Luminescent organic materials are commonly polycyclic aromatic molecules with planar extended π-electron conjugation. In the present work, however, we report a series of novel and simple salicylaldehyde-based tri-Schiff bases (TSBs, 15 samples), which have a non-conjugated trimethylamine bridge but emit strong blue, green, and yellow aggregation-induced emission (AIE) with large Stokes shifts (up to 167 nm) and high fluorescence quantum yields (up to 0.18). Mechanochromic fluorescence enhancement and enantiomers are also found in TSB solids. Moreover, these flexible and tripod-like molecular cages acting as ideal and universal anion hosts can be used to detect anions with turn-on fluorescence signals (fluorescence quantum yields up to 0.51). Combining their advantages of AIE and biocompatibility, TSBs have potential application in living cell imaging. The inherent relationships between their chemical structures and AIE/anion host properties are studied, which provide unequivocal insights for the design of non-conjugated fluorescent materials.

Natural products are usually non-conjugated and chiral, but organic luminescent materials are commonly polycyclic aromatic molecules with extended π-conjugation. In the present work, we combine with the advantages of non-conjugation and chirality to prepare a series of novel and simple salen ligands (41 samples), which have a non-conjugated and chiral (S,S) and (R,R) cyclohexane or 1,2-diphenylethane bridge but display strong blue, green, and red aggregation-induced emission (AIE) with large Stokes shifts (up to 186 nm) and high fluorescence quantum yields (up to 0.35). Through hydrogen and halogen bonds, these flexible salen ligands can be used as universal anion probes and chiral receptors of unprotected amino acids (enantiomeric selectivity up to 0.11) with fluorescence quantum yields up to 0.29 and 0.27, respectively. Moreover, the effects of different chiral bridges on the molecule arrangement, AIE, and anion and chiral recognition properties are also explored, which provide unequivocal insights for the design of non-conjugated chiral and soft fluorescent materials.

We report a class of multiresponsive colorimetric and fluorescent pH probes based on three different reaction mechanisms including cation exchange, protonation, and hydrolysis reaction of K(I), Ca(II), Zn(II), Cu(II), Al(III), and Pd(II) Salen complexes. Compared with traditional pure organic pH probes, these complex-based pH probes exhibited a much better selectivity due to the shielding function of the filled-in metal ion in the complex. Their pH sensing performances were affected by the ligand structure and the central metal ion. This work is the first report of “off–on–on′–off” colorimetric and fluorescent pH probes that possess three different reaction mechanisms and should inspire the design of multiple-responsive probes for important analytes in biological systems.

A laboratory experiment visually exploring two opposite basic principles of fluorescence of aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE) is demonstrated. The students would prepared two salicylaldehyde-based Schiff bases through a simple one-pot condensation reaction of one equiv of 1,2-diamine with 2 equiv of salicylaldehyde. The resulting fluorescent dyes have similar chemical structures but possess ACQ and AIE properties, respectively. Their ACQ/AIE properties and pH sensing applications would then examined by visually qualitative analysis (UV lamp, light-emitting diode, and naked eye) and quantitative analysis (fluorometer). Finally, in a deeper level, X-ray single crystal structure analysis was utilized to reveal the inherent relationships between molecular structures/molecular arrangements and ACQ/AIE properties. This lesson is suitable for many areas of chemistry, especially for organic and analytical chemistry.

A series of novel, simple, and colorful Salen ligands (56 samples), salicylaldehyde-based bis-Schiff bases, linking with different non-conjugated alkyl bridges ((CH2)n, n = 2–9, 12; cyclohexyl) and containing different electron-accepting (–NO2, –F, and –Cl), electron-donating (–OMe, –OH, and –NEt2), or sterically hindering (–t-butyl) substituents or a π-extended system (naphthalene ring) have been designed and synthesized. The photophysical properties of these Salen ligands can be well-tuned by the introduction of side functional substituents, π-extended systems, and central N-alkyl chain bridges. It is unusual that they contain a small π-conjugated system but display strong blue, green, and red aggregation-induced emission (AIE) with large Stokes shifts (up to 162 nm) and high fluorescence quantum yields (up to 0.44 and 0.75 in water and in solid, respectively). Combining with their advantages of AIE and good stability and biocompatibility, the Salen ligands can be potentially used in mechanofluorochromism (crystal-defect-induced emission) and living cell imaging. Moreover, the inherent relationships between their chemical structures and AIE properties are studied, which provide unequivocal insights for the design of AIE-active dyes.

