Photochromic Behavior Research Articles

Reversible photochromism and photoluminescence modulation in (K,Na)NbO3‐based ceramics via oxygen vacancy regulation

AbstractMost of the optical behaviors in photoelectric multifunctional ceramics strongly depend on the addition of rare‐earth elements. However, the limited species of rare‐earth cations available as luminescence centers restrict the further development of photoelectric devices. Additionally, introducing the large atomic‐radii elements usually dramatically deteriorates the piezoelectric properties. Herein, the novel photochromism and luminescence modulation properties have been realized in CuO‐doped [(K0.43Na0.57)0.94Li0.06][(Nb0.94Sb0.06)0.95Ta0.05]O3 (KNLNST) piezoelectric ceramics which show intense sensitivity to visible‐light irradiation. As a strategy for regulating oxygen vacancy concentrations in piezoelectric ceramics, different CuO amounts are doped to generate color centers for photochromism. The photoluminescence phenomenon exhibiting green emission under 375 nm excitation is observed in KNLNST piezoelectric ceramics, which is most commonly realized via rare‐earth doping in luminescent materials. KNLNST–0.3CuO ceramics with the highest oxygen vacancy concentration achieve a reversible luminescence modulation of 46.9% by altering light irradiation and thermal stimulus. Moreover, a hand‐rewritable optical information test demonstrates exceptional reproducibility and fatigue durability. This research displays the controllable photochromism and luminescence behaviors in ceramics via regulating oxygen vacancy concentration by CuO “hard” doping. The lead‐free high performance photoelectric ceramics fulfill the demands of potential applications in anti‐counterfeiting and optical storage devices.

Mesogenic cholesterol-naphthalene dimers: Synthesis, characterization, mesogenic properties, photochromic behaviour and theoretical insights

Synthesis and characterization of two new series of unsymmetrical dimers combining cholesterol with azo (Series I-n) or azomethine (Series II-n) naphthalene moieties were reported. The chemical structures were confirmed using FT-IR, 1H NMR, 13C NMR, mass spectrometry, and elemental analysis. Their thermotropic properties were investigated using POM, DSC and XRD. The analysis revealed that the first lower members (n = 1,2) of each series lack mesogenic properties due to short, flexible spacers, while higher members (n = 3–5,7,9,10) exhibit enantiotropic chiral nematic phases. Notably, higher members with odd spacers (n = 5,7,9) also display SmC* phases, highlighting the influence of spacer length and odd–even effects on molecular interactions. Dimers from series I-n demonstrated slightly higher clearing temperatures (TN-Iso) compared to series II-n. UV–Vis spectroscopy showed trans–cis isomerization, with a photoconversion efficiency of 56.7 % for the I-7 dimer. Computational studies using the B3LYP functional with a 6-31g (d, p) basis set provided insights into the electronic structure through electrostatic potential (ESP) and optical property analysis.

PH regulated two Zn(II) viologen-derived photochromic functional complexes

By adjusting pH, two zero-dimensional (0D) Zn(II)-based functional complexes, namely, [Zn(CEbpy)(TPDC)(H2O)4]·3H2O (1) and (CEbpy)2·[Zn(H2O)6](TPDC)·3H2O (2) have been successfully constructed from thiophene-2,5-dicarboxylic acid (H2TPDC) and 1-carboxyethyl-4,4′-bipyridine (CEbpy). Both 1 and 2 display electron transfer (ET)-mediated photochromic behaviors with different photoresponsive rates. Furthermore, the solid-state photo-controlled luminescence of two compounds is also examined at ambient temperature. Appealingly, upon UV illumination, compound 1 possesses potential as an inkless and erasable printing medium. These results provide potential avenues for rational design and synthesis of functional materials that can be used in photochromic devices by modulating pH.

