Photomodulation of Charge Transfer through Excited-State Processes: Directing Donor-Acceptor Binding Dynamics

Modulating charge transfer (CT) interactions between donor and acceptor molecules may give rise to unique dynamic changes in physicochemical properties, exhibiting great importance in supramolecular chemistry and materials science. In this work, we demonstrate the first instance of reversible photomodulation of donor-acceptor (D-A) CT interaction in the solid state. Pyridinium-based chromophore featuring π-conjugated D-A structures can not only function as a good electron acceptor to undergo photoinduced electron transfer (ET) or engage in intermolecular CT interaction, but also exhibit unique dual emission depending on the excitation wavelengths. The rotatable C-C single bonds within D-A pairs enhance the tunability of molecular structure. Through the synergy of a photoinduced ET and an excited-state conformational change, the intermolecular CT interaction can be switched on and off by alternate light irradiation to enable reversibly modulation of the affinity between donor and acceptor molecules, accompanied by visual color switching and fluorescence on-off as feedback signals.

Photochromism, Thermochromism, and Electrochromism in Solid-State Host-Guest Inclusion Complexes of β-Cyclodextrin with Dialkylcarboxyl-Substituted Viologens

Multi-stimuli-responsive chromic materials have immense potential for utilization. Herein, two supramolecular inclusion complexes were prepared by self-assembly of β-cyclodextrin (β-CD) with dialkylcarboxyl-substituted viologens, N,N'-di(3-carboxy-propyl)-4,4'-bipyridinium dichloride (CPV·Cl2) and N,N'-di(6-carboxy-hexyl)-4,4'-bipyridinium dibromide (CHV·Br2). The self-assembled inclusion complexes CPV2+@β-CD and CHV2+@β-CD2 in the solid-state exhibited naked-eye photochromism, thermochromism, and electrochromism in response to multiple external stimuli including light, temperature, and electric field, respectively. Solid-state UV-vis diffuse reflectance and electron spin resonance (ESR) spectroscopy revealed that the observed photochromism, thermochromism and electrochromism are attributed to the formation of viologen free radicals induced by electron transfer under external stimuli. The excellent stimuli-response chromic properties of the title inclusion complexes support their practical utility in visual display, multiple anticounterfeiting, and multilevel information encryption.

Photochromism and photoswitchable luminescence in a Zn7 cluster-based metal-organic framework with an organic guest

Photochromic materials coupled with photoswitchable luminescence functionalities are of particular interest due to their potential applications in switches and optical memory devices. However, the construction of such materials, especially those with two-color emission states, is still challenging. In this context, a rare Zn₇ cluster-based host-guest MOF material, (bbmp)[Zn₇(IPA)₆(OH)₄(H₂O)₂] (1) (H₂IPA = isophthalic acid, bbmp·2I = 4,4'-([1,1'-biphenyl]-4,4'-diyl)bis(1-methylpyridin-1-ium) diiodide), was prepared by encapsulating an organic cation into an anionic MOF produced from zinc cations and isophthalic acid ligands, which exhibits reversible naked detectable photochromic properties varying from yellow to green upon UV-Vis light irradiation. The photoactive guest bbmp²⁺ and the short O⋯N⁺ distances between the oxygen atoms of the carboxylate groups and the pyridine ring play a crucial role in the photochromism of this compound. More interestingly, the luminescence color of this cluster-based host-guest material can be reversibly switched from green to blue upon irradiation, exhibiting photoswitchable luminescence properties.

Electron transfer photochromism of solid-state supramolecules constructed by cucurbit[n]uril (n = 5–8) and 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride

Solid-state supramolecular complexes of a bispyridinium ethylene derivative with cucurbit[n = 5–8]uril exhibit fast responsive photochromic properties, which are caused by the electron transfer from the host to the guest.

Strategy for optical data encryption and decryption using a D-A type stimuli-responsive AIE material

In this study, we report a new donor-acceptor (D-A) type stimuli-responsive material, (E)-4-(((9-ethyl-9H-carbazol-2-yl)methylene)amino)benzoic acid (C1), which possesses both aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) natures. It glows green photoluminescence which changes into yellow color in response to mechanical stimuli, and fumigation in volatile organic compounds (VOCs) can switch the emission back to the initial state with high reversibility. In addition, the C1 film glows yellow-orange light, but turns into blue emission under continuous fumigation in ethyl acetate vapor. However the vapochromism behaves different when the C1 film is smeared: The emission of the smeared film is similar to the unsmeared but changes into cyan color after fumigation. The differences in vapochromism between smeared and unsmeared film can be easily distinguished by naked eyes. As revealed by SEM, the as-prepared film undergoes a morphology change from ill-shaped particles to microspheres in response to organic vapor, while the smeared film with scratched surface changes into dendritic patterns. According to the morphology study, the vapochromic luminescence can be ascribed to the physical adsorption of ethyl acetate vapor and the resulting change in the ICT process. In light of the unique vapochromism of C1, a new encryption-decryption technique for data recording was developed. Information can be recorded on the C1 film by mechanical writing and simultaneously concealed. It can be only accessed via fumigation in organic vapor, demonstrating a reliable steganography technology.

