Photoinduced Color Research Articles

Metal Coordination Sphere Deformation Induced Highly Stokes-Shifted, Ultra Broadband Emission in 2D Hybrid Lead-Bromide Perovskites and Investigation of Its Origin.

Published studies of layered (2D) (100)-oriented hybrid lead-bromide perovskites evidence a correlation between increased inter-octahedral (Pb-Br-Pb) distortions and the appearance of broadband white light emission. However, the impact of distortions within their constituent [PbBr6 ]4- octahedra has yet to be assessed. Herein, we report two new (100)-oriented 2D Pb-Br perovskites, whose structures display unusually high intra-octahedral distortions, whilst retaining minimal inter-octahedral distortions. Using a combination of temperature-dependent, power-dependent and time-resolved photoluminescence spectroscopic measurements, we show that increased intra-octahedral distortion induces exciton localization processes and leads to formation of multiple photoinduced emissive colour centres. Ultimately, this leads to highly Stokes-shifted, ultrabroad white light emission at room temperature.

Metal Coordination Sphere Deformation Induced Highly Stokes‐Shifted, Ultra Broadband Emission in 2D Hybrid Lead‐Bromide Perovskites and Investigation of Its Origin

AbstractPublished studies of layered (2D) (100)‐oriented hybrid lead‐bromide perovskites evidence a correlation between increased inter‐octahedral (Pb‐Br‐Pb) distortions and the appearance of broadband white light emission. However, the impact of distortions within their constituent [PbBr6]4− octahedra has yet to be assessed. Herein, we report two new (100)‐oriented 2D Pb‐Br perovskites, whose structures display unusually high intra‐octahedral distortions, whilst retaining minimal inter‐octahedral distortions. Using a combination of temperature‐dependent, power‐dependent and time‐resolved photoluminescence spectroscopic measurements, we show that increased intra‐octahedral distortion induces exciton localization processes and leads to formation of multiple photoinduced emissive colour centres. Ultimately, this leads to highly Stokes‐shifted, ultrabroad white light emission at room temperature.

A dual-emitting inorganic-organic hybrid material with emission intensity enchanced via electron-transfer photochromism

A new photochromic inorganic-organic hybrid material [(N-MeIQL)BiCl4] (1, N-MeIQL+ = N-methylisoquinolinium) has been solvothermally synthesized. In 1 the N-MeIQL+ cation is obtained by in situ alkylation of isoquinoline in the acidized methanol solution, and it is firstly reported that N-MeIQL+ acts as an organic template for the design and synthesis of photochromic material. Compound 1 exhibits reversible photochromism between colorless and bright yellow. Moreover, compound 1 displays dual-emitting luminescence and the lifetimes of the two emissions at 410 nm and 640 nm were measured to be 0.71 ns and 0.28 ms, respectively, indicating the fluorescence and phosphorescence characteristics. It is notable that unlike the luminescence quenching of normal photochromic materials in the process of photoinduced coloration, and the emission intensity of 1 can be significantly enhanced via electron-transfer photochromism. The studies of electron-transfer photochromism and photomodulated luminescence provide useful insight into the design of new photoresponsive materials.

Photochromic fluorescence switching in liquid crystalline polynorbornenes with α-cyanostilbene side-chains

Photoinduced fluorescence colour change based on Z/E isomerization and [2+2] dimerization in cyanostilbene liquid crystalline polymers.

Photochromic Barbiturate Pendant-Containing Benzo[b]phosphole Oxides with Co-Assembly Property and Photoinduced Morphological Changes.

A series of novel barbiturate pendant-containing benzo[b]phosphole oxides has been demonstrated to exhibit photochromic and co-assembly properties. Both the open form and the photogenerated closed form of the diarylethene-based benzo[b]phosphole oxides have displayed color changes with new appearance of the bathochromic-shifted lowest-energy absorption bands upon addition of the H-bonding ditopic receptor that is complementary to the barbiturate groups, probably resulting in the formation of a supramolecular polymer. The co-assembly mixtures have displayed dissociation and association processes upon heating and cooling, respectively. Moreover, both the unassembled and co-assembled states of the benzo[b]phosphole oxide have exhibited photoinduced coloration, leading to a near-infrared absorption behavior. Multiaddressable states of the co-assembly mixture with multiple colors have been achieved via the combination of photochromism and thermochromism. More interestingly, photoinduced morphological changes from extensive porous network structures to ring-like aggregates have been observed for the co-assembly upon photoirradiation. The present work provides important insight for further developments of the multiaddressable systems and the supramolecular polymers with photo- and thermo-responsive behaviors, paving the way for the future design of photochromic materials with interesting photocontrollable functions.

Photochromic Lanthanide(III) Materials with Ion Sensing Based on Pyridinium Tetrazolate Zwitterion.