Rapid fabrication of polyoxometalate-enhanced photo responsive films from ethyl cellulose (EC) and polyvinylpyrrolidone (PVP)

Ethyl cellulose (EC), polyvinylpyrrolidone (PVP), and phosphomolybdic acid (PMoA) were the components of a new photochromic hybrid film composed of heteropoly acids (abbreviated EC-PVP/PMoA), created by solvent evaporation. The EC-PVP/PMoA mechanism, visible light photochromic behaviors, and microstructure were closely studied by transmission electron microscopy (TEM), atomic force microscopy (AFM), ultraviolet-visible (UV–visible) spectroscopy, X-ray photo electronics (XPS), and Fourier transmission infrared spectroscopy (FTIR). FTIR spectra show that neither the core structures of EC, PVP, nor the Keggin structure of PMoA in the EC-PVP/PMoA composite hybrid film were damaged during fabrication. In the presence of oxygen, the yellowish transparent hybrid film exhibited reversible photochromism and significant photochromic reactivity, becoming blue in the presence of visible light. Upon heating the film at 90 °C for 1 h its photoresponsive properties were enhanced, making the blue color more vibrant and stable due to polyethylene glycol (PEG). The XPS spectra demonstrated a proton transfer procedure during the photochromic process, and the presence of Mo+5 species indicated a photoreduction interaction between the PMoA particles and the EC-PVP matrix. To the best of our knowledge, this is the first EC-PVP combined hybrid film with POMs with promising prospects for solar UV detection and smart glass windows applications.

Polar Thiazole Derivative-Induced Second Harmonic Response in Hybrid Bismuth Chlorate with Reversible Photochromism.

Inorganic-organic hybrid bismuth halides demonstrate great prospect in the field of second-order nonlinear optical (NLO) crystals because the ns2 electron on Bi(III) could lead to large molecular polarization and high second harmonic generation (SHG) coefficient on a noncentrosymmetric structure. However, researchers cannot yet control the effective arrangements of the bismuth halide functional motif, which results in SHG-active hybrid bismuth halides being rare. Herein, thiazole derivatives with a polar donor-π-acceptor system are designed to explore hybrid bismuth halide NLO crystals. The protonated 2-(4-hydroxyphenyl) thiazole (denoted as hpt) interacts with the (BiCl6) motif via H···Cl hydrogen bonds to prevent antiparallel arrangements, which therefore enhance the NLO property. (Hhpt)3[BiCl6] crystallizes in the monoclinic polar P21 space group, which exhibits a strong SHG response and reversible photochromic behavior. Structural analysis and theory calculations reveal that the synergistic effect between the polar thiazole derivative and the distorted inorganic [BiCl6]3- motif is responsible for the SHG response. This is the first report of polar thiazole derivative-induced SHG-active hybrid bismuth halide coupled with reversible photochromism.

Photoluminescent Multicolor Carbon Dots for UV Detection and Dynamic Anticounterfeiting.

Carbon dots (CDs) are an emerging type of fluorescent carbon nanomaterial with broad application prospects. Among them, photochromic CDs have been widely used in the field of optoelectronic devices but rarely in ultraviolet (UV) detection. In this work, we successfully developed photochromic CDs that exhibit reversible emission under light stimulation in an amine solvent system. Notably, the CDs showed ultrafast photochromic behavior in diethylamine solvent, shifting the fluorescence color from cyan-green to orange-red after 2 s of irradiation, with the solution color changing from pale yellow to pale purple. Furthermore, this performance could recover without additional stimuli, simply by standing for several tens of seconds. Structural analysis indicated that rapid photochromism arises from changes in the surface functional group radicals of the CDs, with the reversibility attributed to fluctuation in these radicals. Leveraging the excellent photochromic properties of CDs, we further developed a device for detecting UV indices in sunlight. This opens up broad prospects for developing high-performance UV detection devices based on CDs.

Multi-Stimuli-Responsive Carbon Dots with Intrinsic Photochromism and In Situ Radical Afterglow.