Correction for ‘Fluorescent metal ion chemosensors via cation exchange reactions of complexes, quantum dots, and metal–organic frameworks’ by Jinghui Cheng, et al., Analyst, 2015, DOI: 10.1039/c5an01398d.

Due to their wide range of applications and biological significance, fluorescent sensors have been an active research area in the past few years. In the present review, recent research developments on fluorescent chemosensors that detect metal ions via cation exchange reactions (transmetalation, metal displacement, or metal exchange reactions) of complexes, quantum dots, and metal–organic frameworks are described. These complex-based chemosensors might have a much better selectivity than the corresponding free ligands/receptors because of the shielding function of the filled-in metal ions. Moreover, not only the chemical structure of the ligands/receptors but also the identity of the central metal ions have a tremendous impact on the sensing performances. Therefore, sensing via cation exchange reactions potentially provides a new, simple, and powerful way to design fluorescent chemosensors.

A series of luminescent salicylaldehyde azines (SAs) containing different electron-accepting substituents (–NO2, –F, and –Cl), electron-donating substituents (–OMe and –NEt2), and a π-extended system (naphthalene ring) are prepared for the application of fluorescent pH probes. These SAs inheriting the aggregation-induced emission (AIE) features display strong blue, green, and red fluorescence with large Stokes shifts in water and solid medium. Combining the advantages of AIE and the chemical reactivity of phenol towards OH−/H+, most of the SAs can be used as ratiometric fluorescent pH probes with a broad pH range (2–14) in water and solid medium (test paper). Moreover, the inherent relationship between their chemical structures and AIE properties/pKa values (7.5–13.3) is studied, which provides unequivocal insights into the design of AIE-active dyes and their applications.

We report our systematic studies of novel, simple, selective, and sensitive optical (both colorimetric and fluorescent) chemosensors for detecting Al3+ based on transmetalation reactions (metal displacement or exchange reactions) of a series of K(I), Ca(II), Zn(II), Cu(II), and Pt(II) complexes containing different ligands of salen-based Schiff bases. Both the chemical structure of the salen ligand and the identity of the central metal ion have a tremendous impact on the sensing performance, which is mainly determined by the stability constant of the complex. Moreover, the selectivities of the salen-complex-based chemosensors are much better than those of the corresponding free salen ligands because of the shielding function of the filled-in metal ion in the complex. Therefore, the present work potentially provides a new and simple way to design optical probes via complex-based transmetalation reactions.

Room-temperature phosphorescent materials that emit light in the visible (red, green, and blue; from 400 to 700 nm) have been a major focus of research and development during the past decades, due to their applications in organic light-emitting diodes (OLEDs), light-emitting electrochemical cells, photovoltaic cells, chemical sensors, and bio-imaging. In recent years, near-infrared (NIR) phosphorescence beyond the visible region (700–2500 nm) has emerged as a new, promising, and challenging research field with potential applications toward NIR OLEDs, telecommunications, night vision-readable displays. Moreover, NIR phosphorescence holds promise for in vivo imaging, because cells and tissues exhibit little absorption and auto-fluorescence in this spectral region. This review describes the overall progress made in the past ten years on NIR phosphorescent transition-metal complexes including Cu(I), Cu(II), Cr(III), Re(I), Re(III), Ru(II), Os(II), Ir(III), Pt(II), Pd(II), Au(I), and Au(III) complexes, with a primary focus on material design complemented with a selection of optical, electronic, sensory, and biologic applications. A critical comparison of various NIR phosphorescent materials reported in the literature and a blueprint for future development in this field are also provided.

Both photo-induced cis–trans-isomers of a maleonitrile-based Salen ligand can be used as pH probes covering a broad pH range through three different mechanisms but upon undergoing the formation of a stable complex and Cu2+-promoted hydrolysis, respectively, they exhibit totally different responses and mechanisms for sensing Cu2+.

