We report the formation of macroscopic wires up to centimeters in length from a series of structurally flexible, covalently tethered small-molecular fluorophore-quencher dyads (FQDs, average MW = 425 Da), comprised of carbazole, melatonin, and cyanobenzoate moieties. These FQDs are nonemissive in organic solutions but become moderately to highly luminescent (ΦF = 0.037–0.39) upon formation of wires with emission maxima in the blue region (446–483 nm). The blue photoluminescence (PL) is ascribed to a combination of singlet charge transfer, localized triplet state, and possibly delayed fluorescence emissions with intrinsic luminescence lifetimes ranging from 0.228 to 21333 μs, based on luminescence, transient absorption measurements, X-ray diffraction, and calculations.

We report the formation of macroscopic wires up to centimeters in length from a series of structurally flexible, covalently tethered small-molecular fluorophore-quencher dyads (FQDs, average MW = 425 Da), comprised of carbazole, melatonin, and cyanobenzoate moieties. These FQDs are nonemissive in organic solutions but become moderately to highly luminescent (ΦF = 0.037–0.39) upon formation of wires with emission maxima in the blue region (446–483 nm). The blue photoluminescence (PL) is ascribed to a combination of singlet charge transfer, localized triplet state, and possibly delayed fluorescence emissions with intrinsic luminescence lifetimes ranging from 0.228 to 21333 μs, based on luminescence, transient absorption measurements, X-ray diffraction, and calculations.

Room-temperature phosphorescence (RTP) from purely organic systems is of practical importance in biological imaging, oxygen sensing and displaying technologies. The key step to obtaining RTP from organic molecules is efficient intersystem crossing (ISC), which is usually low compared to inorganic materials. Here we show that protonation of a dye molecule, a thioflavin derivative, in strongly polar polyurethane can be used to effectively harness RTP. Prior to protonation, the predominant transition is π–π* for the polymer, which has nearly undetectable RTP due to the large singlet–triplet energy splitting (0.87 eV); when Brønsted acids are gradually added to the system, increasingly strong RTP is observed due to the presence of a new intramolecular charge-transfer state (ICT). The ICT state serves to lower the singlet–triplet energy gap (0.46 eV). The smaller gap results in more efficient ISC and thus strong RTP under deoxygenated conditions. The thioflavin–polyurethane system can be tuned via proton concentration and counterions and opens new doors for RTP-based polymeric sensors and stimuli-responsive materials.

Room-temperature phosphorescence (RTP) from purely organic systems is of practical importance in biological imaging, oxygen sensing and displaying technologies. The key step to obtaining RTP from organic molecules is efficient intersystem crossing (ISC), which is usually low compared to inorganic materials. Here we show that protonation of a dye molecule, a thioflavin derivative, in strongly polar polyurethane can be used to effectively harness RTP. Prior to protonation, the predominant transition is π–π* for the polymer, which has nearly undetectable RTP due to the large singlet–triplet energy splitting (0.87 eV); when Brønsted acids are gradually added to the system, increasingly strong RTP is observed due to the presence of a new intramolecular charge-transfer state (ICT). The ICT state serves to lower the singlet–triplet energy gap (0.46 eV). The smaller gap results in more efficient ISC and thus strong RTP under deoxygenated conditions. The thioflavin–polyurethane system can be tuned via proton concentration and counterions and opens new doors for RTP-based polymeric sensors and stimuli-responsive materials.

A series of pyridinium ring substituted β-diketones were found to exhibit aggregation-induced emission (AIE) and water-vapour recoverable mechanochromic luminescence in the solid state. Certain water-soluble β-diketones could selectively bind to cellulose-based materials, where strong fluorescence similar to AIE was turned on.

The crystals of 1-(4-methoxyphenyl)-3-(pyridin-2-yl)propane-1,3-dione exhibited rarely seen curved fractal structures under an optical microscope. Weak intermolecular interactions may result in dislocation during crystal growth and thus the irregular crystalline structures. The micrographs of the crystals were exploited for their artistic value and were transferred onto fabrics for application in fashion design.