The combination of advanced photoluminescence characteristics to photochromism is highly attractive in preparing high-performance multifunctional photo-responsive materials for optoelectronic applications. However, this is rather challenging in material design owing to the limited mechanism understanding and construction principles. Here, an effective strategy to integrate photochromism and afterglow emission in carbon dots (CDs) is proposed through embedding naphthaleneimide (NI) structure in CDs followed by polyvinylpyrrolidone (PVP) encapsulation. The NI-structured CDs-PVP shows intrinsic photochromism owing to the in situ formation of NI-radical anions and controllable multi-stimuli-responsive afterglow behaviors related to the oxygen-trigged triplet exciton quenching and Förster resonance energy transfer (FRET) from the pristine CDs to the photoactivated CDs radicals. Notably, a wide range of appearance colors from colorless to brown, luminescence color transition from blue to yellow, and much elongated afterglow lifetime up to 253ms are observed. With the extraordinary stimuli-chromic and stimuli-luminescent CDs-PVP film dynamically responsive to multiple external stimuli, reversible secure snapchat, data encryption/decryption and synaptic imaging recognition are realized. These findings demonstrate a fundamental principle to design multi-stimuli-responsive photochromic CDs with afterglow, providing important understandings on the synergic mechanism of dynamic photochromism and emission behaviors and thereby expanding their applications in advanced information anti-counterfeiting and artificial intelligence.

Light/Force-Responsive Room Temperature Phosphorescence in a Zinc-Organic Coordination Polymer.

A zinc-organic hybrid (1) with multifunctional room temperature phosphorescence (RTP) was synthesized. 1 presents light/force-sensitive RTP properties due to the photochromic behavior from gray to light yellow and the transition from crystalline to amorphous state, respectively. Furthermore, inkless printing and information encryption models were successfully constructed to prove their widespread application prospect.

Synthesis, structure, and photochromic properties of Zn-coordination polymers derived from hexyl viologen cations

This study presents the synthesis, structural characterization, and investigation of the photochromic properties of two Zn-coordination polymers, formulated as {(HV)0.5[Zn(MIC)(HMIC)]·H2O}n (1) and {(HV)2[Zn4(BTEC)3]}n (2) (H2MIC = 5-methylisophthalic acid, H4BTEC = 1,2,4,5-benzenetetracarboxylic acid, HVBr2 = 1,1′-bis(n-hexyl)-4,4′-bipyridinium dibromide), derived from hexyl viologen cations. Compound 1 is characterized by a one-dimensional (1D) structure, while compound 2 exhibits a three-dimensional (3D) architecture, both crystallizing in distinct space groups and showcasing unique assembly and composition. Detailed structural analysis reveals the coordination modes of Zn(II) ions within these polymers. The photochromic behavior of two compounds under UV light exposure was thoroughly examined through ultraviolet-visible (UV–vis) and electron paramagnetic resonance (EPR) spectroscopy, demonstrating reversible color changes from light yellow to blue upon irradiation, with distinct rates of decolorization indicative of the stability of photo-induced viologen radicals. This reversibility was successfully replicated over multiple cycles, underscoring the durability of the photochromic transition. Furthermore, powder X-ray diffraction (PXRD) data confirmed the maintenance of crystal structures throughout the photochromic process, dismissing photolysis or photo-induced isomerization as underlying mechanisms. Instead, the observed photochromism is attributed to efficient electron transfer facilitated by the proximity between electron-donating and electron-withdrawing groups within the coordination polymers. This study highlights the influence of structural parameters on the photochromic properties of viologen-based coordination polymers, offering insights into the design of photo-responsive materials.

Metal Ion Modulated Electron Transfer in Metalloviologen Compounds: Photochromism and Differentiable Amine Detection.

Different types of electron transfers (ETs) underlie the versatile use of various solid viologen-derived compounds, which is still insufficiently understood and difficult to control. Here, we demonstrate an effective strategy for modulating the key ET process in crystalline metalloviologen compounds (MVCs). By adjusting the coordinated transition metal ions bearing different electronic structures (e.g., d5, d7, d10), three MVCs (i.e., Mn-1, Co-2, and Cd-3) with highly consistent coordination environments have been synthesized successfully. Surprisingly, whether the photochromism (energy-induced ET mechanism) or the specific analyte recognition (molecule-induced ET mechanism), compound Cd-3 exhibits obvious photochromic behavior and differential dimethylamine detection. Combined detailed structural analysis with theoretical calculations, such unique ion-dependent properties, were correlated to the fine modulation of the electron density of the bipyridinium cores by metal ions. Additionally, thanks to the delicate recognition of dimethylamine vapor, a convenient test strip Cd-3-PAN was prepared as a sensitive biogenic amine sensor for evaluating the real-time freshness of seafood.