A series of colorful Salen-type Schiff bases derived from the different diamine bridges including 1,2-ethylenediamine (Et1–Et3), 1,2-cyclohexanediamine (Cy1–Cy3), 1,2-phenylenediamine (Ph1–Ph4), dicyano-1,2-ethenediamine (CN1–CN3), phenazine-2,3-diamine (Phen1–Phen3), and naphthalene-2,3-diamine (Naph1 and Naph2) have been designed and prepared. The presence of electron-accepting substituents, electron-donating substituents, donor–acceptor (DA) systems, and/or π-extended systems leads to not only full absorption and emission spectra (300–700 nm) in the visible region but also high fluorescence quantum yields up to 0.83 in solution. The experimental results and density functional theory (DFT) calculations have proved that the highest occupied molecular orbital (HOMO) levels, the lowest unoccupied molecular orbital (LUMO) levels, and consequently the energy gaps of these Salen ligands can be well tuned. The LUMO levels of these Salen ligands are mainly affected by the diamine bridges, whereas both the HOMO and LUMO levels are influenced by the phenol fragments. Adding Cu2+ and Co2+ to CN1 solution leads to a drastic color change from pink into brownish red and purple, respectively, which is useful not only for the ratiometric detection but also for rapid visual sensing even by the naked eye. Moreover, the properties of the cis–trans isomer of CN1 are examined. The Salen ligands have coordination chemistry similar to other well-known tetradentate porphyrin ligands as well as much easier preparation and rich photophysical properties.

5,15-Dialkyl-substituted porphyrins that are symmetrically capped with ethyl (C2-Por), butyl (C4-Por) and hexyl (C6-Por) were synthesized and characterized. Molecular structure versus physical property relationship has been established through the analysis of planar charge transport using thin film transistor (TFT) structure.

A series of water-soluble sulfonato-Salen-type ligands derived from different diamines including 1,2-ethylenediamine (Et-1–Et-4), 1,2-cyclohexanediamine (Cy-1 and Cy-2), 1,2-phenylenediamine (Ph-1–Ph-3 and PhMe-1–PhMe-4), and dicyano-1,2-ethenediamine (CN-1) has been designed and prepared. Sulfonate groups of ligands ensure good stability and solubility in water without affecting their excited state properties. These ligands exhibit strong UV/Vis-absorption and blue, green, or orange fluorescence. Time-dependent-density functional theory calculations have been undertaken to reveal the influence of ligand nature, especially sulfonate groups, on the frontier molecular orbitals. Since their fluorescence is selectively quenched by Cu2+, the sulfonato-Salen-type ligands can be used as highly selective and sensitive turn-off fluorescence sensors for the detection of Cu2+ in water and fluorescence imaging in living cells.Graphical abstractHighlights► Sulfonate groups ensure good stability and solubility in water. ► Sulfonate groups have little effect on the photophysical properties. ► This is confirmed by the TD-DFT calculations and experimental results. ► The strong blue, green, or orange fluorescence is selectively quenched by Cu2+. ► The ligands are sensitive fluorescence sensors for Cu2+ in water and living cells.

A series of new platinum(II) 5,15-bis(pentafluorophenyl)-10,20-bis(phenyl)porphyrin–9,9-dioctylfluorene copolymers, in which the relative intensities of the blue fluorescence and red phosphorescence can be easily tuned by the initial feed ratio of the two monomers or energy transfer between the fluorescent and phosphorescent units, have been designed and prepared for the application in ratiometric dual emissive oxygen sensing. To the best of our knowledge, this is the first example of a ratiometric oxygen sensor based on dual fluorescent/phosphorescent polymers or copolymers containing transition-metal complexes. It also provides an alternative and easy way to achieve dual emissive oxygen sensing.

The quantitative determination of oxygen concentration is essential for a variety of applications ranging from life sciences to environmental sciences. Optical oxygen sensing allows non-invasive measurements with biological objects, parallel monitoring of multiple samples, and imaging. In general, ratiometric optical oxygen sensing is more desirable, due to its advantages of selectivity, insensitivity to ambient or scattered light, and elimination of instrumental fluctuation. Moreover, it can provide the perceived colour change, which would be useful not only for the ratiometric method of detection but also for rapid visual sensing. Mainly focusing on material design for ratiometric measurement, this review describes the overall progress made in the past ten years on ratiometric optical ground-state triplet oxygen sensing and offers a critical comparison of various methods reported in the literature. It also provides a development blueprint for ratiometric optical oxygen sensing.