Many organic molecules exhibit reversible, force-induced emission change known as mechanochromic luminescence (ML) and can potentially be used as mechanosensors. Here we provide an example on how ML behaviours can be modulated by side-chain substituents which serve as the directing group, as well as substrates which may disrupt the function of the directing group.

Many organic molecules exhibit reversible, force-induced emission change known as mechanochromic luminescence (ML) and can potentially be used as mechanosensors. Here we provide an example on how ML behaviours can be modulated by side-chain substituents which serve as the directing group, as well as substrates which may disrupt the function of the directing group.

A new strategy to use conjugated polymers to conduct fluorescent enhancement sensing has been developed. Chiral 1,1′-bi-2-naphthol-based binding sites are linked by p-phenylene units to construct a conjugated polymer whose fluorescence is quenched by the aldehyde groups introduced at each binding site. Interaction of this polymer with chiral amino alcohols in the presence of Zn(II) leads to highly enantioselective fluorescent enhancement. It is found that the chiral conjugated polymer shows greatly enhanced enantioselectivity over the corresponding small molecular sensor under the same conditions. This work provides the first example that a conjugated polymer is used to greatly increase the enantioselectivity of a small molecular sensor in chiral recognition. Simultaneous determination of the concentration and enantiomeric composition of chiral substrates by a fluorescent measurement has been achieved by combining the polymer with salicylaldehyde in the assay.

Polyacrylates bearing dinitrobenzoate side groups undergo sol–gel–sol transformations in DMF or THF solutions regulated by alternating UV light and dark conditions. The formation and recombination of radical ionic species via photoinduced electron transfer may be responsible.

A small fluorescence ratiometric probe consisting of a single dye species, N-methyl-6-hydroxyquinolinium (MHQ), and coupled enzymatic substrates, exhibits a dramatic colour change (deep blue to red) and possesses a huge response ratio (over 2000 fold) upon specific recognition of target enzymes. Such dramatic responses are attributed to the excited-state proton transfer processes of MHQ molecules in water. Here the detection of β-galactosidase and porcine pancreatic lipase is successfully demonstrated and this class of molecules has the potential to be developed as a “naked-eye” probe in vitro.

Enhanced spin–orbit coupling through external heavy-atom effect (EHE) has been routinely used to induce room-temperature phosphorescence (RTP) for purely organic molecular materials. Therefore, understanding the nature of EHE, i.e., the specific orbital interactions between the external heavy atom and the luminophore, is of essential importance in molecular design. For organic systems, halogens (e.g., Cl, Br, and I) are the most commonly seen heavy atoms serving to realize the EHE-related RTP. In this report, we conduct an investigation on how heavy-atom perturbers and aromatic luminophores interact on the basis of data obtained from crystallography. We synthesized two classes of molecular systems including N-haloalkyl-substituted carbazoles and quinolinium halides, where the luminescent molecules are considered as “base” or “acid” relative to the heavy-atom perturbers, respectively. We propose that electron donation from a π molecular orbital (MO) of the carbazole to the σ* MO of the C–X bond (π/σ*) and n electron donation to a π* MO of the quinolinium moiety (n/π*) are responsible for the EHE (RTP) in the solid state, respectively.

Alkyl-substituted tetra-coordinate organoboronium bisdiketone complexes exhibit dramatic luminescence thermochromism in organic solvents. In glass-forming alcohols, these complexes exhibit a reversible aqua blue to orange-red to greenish yellow luminescence emission colour change upon cooling.