Efficient Reversible Upconversion Luminescence Modulation based on Photochromism of Lanthanides‐Doped BaMgSiO4 Glass Ceramics Toward Optical Storage Application

AbstractTransparent glass with photochromic and luminescent properties has recently garnered considerable interest in optical storage, while the photochromic transparent glass system is still limited. Herein, it is proposed that combining transparent glass matrix with photochromic nanocrystals is an efficient strategy to develop a broader range of photochromic luminescent glass systems. The Yb3+/Tb3+ co‐doped BaMgSiO4 glass ceramics are successfully prepared by a two‐step melt‐quenching approach followed by a thermal treatment in a reducing atmosphere. The glass ceramics exhibit high transparency and distinct coloration, showing a color change between colorless and pink by alternating irradiation between 365 nm ultraviolet light and 473 nm laser. The formation of color centers in the BaMgSiO4 nanoparticles drives the coloration of such glass ceramics. Based on their photochromic and photobleaching behavior, the efficient reversible upconversion luminescence modulation can be implemented with the maximum luminescence modulation rate of 79.83%. The excellent reproducibility and anti‐fatigue of such upconversion luminescence modulation demonstrate the promising application of Yb3+/Tb3+ co‐doped BaMgSiO4 glass ceramics as an optical storage medium.

Multi Stimuli Responsive Dual Aggregation-Induced Emission and Photochromic Behavior of a Tetraphenyl Substituted Triphenylamine Derivative and its Application as Anti-counterfeiting Agent.

A multi-stimuli responsive tetraphenyl substituted tripehnylamine-based aggregation induced emissive (AIE) material coupled with spiropyran was prepared. Owing to the presence of AIE and photochromic moiety, the molecule exhibits emissive aggregates, photochromism, and acidochromism. The multiple stimuli sensitive behavior of the molecule was explored for anti-counterfeiting behavior on TLC plate and commercial banknotes. The fluorogenic and photogenic response under UV and visible light established the potential of the candidate as a new generation encryption material.

Structures and photochromic behavior of viologen-based compounds: Insights from Mn(II) and Cd(II) complexes

This study investigates the crystal structures and photochromic properties of two viologen-based compounds [Mn(CMbpy)(BTEC)0.5(H2O)4]·0.5(H2BTEC)·3H2O (1) and [Cd(CMbpy)(1,2,4-BTC)(H2O)]n (2) [CMbpyBr = 1-(1-cyclohexylmethane)-4,4′-bipyridinium bromide, H4BTEC=1,2,4,5-benzenetetracarboxylic acid, 1,2,4-H3BTC=1,2,4-benzenetricarboxylic acid]. X-ray diffraction analysis identified compound 1 as a triclinic ionic lattice featuring Mn(II) in an octahedral coordination with CMbpy cations, whereas compound 2 crystallizes in an orthorhombic space group forming a three-dimensional MOF stabilized by CMbpy cations. Compound 1 demonstrated no color change, while compound 2 exhibited a rapid transition to a blue state upon UV irradiation, with an accompanying emergence of new absorption peaks, indicative of viologen radical formation. This behavior was corroborated by electron paramagnetic resonance (EPR) spectroscopy. Our findings provide a profound understanding of the structure–function relationship in these viologen complexes, highlighting the synergy between metal coordination, hydrogen bonding, and the strategic positioning of donor–acceptor components.