Ratiometric fluorescence/phosphorescence probes for Pt2+ based on the fluorescence quenching from the ligand of Salen-type Schiff bases and phosphorescence enhancement from the resulting Pt(II) complexes have been demonstrated. For phosphorescence enhancement, a good linearity (correlation coefficient R2 = 0.996, n = 10) was established with detection limit of 11.6 ± 0.2 ppb. The detection limit would be decreased to 2.31 ± 0.03 ppb if degassing was adopted. To the best of our knowledge, this value is one of the most sensitive probes for Pt2+. This system also has good selectivity with little interference from Sr2+, Sn2+, Pb2+, Cr3+, Mn2+, Al3+, Fe3+, Ce3+, Ag+, Li+, Mg2+, Na+, Cd2+, Cu2+, Ca2+, Zn2+, K+, Co2+, Ni2+, Au3+, Ir3+, and Pd2+. Moreover, this system is suitable for detection of different Pt(II) sources, such as PtCl2, K2PtCl4, Pt(COD)Cl2 (COD = 1,5-cyclooctadiene), and cis-platin. It potentially provides a new and simple way to detect some useful transition metal ions, such as Cu+, Au+, Pd2+, and Ru2+.

5,15-Dialkyl-substituted porphyrins that are symmetrically capped with ethyl (C2-Por), butyl (C4-Por) and hexyl (C6-Por) were synthesized and characterized. Molecular structure versus physical property relationship has been established through the analysis of planar charge transport using thin film transistor (TFT) structure.

Both photo-induced cis–trans-isomers of a maleonitrile-based Salen ligand can be used as pH probes covering a broad pH range through three different mechanisms but upon undergoing the formation of a stable complex and Cu2+-promoted hydrolysis, respectively, they exhibit totally different responses and mechanisms for sensing Cu2+.

Room-temperature phosphorescent materials that emit light in the visible (red, green, and blue; from 400 to 700 nm) have been a major focus of research and development during the past decades, due to their applications in organic light-emitting diodes (OLEDs), light-emitting electrochemical cells, photovoltaic cells, chemical sensors, and bio-imaging. In recent years, near-infrared (NIR) phosphorescence beyond the visible region (700–2500 nm) has emerged as a new, promising, and challenging research field with potential applications toward NIR OLEDs, telecommunications, night vision-readable displays. Moreover, NIR phosphorescence holds promise for in vivo imaging, because cells and tissues exhibit little absorption and auto-fluorescence in this spectral region. This review describes the overall progress made in the past ten years on NIR phosphorescent transition-metal complexes including Cu(I), Cu(II), Cr(III), Re(I), Re(III), Ru(II), Os(II), Ir(III), Pt(II), Pd(II), Au(I), and Au(III) complexes, with a primary focus on material design complemented with a selection of optical, electronic, sensory, and biologic applications. A critical comparison of various NIR phosphorescent materials reported in the literature and a blueprint for future development in this field are also provided.

A series of novel, simple, and colorful Salen ligands (56 samples), salicylaldehyde-based bis-Schiff bases, linking with different non-conjugated alkyl bridges ((CH2)n, n = 2–9, 12; cyclohexyl) and containing different electron-accepting (–NO2, –F, and –Cl), electron-donating (–OMe, –OH, and –NEt2), or sterically hindering (–t-butyl) substituents or a π-extended system (naphthalene ring) have been designed and synthesized. The photophysical properties of these Salen ligands can be well-tuned by the introduction of side functional substituents, π-extended systems, and central N-alkyl chain bridges. It is unusual that they contain a small π-conjugated system but display strong blue, green, and red aggregation-induced emission (AIE) with large Stokes shifts (up to 162 nm) and high fluorescence quantum yields (up to 0.44 and 0.75 in water and in solid, respectively). Combining with their advantages of AIE and good stability and biocompatibility, the Salen ligands can be potentially used in mechanofluorochromism (crystal-defect-induced emission) and living cell imaging. Moreover, the inherent relationships between their chemical structures and AIE properties are studied, which provide unequivocal insights for the design of AIE-active dyes.

Luminescent organic materials are commonly polycyclic aromatic molecules with planar extended π-electron conjugation. In the present work, however, we report a series of novel and simple salicylaldehyde-based tri-Schiff bases (TSBs, 15 samples), which have a non-conjugated trimethylamine bridge but emit strong blue, green, and yellow aggregation-induced emission (AIE) with large Stokes shifts (up to 167 nm) and high fluorescence quantum yields (up to 0.18). Mechanochromic fluorescence enhancement and enantiomers are also found in TSB solids. Moreover, these flexible and tripod-like molecular cages acting as ideal and universal anion hosts can be used to detect anions with turn-on fluorescence signals (fluorescence quantum yields up to 0.51). Combining their advantages of AIE and biocompatibility, TSBs have potential application in living cell imaging. The inherent relationships between their chemical structures and AIE/anion host properties are studied, which provide unequivocal insights for the design of non-conjugated fluorescent materials.