Lanthanide(III) ion (Ln(III)) sensing has become a major research area owing to its intriguing prospect in clinic, biology, and environmental studies. However, the existing methods have limitations like requirement of expensive instrumentation, long analytical times, and sample pretreatments, revealing the necessity of other methods. In this work, by using N-methyl-4-pyridinium tetrazolate (mptz) zwitterion as an electron acceptor, we obtained several new Ln(III) compounds with electron-transfer (ET) photochromic properties: [Ln(NO3)3(H2O)4]·mptz [Ln = Sm (1), Eu (2), Gd (3), La (4), Ce (5), Pr (6), Nd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14)]. Notably, different Ln(III) ions in these compounds can be visually identified by their different photoinduced color changes related to the ET process. This work may not only contribute to the more understanding of the structure–photosensitivity relationships of pyridinium-based compounds, but also provide a new approach for Ln(III) ion sensing.

Mechano- and Photoresponsive Behavior of a Bis(cyanostyryl)benzene Fluorophore.

The mechanoresponsive behavior and photochemical response of a new bis(cyanostyryl)benzene fluorophore (CSB-5) were investigated. Green fluorescence with λem,max of 507 nm was found for CSB-5 in chloroform solution, mirroring the behavior of a previously reported similar dye (CSB-6). Alternatively, crystalline samples of CSB-5 exhibited orange fluorescence with λem,max of 620 nm, attributable to excimer emission. Although the emission color change was not clearly noticeable by naked eye, CSB-5 exhibited mechanochromic luminescence, due to transformation into the amorphous state upon grinding the crystalline powder. Interestingly, rubbed films of CSB-5 prepared on glass substrates exhibited a pronounced emission color change from orange to green when exposed to UV light. This response is the result of a photochemical reaction that occurs in the amorphous state and which causes a decrease of the excimer emission sites so that the emission color changes from excimer to monomer. The crystalline material did not display such a photoinduced emission color change and the difference in photochemical reactivity between crystalline and amorphous states was exploited to pattern the emission color of rubbed films.

Photochromic Systems with Dynamic Adaptive Photobleaching

Results of own investigations on photochromic systems bleaching reversibly under sun light depending on the intensity of excitant radiation are described. The results are aimed to be used for the development of camouflage coatings and civil cloths with photoinduced dynamic automatic color change.

Fast photo-induced color changes of Ag particles deposited on single-crystalline TiO2 surface

It is well known that surface-plasmon enhanced photo-electrochemical effect or photo-thermal effect of metallic particles on a semiconductor substrate or in a suspension may result in color changes. Such character could be potentially applicable to colorimetric sensors, optical filters, and data storage devices. However, usually the response time for color changes is too long to be practically applied. In this letter, we found that the response rate of color changes could be controlled by the annealing condition of the semiconductor substrate, and changes larger than 10% in spectra were observed after only 1-min exposure to light. Furthermore, such fast response was applied to realize wavelength-dependent “write” and “read” applications with high spatial resolution.

Significant enhancement of conductance of a hybrid layered molybdate semiconductor by light or heat.

Introduction of the viologen cation, a well-known electron acceptor, into molybdate layers led to a dramatic change in the conductance of the 2D hybrid material in the process of photo-induced or thermo-induced coloration, which provides an alternative approach to manipulate the electronic properties of 2D semiconductors via external optical/thermal stimuli.

UV-induced formation of color centers in dispersed TiO2 particles: Effect of thermal treatment, metal (Al) doping, and adsorption of molecules

Titanium dioxide is one of the most popular photoactive solids used as a photocatalyst, and as a self-cleaning, bactericidal, superhydrophilic (hydrophobic) and photochromic material when exposed to UV light. The primary steps of titania’s response to UV light are responsible for its photoactivity that involve generation of photocarriers subsequent to photoexcitation in its intrinsic (band-to-band transitions) and extrinsic absorption regions (defects-to-conduction band transitions). Photoexcitation into extrinsic absorption bands related mainly to extrinsic defects introduced deliberately through doping and to pre-existing intrinsic defects is an important step in the creation of visible-light-active (VLA) photocatalysts and other VLA titania-based materials. Despite the useful effects of doping, doping can also be detrimental because it can enhance the loss of otherwise active photocarriers through recombination with increasing dopant concentration. This article reports on AlxTi(1-x)O2 materials at different levels of Al doping that preliminary testing revealed significant photoinduced coloration. Materials were characterized by X-ray diffraction, Raman spectroscopy, XPS spectroscopy, SEM/EDX spectroscopic microanalyses, diffuse reflectance spectroscopy, and by the BET method for specific surface area determination. The principal objectives were to examine the effect(s) of thermal treatment of pristine titania (up to 1000°C) and Al-doped titania that led to the formation of intrinsic and extrinsic (Al in Ti positions) defects in the subsurface region and in the bulk lattice, together with the effect(s) that adsorbed and photoadsorbed oxygen and hydrogen molecules impart on the UV-induced coloration of dispersed titania particles. Results confirm that both thermal treatment and Al doping of titania caused noticeable UV coloration of the titania particles. The ultimate level of coloration of TiO2 under illumination in air was an order of magnitude greater than in vacuum. Results also demonstrate that UV-induced photocoloration of titania could delineate between the subsurface and bulk areas in titania particles, and could assess the depth of the subsurface region from the depth of the space charge region. The relevance of using the photocoloration of titania and other metal oxides as a simple tool to determine whether or not a so-called photocatalyzed reaction is truly catalytic has also been inferred.