Syntheses, characterizations and photochromic properties of 1D→2D polycatenated cationic coordination polymers based on the flexible tripodal zwitterionic ligand

Two new coordination compounds, namely, {[ZnBr(µ3-tazna)]NO3·4H2O}n (1) and {[CdBr(µ3-tazna)]ClO4·2H2O}n (2) were solvothermally synthesized from the 1,1′,1′′-(((1,3,5-triazin-2,4,6-triyl)tris(benzene-4,1-diyl))tris(methylene))tris(3-carboxypyridin-1-ium) bromide (H3taznaBr3) ligand and characterized by IR spectroscopy, elemental analysis, X-ray single-crystal diffraction, X-ray powder diffraction (PXRD), and thermal analyses techniques. Single-crystal X-ray diffraction analysis reveals that compounds 1 and 2 crystallize in the triclinic system, P-1 space group. In 1 and 2, Zn(II) and Cd(II) centers are bridged by tazna ligands, displaying a rare 1D → 2D structure formed by the catenation of a 1D double chain. The photochromic behavior of compound 2 was investigated under 365 nm UV light, exhibiting a color change from white to yellow. Furthermore, the thermal and optical absorption properties of compounds 1 and 2 were investigated.

Reversible Photochromism of 4,4'-Disubstituted 2,2'-Bipyridine in the Presence of SO3.

We report an unusual photochromic behavior of 4,4'-disubstituted-2,2'-bipyridine. It was found that in the presence of a SO3 source and HCl, 2,2'-bipyridine-4,4'-dibutyl ester undergoes a color change from yellow to magenta in solution with maximum absorbance at 545 nm upon irradiation with 395 nm light. The photochromism is thermally reversible in solution. Different from the known bipyridine-based photoswitching pathways, the photo response does not involve any metal which form colored complexes or the formation of colored free radical cations like the photo-reduction of viologens. A combination of experimental and computational analysis was used to probe the mechanism. The results suggest the colored species to be a complex formed between N-oxide of the 2,2'-bipyridine-4,4'-dibutyl ester and SO2; the N-oxide and SO2 are formed from photoactivated oxidation of the bipyridine with SO3 serving as the oxygen source. This complex represents a new addition to the library of photoswitches that is easy to synthesize, reversible in solution, and of high fatigue resistance, making it a promising candidate for applications in photo-switchable materials and SO3 detection. We also demonstrated experimentally similar photochromic behaviors with 2,2'-bipyridine-containing polymers.

A different photochromic mechanism of spirooxadiazine: Electronic structure calculations and nonadiabatic dynamics simulations

Previously, we conducted electronic structure calculations and surface hopping dynamics to investigate the photochromic mechanism of spirooxazine (SO). Herein, a very similar calculation strategy was employed to simulate the excited-state ring-opening, relaxation, and subsequent isomerization of spirooxadiazine (SOD). Interestingly, we discovered a significantly different photochromic behavior compared to SO. Firstly, the bright state of SOD is the S2(ππ*) state, while the S1 state is a weakly absorbing state with a nπ* character. This is in stark contrast to SO, of which the S1 state is a ππ* state. Secondly, we identified a new excited-state decay channel along the C2N3 dissociation within the benzoxadiazine ring, in addition to the closed-loop and C2O7 cleavage relaxation pathways. Thirdly, the out-of-plane hydrogen (HOOP) oscillations were no longer operative in the relaxation processes, as the CH group was replaced by the N atom in SOD. Finally, several nonadiabatic trajectories will lead to the ‘N2 release’ product in the ground state, which might result in deactivation of SOD and affect the light fatigue resistance.

Transparent insulating photochromic PU/PTA films for Wide-Spectrum modulated smart windows

Photochromic materials have recently attracted widespread attention in the research of smart windows because they can function without additional devices. In this study, an innovative photochromic thermal insulating hybrid film based on a polyurethane matrix was prepared via a one-pot method, incorporating phosphotungstic acid. The film demonstrates excellent transparency, good mechanical strength, and a significant modulation of the solar spectrum, transitioning from colorless to dark blue under sunlight without the need for additional electron donors. This photochromic behavior is reversible and reproducible without degradation, resulting in a transmittance variation of up to 71 % at 550 nm, which is the most sensitive to the human eye, and more than 50 % modulating performance in 1800 ∼ 2500 nm. In addition, thermal insulation experiments proved that the smart window device prepared by the film could achieve a reduction of 9.3 °C under daylight. Overall, the photochromic films prepared using the one-pot method show good performance involving mechanical strength, efficient photochromic response, broad spectral range modulation, high coloring efficiency and cycling stability, and have promising applications in energy-saving buildings, anti-counterfeiting and smart displays.