Single-component materials with both fluorescence and room-temperature phosphorescence (RTP) are useful for ratiometric sensing and imaging applications. On the basis of a general design principle, an amino-substituted benzophenone is covalently incorporated into waterborne polyurethanes (WPU) and results in fluorescence and RTP single-component dual-emissive materials (SDMs). At different aminobenzophenone concentrations, the statistical, thermal, and optical properties of these SDMs are characterized. Despite their similar thermal behaviors, the luminescence properties as a function of the chromophore concentration are quite different: increasing concentrations led to progressively narrowed singlet–triplet energy gaps. The tunability of fluorescence and RTP via chromophore concentration is explained by a previously proposed model, polymerization-enhanced intersystem crossing (PEX). The proposal of PEX is based on Kasha’s molecular exciton theory with a specific application in polymeric systems, where the polymerization of luminophores results in excitonic coupling and enhanced forward and reverse intersystem crossing. The mechanism of PEX is also examined by theoretical calculations for the WPU system. It is found that the presence of K1 aggregates indeed enhances the crossover from singlet excited states to triplet ones.Keywords: dual-emissive; fluorescence; room-temperature phosphorescence; single-component; waterborne polyurethane

A 1,1′-bi-2-naphthol-based macrocyclic compound was found to show large fluorescent enhancement at a greatly red-shifted wavelength in the presence of one equiv. Hg2+ but not with any other metal ions. This change was visually observable from blue-greenish emission to bright yellow emission. The fluorescence was quenched when more than 2.6 equiv. Hg2+ was added.

A highly fluorescent polymer consisting of repeating pendant dye molecules, difluoroboron dibenzoylmethane (BF2dbm), and an end-capped Rhodamine B (RhB) exhibits efficient energy transfer (EnT) owing to long-lived polymer excitons. External stimuli such as solvation and temperature can dramatically affect the efficiency of EnT and thus change the emission color.

Materials with both fluorescence and room-temperature phosphorescence (RTP) can be useful in the field of optoelectronics. Here we present a general strategy, taking advantage of carbonyl compounds, which have been known to possess efficient intersystem crossing with high triplet state yield, as well as a strongly fluorescent intramolecular charge-transfer (ICT) state, to produce materials with both fluorescence and RTP at the same time, or dual-emission. In the presented model systems, in order to generate a suitable ICT state, Lewis acid binding to aromatic ketone derivatives has been proved to be a viable method. We have selected AlCl3, BCl3, BF3, and GdCl3 as binding Lewis acids, in that they exhibit sufficiently strong binding affinity toward the aromatic ketone derivatives to afford stable complexes and yet do not possess low-lying electronic transitions vs the ligands. We have successfully observed dual-emission from these designed complexes in polymers, which act to suppress competitive thermal decay at room temperature. One of the complexes is particularly interesting as it is dual-emissive in the crystalline state. Single-crystal XRD reveals that the molecule forms multiple hydrogen bonds with its neighbors in crystals, which may significantly enhance the rigidity of the environment.Keywords: aromatic ketone; charge-transfer state; dual-emission; fluorescence; Lewis acid; room-temperature phosphorescence;

Poly(ethylene glycol)-modified β-diketone macroligand is developed to sensitize europium(III) ions in water. High luminescence intensity characteristic of Eu3+ was achieved due to spontaneous formation of micelle-like structure in which the hydrophobic core prevents luminescence-quenching by water molecules. The pH is found to induce a quantitative ratio change in two fluorescence bands from both ligand and Eu3+.

Being responsive and adaptive to external stimuli is an intrinsic feature characteristic of all living organisms and soft matter. Specifically, responsive polymers can exhibit reversible or irreversible changes in chemical structures and/or physical properties in response to a specific signal input such as pH, temperature, ionic strength, light irradiation, mechanical force, electric and magnetic fields, and analyte of interest (e.g., ions, bioactive molecules, etc.) or an integration of them. The past decade has evidenced tremendous growth in the fundamental research of responsive polymers, and accordingly, diverse applications in fields ranging from drug or gene nanocarriers, imaging, diagnostics, smart actuators, adaptive coatings, to self-healing materials have been explored and suggested. Among a variety of external stimuli that have been utilized for the design of novel responsive polymers, enzymes have recently emerged to be a promising triggering motif. Enzyme-catalyzed reactions are highly selective and efficient toward specific substrates under mild conditions. They are involved in all biological and metabolic processes, serving as the prime protagonists in the chemistry of living organisms at a molecular level. The integration of enzyme-catalyzed reactions with responsive polymers can further broaden the design flexibility and scope of applications by endowing the latter with enhanced triggering specificity and selectivity. In this tutorial review, we describe recent developments concerning enzyme-responsive polymeric assemblies, nanoparticles, and hydrogels by highlighting this research area with selected literature reports. Three different types of systems, namely, enzyme-triggered self-assembly and aggregation of synthetic polymers, enzyme-driven disintegration and structural reorganization of polymeric assemblies and nanoparticles, and enzyme-triggered sol-to-gel and gel-to-sol transitions, are described. Their promising applications in drug controlled release, biocatalysis, imaging, sensing, and diagnostics are also discussed.