Distinct and Selective Amine- and Anion-Responsive Behaviors of an Electron-Deficient and Anion-Exchangeable Metal-Organic Framework

Smart materials that respond to chemical stimuli with color or luminescence changes are highly desirable for daily-life and high-tech applications. Here, we report a novel porous metal-organic framework (MOF) that shows multiple, selective, and discriminative responsive properties owing to the combination of different functional ingredients [tripyridinium chromogen, Eu(III) luminophore, cationic framework, and special porous structure]. The MOF contains two interpenetrated three-dimensional cationic coordination networks built of a tetrahedral [Eu₄(μ₃-OH)₄] cluster and a tripyridinium-tricarboxylate zwitterionic linker. It shows reversible and discriminative chromic response to aliphatic amines and aniline through different host-guest interactions between electron-deficient pyridinium and electron-rich amines. The size- and shape-selective response to aliphatic amines is ascribed to the radical formation through host-guest electron transfer, whereas the response to aniline is ascribed to the formation of sandwich-type acceptor-donor-acceptor complexes. The MOF is capable of reversible anion exchange with various anions and shows selective and discriminative ionochromic response to iodide, bromide, and thiocyanate, which is attributed to charge-transfer complexation. The above chromic behaviors are accompanied by efficient quenching of Eu(III) photoluminescence. The MOF represents a multi-stimuli dual-output responsive system. It can be used for discrimination and identification of anions and amines. The potential use in invisible printing, reusable sensory films, and optical switches was demonstrated by the ink and the membrane made of the MOF and organic polymers.

Synergism of photocycloaddition and photoinduced electron transfer for multi-state responsive materials with high-stability and reversibility

A dual-photoresponsive coordination polymer displaying color-distinguishable olefin/cyclobutane-linked pyridinium radical states through photoinduced electron transfer and photocycloaddition has been successfully assembled based on the monoquaternized trans-1,2-bis(4-pyridyl)ethylene and electron-rich benzenetetra-carboxylate anion. The synergy of charge-transfer and cation-π interactions within the framework endows the compound with excellent stability toward high temperature (350 °C), acidic/basic environments (pH = 2-12) and organic solvents. Reversible recovery from the radical states to their initial states, and reverse-cyclization can be achieved upon heat-treatment.

Synthesis and photophysical studies on a new fluorescent phenothiazine-based derivative

A new typical phenothiazine compound functionalized with thienyl-indandione derivative (PTZTID) was synthesized and characterized using spectral analysis (ultraviolet-visible (UV-vis) light, infrared (IR), ¹ H nuclear magnetic resonance (NMR) and ¹³ C NMR tools). The UV-vis absorption spectra of the PTZTID solution in 1,4-dioxane showed two absorption bands attributed to localized aromatic π-π* transitions of conjugated aromatic moieties and intramolecular charge transfer with the characteristics of a π-π* transition. The fluorescence spectra exhibited a maximum emission wavelength at 580 nm. The effect of concentration on photophysical properties took the form of a minor hypsochromic shift, which was attributed to some extent to the occurrence of H-type aggregation of the PTZTID derivative. Binary solvent effects on the spectroscopic behaviour of PTZTID were measured at different H₂ O/1,4-dioxane ratios. Similarly, when increasing the water content, a hypsochromic shift was observed that resulted from H-type aggregation. Furthermore, geometry and electronic configurations of PTZTID were studied at density functional theory /B3LYP level and indicated that the compound had a nonplanar (butterfly structure).

Photochromic and luminescent switchable iodoargentate hybrids directed by solvated lanthanide cations

A series of photochromic and luminescent switchable iodoargentate hybrids have been constructed by using solvated lanthanide cations as SDAs.

Gemini-Type Supramolecular Amphiphile Based on a Water-Soluble Pillar[5]arene and an Azastilbene Guest and Its Application in Stimuli-Responsive Self-Assemblies

Supramolecular amphiphiles are a type of intriguing building blocks to fabricate self-assembled nanostructures that can be applied in diverse fields. Gemini-type supramolecular amphiphiles, containing two hydrophobic tails and two hydrophilic head groups linked by a spacer, are good candidates to fabricate many advanced materials that are able to apply in surface modification, drug/gene delivery, and solubilization. Pillararenes, the fifth generation of macrocyclic host molecules, have been used to fabricate many supramolecular amphiphiles that played important roles in biomedical fields and materials science. However, compared with single-chain and bola-type supramolecular amphiphiles, the studies of gemini-type supramolecular amphiphiles based on pillararenes are very rare. Herein, a new strategy to prepare gemini-type supramolecular amphiphiles was reported. A new acid-responsive host?guest recognition motif in water on the basis of a 4,4?-azastilbene derivative (G1) and a water-soluble pillar[5]arene (WP5) was fabricated. The gemini-type supramolecular amphiphile was constructed by an azastilbene amphiphilic guest (G2) and WP5. Then its application in stimuli-responsive self-assemblies was investigated. G2 self-assembled into nanoribbons in water. Upon addition of WP5, the gemini-type supramolecular amphiphile formed, leading to the formation of disklike micelles. After further addition of hydrochloric acid, the morphology changed into nanosheets.