Photocycloaddition of Zero-Dimensional Metal Halide Hybrids with Reversible Photochromism.

In this work, two zero-dimensional (0D) metal halide hybrids L2ZnBr4 [1, L = (E)-4-(2-(1H-pyrrol-3-yl)vinyl)-1-methylpyridin-1-ium] and L6Pb3Br12 (2) were prepared, which demonstrated photochromism and photoinduced cracking. Upon irradiation at 450 nm, a single crystal-to-single crystal transformation occurred as a result of the [2 + 2] photocycloaddition of L. Interestingly, compared to the complete photocycloaddition of L in 1, only two-thirds of L monomers could be photodimerized in 2 because of the difference in L orientation. 1 shows reversible photochromic behavior including rapid response time, few cracks, high conversion rate, and good reaction reversibility, while 2 exhibits no significant color change but distinct photoinduced cracking because of the large local lattice strain induced by inhomogeneous and anisotropic deformation. Moreover, the photocycloaddition of L results in the distinct shift of photoluminescence of 1 and 2, attributed to the variation in conjugation of π electrons and distortion of metal halide clusters. As a proof-of-concept, reversible optical writing is demonstrated for 1. These findings provide new insights into the design of stimuli-responsive multifunctional materials.

Cucurbit[7]Uril‐Based Self‐Assembled Supramolecular Complex with Reversible Multistimuli‐Responsive Chromic Behavior and Controllable Fluorescence

AbstractMultistimuli‐responsive color‐changing materials have potential applications in areas such as chemical sensing and anti‐counterfeiting. In this work, a fluoro‐phenyl‐acetyl‐substituted viologen derivative (FPAV·Cl2) is synthesized, which can form a supramolecular inclusion complex with cucurbit[7]uril (Q[7]) via self‐assembly. Significantly, the resulting inclusion complex FPAV2+@Q[7] in the solid‐state exhibits visual color changes in response to multiple external stimuli including light, heat, and vapors of ammonia and organic amines. The electron spin resonance (ESR) and ultraviolet–visible (UV–vis) spectra analysis reveals that the photochromic, thermochromic, and vapochromic behavior of FPAV2+@Q[7] is due to the generation of FPAV+• free radicals resulting from electron transfer. Furthermore, in the solid‐state, FPAV2+@Q[7] shows controllable fluorescence regulated by its chromic process. Based on the multi stimuli‐responsive color‐changes and fluorescence properties of FPAV2+@Q[7], its practical applications in erasable inkless printing, multiple anti‐counterfeiting processes, visible ammonia/organic amines detection, and information encryption are demonstrated. The supramolecular strategy of encapsulating viologen derivatives into macrocycles provides a general method for fabricating multi stimuli‐responsive chromic materials with multiple practical applications.

Long-lasting, fast-switchable photochromism in Na0.5Bi0.5TiO3 induced by photocatalytic memory effect, and its subsequently enhanced piezoelectric response and catalytic performances

A ferroelectric ceramic material of Na0.5Bi0.5TiO3 was found to possess a pronounced, long-lasting, and fast-switchable photochromic behavior responsive to visible light. The working mechanism of its photochromism could be attributed to an interesting photocatalytic memory effect, during which photogenerated electrons were trapped by Bi3+/Ti4+ and oxygen vacancies generated in Na0.5Bi0.5TiO3. The release kinetics of trapped electrons could be modulated by controlling environment conditions to make its photochromism phenomenon both long-lasting and fast-switchable to meet various application requirements. In the meantime, the photochromism process changed its inner electronic structure, resulting in stronger oxidation capability, increased piezoelectric response, improved charge carrier separation and transfer, and more active oxygen species production. Thus, its photocatalytic, piezocatalytic and photopiezocatalytic performances were all significantly enhanced as demonstrated by its TC-HCl degradation performances. This work offered a novel approach to search for and design inorganic photochromic materials with good reversibility for various technical applications.