Mechanochromic luminescence (ML) refers to the luminescence color and/or intensity change of solid-state materials induced by mechanical perturbations. For organic molecular solids, this phenomenon is related to the specific packing modes and orientations of individual fluorophores, which could give rise to different excited-state interactions. The molecular solids of difluoroboron dibenzoylmethane (BF2dbm) derivatives were previously found to exhibit reversible ML at room temperature and are promising as self-healing optical materials. In this report, we aim to shed some light on the mechanism of BF2dbm ML by trying to understand the excited-state interactions among solid-state BF2dbm molecules and elucidate how these interactions change upon mechanical stimulation. We first investigated the optical properties of monomeric, dimeric, and polymeric BF2dbm derivatives in optically dilute solutions and demonstrated unambiguously that BF2dbm moieties have a propensity to form H-aggregates. Next, we studied the physical properties of these boron complexes in the solid state including their crystal structures, fluorescence emissions, and mechanochromic luminescence. By correlating solution data with the solid-state characterization results, it was concluded that two coupled processes, force-induced emissive H-aggregate formation and energy transfer to the emissive H-aggregates, are responsible for the observed BF2dbm ML in the solid state.

Commercially available avobenzone readily reacts with boron trichloride or tribromide to afford boron-bisavobenzone salts. The complexes were found to exhibit unusual solvent-dependent fluorescence in solution and hydrochromic fluorescence in the solid state: a blue-to-orange emission colour change was observed upon water vapour exposure. The phenomenon was likely caused by interference of the boron coordination sphere by water molecules.

We report on the fabrication of a new type of polymeric fluorescent Hg2+ probe covering a broad effective concentration range from nanomolar to micromolar levels and exhibiting considerably enhanced detection selectivity. Two amphiphilic diblock copolymers colabeled with Hg2+-reactive caged dye (RhBHA) and Hg2+-catalyzed caged fluorophore (HCMA) in the hydrophilic segments, PS-b-P(DMA-co-HCMA) and PS-b-P(DMA-co-RhBHA), were synthesized via sequential reversible addition–fragmentation chain transfer (RAFT) polymerization, where PS, DMA, HCMA, and RhBHA are polystyrene, N,N-dimethylacrylamide, hydrazone-caged coumarin, and rhodamine B (RhB) derivatives, respectively. The two amphiphilic diblock copolymers can spontaneously self-assemble into mixed micelles in aqueous solution possessing hydrophobic PS cores and HCMA and RhBHA moieties colabeled hydrophilic PDMA coronas. Fluorescence emissions of caged RhBHA and HCMA moieties can effectively turn on in the presence of low and high Hg2+ concentrations via Hg2+-induced ring-opening reaction and Hg2+-catalyzed hydrolysis mechanisms, respectively. The drastically different, but self-complementary reaction kinetics and optimum working concentration ranges of RhBHA and HCMA moieties endow the sensing system with high selectivity and broad sensing concentration range (from nanomolar to micromolar). In addition, the Hg2+-sensing platform can be further employed for the fluorescent ratiometric detection of Cu2+ ion via its selective quenching of the emission of acyclic RhBHA moieties. This work presents a new example of ensembling two partially selective chemical reaction-based fluorometric sensing motifs to achieve enhanced metal ion sensing selectivity and broadened working concentration ranges, which can be further generalized for the construction of other highly selective and broad dynamic range sensing systems.