Bifunctional Cu2+/Fe3+ Probe with Independent Signal Outputs Based on a Photochromic Diarylethene with a Dansylhydrazine Unit

A novel dual-response fluorescent sensor based on a diarylethene photoswitching unit and a dansylhydrazine functional group has been synthesized. The compound exhibited high selectivity for Fe³⁺ and Cu²⁺ with independent fluorescence signal outputs. In the presence of Fe³⁺, the sensor formed a 1:1 metal complex, resulting in a remarkable "turn-off" fluorescence signal. On the other hand, its fluorescence intensity was notably enhanced (turn-on) and a color change from bright yellow to bright blue was observed when the sensor interacted with Cu²⁺, which was due to the hydrolysis reaction of the dansyl acid dye, as confirmed by high-resolution mass spectrometry-electrospray ionization and infrared spectrum. The detection limits were 9.73 × 10^-8 mol L^-1 for Fe³⁺ and 3.49 × 10^-7 mol L^-1 for Cu²⁺, respectively. From the unimolecular platform, two molecular logic circuits were constructed using the fluorescence emission intensity at 557/494 nm (Fe³⁺/Cu²⁺) as the outputs and the combined stimuli of Fe³⁺/ethylenediaminetetraacetic acid, Cu²⁺, and UV/vis as the inputs. In addition, the sensor was successfully used to determine Fe³⁺ in water samples from Ganjiang River and soil samples from Nanchang fields.

Reversible two-channel mechanochromic luminescence for a pyridinium-based white-light emitter with room-temperature fluorescence–phosphorescence dual emission

For a pyridinium-based emitter with fluorescence–phosphorescence dual emission, mechanical grinding induces phosphorescence disappearance and mechanical pressing induces extraordinary phosphorescence enhancement.

Reversible dielectric switching behavior of a 1D coordination polymer induced by photo and thermal irradiation

Switchable dielectric relaxation is achieved through photocycloaddition/reversion within a 1D coordination polymer on thermal and photo irradiation.

Reversible mechanochromism and aggregation induced enhanced emission in phenothiazine substituted tetraphenylethylene

Reversible mechanofluorochromism (MFC) and aggregation induced emission (AIE) properties of phenothiazine functionalized tetraphenylethylene derivatives were investigated.

Three birds, one stone - photo-/piezo-/chemochromism in one conjugated nanoporous ionic organic network

A nanoporous material bearing a high ion density and inherent organic radical character was synthesized by a facile one-pot process, which exhibits photo-, piezo- and chemochromism, driven by the diverse electron transfer processes between the acceptor framework and different electron donors. The responsive behavior is useful for its sensing application, as demonstrated here for pressure, anion and gas sensing.

Hierarchically controllable photoreaction of a coordination polymer based on quaternized 1,2-bis(4'-pyridyl)ethylene

A multiply photoactive compound has been developed based on quaternized 1,2-bis(4'-pyridyl)ethylene, which exhibits a hierarchical photoreaction by controlling light intensity. Weak ultraviolet light can effectively promote the cycloaddition reaction with fluorescence enhancement, while stronger ultraviolet light can stimulate a complex response of cycloaddition and electron-transfer which shows turn-off luminescence and coloration. Photofacilitated bleaching of the colored products has been observed for the first time in the pyridinium family.

Vapour-driven crystal-to-crystal transformation showing an interlocking switch of the coordination polymer chains between 1D and 3D

A rare crystal-to-crystal transformation occurs between 1D coordination polymer chains and 3D mechanically interlocked structures via reversible opening and closing of the homogeneous chains without any change in the coordination mode and composition.

Aromatic sulfonium polyoxomolybdates: tuning the photochromic properties through substitutions on the counter ion moiety

A series of aromatic sulfonium Keggin hybrids have been developed which exhibited excellent photochromic properties under UV irradiation. The photo-coloration speed of these hybrids could be fine-tuned by changing the substituents on the sulfonium counter ion moiety.

Post-cycloaddition modification of a porous MOF for improved GC separation of ethanol and water

An olefin-containing porous framework undergoes [2 + 2] cycloaddition to give post-modified pore space, accompanied by turn-on luminescence upon light irradiation. The photochemical modification reinforces the supramolecular network and improves GC separation performance of ethanol and water.

Multi-stimuli responsive fluorescent behaviors of a donor–π–acceptor phenothiazine modified benzothiazole derivative

Donor–π–acceptor type phenothiazine modified benzothiazole derivative PVBT exhibited multi-stimuli responsive fluorescent properties.