and 6–10 were carried out in several organic solvents as well as in water and as a function of pH using UV-Vis absorption and fluorescence spectroscopies. The DNA binding affinities of 1–5 in aqueous solution at pH 7.4 were also investigated using several UV-Vis absorption and fluorescence experiments by using calf thymus DNA (ct-DNA). These molecules exhibited significant DNA binding affinities; where large binding values (Kb) in the range of 106 M−1 were determined, even in competitive media (50 mM and 160 mM NaCl at pH 7.4). Thermal denaturation measurements also showed that 1–5 significantly stabilised the DNA helix. Using linear and circular dichroism we further demonstrated that the DNA binding interaction occurs both by intercalation and by groove binding. The Tröger's bases were further shown to be rapidly taken up into cells using confocal fluorescence spectroscopy; and cytotoxic studies in HeLa and MCF-7 cells showed that most of the Tröger's bases were effective cytotoxic agents with EC50 values of between 1.1–12 μM and that all the active compounds induced programmed cell death by apoptosis, where up to 70% cellular death was observed after 24 h of incubation for 4.

The investigation into the luminescence properties of a lanthanide-binding peptide, derived from the Ca-binding loop of the parvalbumin, and modified by incorporating a 1,8-naphthalimide (Naph) chromophore at the N-terminus is described. Here, the Naph is used as a sensitising antenna, which can be excited at lower energy than classical aromatic amino acids, such as tryptophan (the dodecapeptide of which was also synthesised and studied herein). The syntheses of the Naph antenna, its solid phase incorporation into the dodecapeptide, and the NMR investigation into the formation of the corresponding lanthanide complexes in solution is presented. We also show that this Naph antenna can be successfully employed to sensitize the excited states of both europium and terbium ions, the results of which was used to determined the stability constants of their formation complexes, and we demonstrated that our peptide ‘loop’ can selectively bind these lanthanide ions over Ca(II).

The design and synthesis of dinuclear-lanthanide complexes possessing triazole-based bridges, formed by using copper catalysed 1,3-cycloaddition reactions between heptadentate alkyne functionalised cyclen europium or terbium complexes and di-azides (CuAAC reactions), are described. While this click reaction worked well for the formation of the homo-Eu(III) and Tb(III) bis-tri-arm cyclen N,N-dimethyl acetamide complexes, 2Eu and 2Tb, and for the homo-Eu(III) chiral N-methylnaphthalene based complexes 3Eu (S,S,S) and 4Eu (R,R,R), the formation of the Eu(III) complex of the primary amide analogue of 2, namely 1Eu, was not successful, clearly demonstrating the effect that the nature of the pendant arms has on this reaction. Furthermore, the click reactions between the free alkyne cyclen bis-derivatives (5–8) and the di-azide were unsuccessful, most likely due to the high affinity of the cyclen macrocycles for Cu(II). The Eu(III) complexes of 2–4 and 2Tb all gave rise to sensitised metal ion centred emission upon excitation of the triazole or the naphthalene antennae in methanol solution, and their hydration states were determined, which showed that while the Eu(III) mono-nuclear complexes had q ∼ 2, the click products all had q ∼ 1. In the case of 3Eu (S,S,S) and 4Eu (R,R,R), the circular polarised emission (CPL) was also observed for both, demonstrating the chiral environment of the lanthanide centres.

The synthesis, characterisation and solid state crystal structure of a cationic 4-amino-1,8-naphthalimide derivative (1) are described. The photophysical properties of 1 are shown to vary with the solvent polarity and H-bonding ability. The fluorescence of 1 is enhanced and blue-shifted in its 1:1 complex with 5′-adenosine-monophosphate while it is partially quenched and red-shifted in its complex with 5′-guanosine-monophosphate. Linear and circular dichroism measurements show that 1 binds to double-stranded DNA by intercalation. Comparative UV-visible and fluorescence studies with double stranded synthetic polynucleotides poly(dA–dT)2 and poly(dG–dC)2 show that 1 binds much more strongly to the AT polymer; 1 also has a strong preference for A–T rich sequences in natural DNA. Thermal denaturation measurements also reveal a much greater stabilisation of the double-stranded poly(dA–dT)2 than of natural DNA.

The thiourea based 4-amino-1,8-naphthalimide molecules 1–5 were designed as fluorescent anion sensors and their photophysical properties investigated upon recognition of biologically relevant anions such as acetate, dihydrogen phosphate and fluoride in DMSO. Synthesised in a single step from their respective aniline precursors, 6–9, these molecules were designed on the fluorophore–spacer–receptor principle, where in the case of sensors 1–3 the thiourea anion recognition moieties were connected to the fluorophore via the 4-amino moiety, while sensors 4 and 5 had the thiourea moieties connected to the ‘imide’via a CH2 spacer. The current study showed that 1–5 operated as photoinduced electron transfer (PET) sensors, as no significant changes were observed in their absorption spectra, while their fluorescence emissions were quenched upon recognition of ions such as AcO−, H2PO4− and F−, which demonstrates that bidirectional PET sensing occurs in such naphthalimide based anion sensors.

This article gives some highlights of the recent advances in the development of novel lanthanide based complexes, conjugates and self-assembly structures formed from the use of organic ligands and organo-metallic (transition metal) complexes, that are designed with the aim of capitalising on the high coordination requirement of the lanthanide ions. The examples shown, demonstrate the versatility of the lanthanide ions as luminescent probes and sensors that emit at long wavelength either in the visible or the near infrared (NIR) part of the electromagnetic spectrum.

The design, synthesis and physical evaluation of 1, a visible colorimetric ‘naked eye’ pyridyl based bis-amidothiourea sensor for anions, is described. This charge neutral sensor gives rise to significant changes in the absorption spectra upon interactions with several important biological anions such as AMP and ADP in 4 : 1 DMSO–H2O solution, while ATP was not detected. These colorimetric changes are due to the formation, or the combination of hydrogen bonding complexes and/or deprotonation between these anions and 1.

The design and synthesis of two novel fluorescent sensors based on the photoinduced electron transfer (PET) concept, 1 and 2, for the detection of zinc under competitive media is described. These sensors are based on the 4-amino-1,8-naphthalimide fluorophore, which has an absorption band centred at 450 nm and emits in the green with λmax ∼550 nm. By functionalizing the chromophore with a simple benzyl or ethyl-aryl based iminodiacetate receptor at the 4-position, both high selectivity and sensitivity were achieved for the sensing of Zn(II) over other competitive transition and Group I and II metal ions. These sensors were also shown to be pH independent, with a pKa of 2.3 being determined for 1, which allows these to be used in highly competitive pH media. Upon sensing of Zn(II) the fluorescence emission spectrum is ‘switched on’ demonstrating the suppression of PET from the receptor to the fluorophore. For 1, the sensing of Zn(II) was achieved with Kd = 4 nM when measured in pH 7.4 buffered solution, in the presence of 1.1 mM of EGTA.