A series of pyridinium ring substituted β-diketones were found to exhibit aggregation-induced emission (AIE) and water-vapour recoverable mechanochromic luminescence in the solid state. Certain water-soluble β-diketones could selectively bind to cellulose-based materials, where strong fluorescence similar to AIE was turned on.

Alkyl-substituted tetra-coordinate organoboronium bisdiketone complexes exhibit dramatic luminescence thermochromism in organic solvents. In glass-forming alcohols, these complexes exhibit a reversible aqua blue to orange-red to greenish yellow luminescence emission colour change upon cooling.

A 1,1′-bi-2-naphthol-based macrocyclic compound was found to show large fluorescent enhancement at a greatly red-shifted wavelength in the presence of one equiv. Hg2+ but not with any other metal ions. This change was visually observable from blue-greenish emission to bright yellow emission. The fluorescence was quenched when more than 2.6 equiv. Hg2+ was added.

Polyacrylates bearing dinitrobenzoate side groups undergo sol–gel–sol transformations in DMF or THF solutions regulated by alternating UV light and dark conditions. The formation and recombination of radical ionic species via photoinduced electron transfer may be responsible.

Being responsive and adaptive to external stimuli is an intrinsic feature characteristic of all living organisms and soft matter. Specifically, responsive polymers can exhibit reversible or irreversible changes in chemical structures and/or physical properties in response to a specific signal input such as pH, temperature, ionic strength, light irradiation, mechanical force, electric and magnetic fields, and analyte of interest (e.g., ions, bioactive molecules, etc.) or an integration of them. The past decade has evidenced tremendous growth in the fundamental research of responsive polymers, and accordingly, diverse applications in fields ranging from drug or gene nanocarriers, imaging, diagnostics, smart actuators, adaptive coatings, to self-healing materials have been explored and suggested. Among a variety of external stimuli that have been utilized for the design of novel responsive polymers, enzymes have recently emerged to be a promising triggering motif. Enzyme-catalyzed reactions are highly selective and efficient toward specific substrates under mild conditions. They are involved in all biological and metabolic processes, serving as the prime protagonists in the chemistry of living organisms at a molecular level. The integration of enzyme-catalyzed reactions with responsive polymers can further broaden the design flexibility and scope of applications by endowing the latter with enhanced triggering specificity and selectivity. In this tutorial review, we describe recent developments concerning enzyme-responsive polymeric assemblies, nanoparticles, and hydrogels by highlighting this research area with selected literature reports. Three different types of systems, namely, enzyme-triggered self-assembly and aggregation of synthetic polymers, enzyme-driven disintegration and structural reorganization of polymeric assemblies and nanoparticles, and enzyme-triggered sol-to-gel and gel-to-sol transitions, are described. Their promising applications in drug controlled release, biocatalysis, imaging, sensing, and diagnostics are also discussed.

Mechanochromic luminescence (ML) refers to the luminescence color and/or intensity change of solid-state materials induced by mechanical perturbations. For organic molecular solids, this phenomenon is related to the specific packing modes and orientations of individual fluorophores, which could give rise to different excited-state interactions. The molecular solids of difluoroboron dibenzoylmethane (BF2dbm) derivatives were previously found to exhibit reversible ML at room temperature and are promising as self-healing optical materials. In this report, we aim to shed some light on the mechanism of BF2dbm ML by trying to understand the excited-state interactions among solid-state BF2dbm molecules and elucidate how these interactions change upon mechanical stimulation. We first investigated the optical properties of monomeric, dimeric, and polymeric BF2dbm derivatives in optically dilute solutions and demonstrated unambiguously that BF2dbm moieties have a propensity to form H-aggregates. Next, we studied the physical properties of these boron complexes in the solid state including their crystal structures, fluorescence emissions, and mechanochromic luminescence. By correlating solution data with the solid-state characterization results, it was concluded that two coupled processes, force-induced emissive H-aggregate formation and energy transfer to the emissive H-aggregates, are responsible for the observed BF2dbm ML in the solid state.