A luminescent zinc coordination complex cocrystallized with benzenetetracarboxylic acid showing photoinduced coloration from grey to orange

Tailoring photoinduced color of a donor-acceptor molecular system is achieved by introducing the macrocycle electron-deficient ligand into the structure of a zinc phosphate binuclear unit. Fluorescence quenching occurs accompanying with the light irradiation process due to energy transfer between the photoactive components.

Synthesis, Crystal Structure, Magnetic Property and Photo-Induced Coloration of a One-Dimensional Chain Complex

A novel photoactive complex was constructed from two non-photoactive ligands and cobalt (II) ions. Upon ultra violet (UV) irradiation (365 nm), the color of complex 1 changes from orange to violet. The ESR spectrum indicates that the photoactive phenomenon of complex 1 originates from an intermolecular energy transfer between the H5DDCPBA ligand and phenanthroline ligand. This photoactive complex shows high thermal stability according to the investigation of thermogravimetric analyses. In addition, the temperature dependence of magnetic susceptibilities for the orange complex 1 was also investigated systematically.

Thiophene-S,S-dioxidized diarylethenes for light-starting irreversible thermosensors that can detect a rise in heat at low temperature

Thiophene-S,S-dioxidized diarylethenes having a trialkylsilyl group at the reactive positions underwent photoinduced coloration and thermal bleaching even at −40 to −30 °C.

Photoinduced solid-state coloring behavior of boronium complexes

Boronium complexes bearing a 9-borabicyclononane framework with a bipyridine-type ligand display photoinduced solid-state coloring behavior. While the identity of the substituents on the boron atom is critical to gain photoresponsive capability, modifying the nitrogen-containing ligand structure and its substituents provides a wide variation in the photoinduced solid color.

Spectra of thermoprogrammed annealing of photoinduced color centers

The kinetics of photoinduced formation and thermoprogrammed annealing of color centers in photochromic rutile ceramics has been studied in situ with the aid of a specially designed attachment for a spectrofluorimeter. Using a regime of constant heating rate, the spectra of color center annealing have been measured and the energy depths of hole traps responsible for the annealing of these centers have been determined.

Photochromism of Acetyl‐Cyclophanochromene: Intriguing Stabilization of Photogenerated Colored o‐Quinonoid Intermediates

AbstractThe photochromism of a chromene annulated on [2.2]paracyclophane, Ac‐CPC, was investigated. Whereas the parent chromene 2,2‐diphenylbenzopyran shows photoinduced coloration only at very low temperatures, acetyl‐cyclophanochromene Ac‐CPC was found to exhibit room‐temperature photochromism. With no extended conjugation due to absence of any substituents in two cofacially‐oriented aryl rings, the through‐space delocalization of π‐electrons, i.e., the phane effect, manifests nicely leading to the stabilization of the photogenerated colored o‐quinonoid intermediates. The interaction between cofacial aryl rings is supported by the distance between them, as revealed by X‐ray crystal structure analysis. Additionally, electrophile···nucleophile interactions are surmised to be important for intriguingly longer persistence of the photogenerated intermediates of Ac‐CPC relative to the lifetime of the photogenerated intermediate generated from the parent cyclophanochromene, which is devoid of an electron‐withdrawing acetyl group.

Photosensitivity in Ce/Sm-decorated Keggin polyoxomolybdates: an interesting photoinduced coloration phenomenon of mixed-metal clusters involving energy transfer from rare-earth fluorescence

Ce/Sm ions are covalently grafted to a Keggin polyoxomolybdate cluster [PMo12O40]3− to yield interesting fluorescent-photodarkening molecular systems. Their photoresponce processes are observed to be activated via energy transfer from luminescent fragments to photocoloration components.

Photoinduced color change and photomechanical effect of naphthalene diimides bearing alkylamine moieties in the solid state.

Photoinduced color change of naphthalene diimides (NDIs) bearing alkylamine moieties has been observed in the solid state. The color change is attributed to the generation of a NDI radical-anion species, which may be formed through a photoinduced electron-transfer process from the alkylamine moiety to the NDI. The photosensitivity of NDIs is highly dependent on the structures of the alkylamine moieties. Crystallographic analysis, kinetic analysis, UV/Vis/NIR spectroscopic measurements, and analysis of the photoproduct suggested that a radical anion was formed through an irreversible process initiated by proton abstraction between an amine radical cation and the neutral amine moiety. The radical anions formed stacks including mixed-valence stacks and radical-anion stacks, as shown by the broad absorption bands in near-IR spectra. These photosensitive NDIs also showed crystal bending upon photoirradiation, which may be associated with a change in the intermolecular distance of the NDI stacks by the formation of monomeric radical anions, mixed-valence stacks, and radical-anion stacks.

Photo- and Thermochromic Spirans. 39. *Study of the Complexation of a New Crown-containing Benzoxazine Spiropyran with Metal Cations

A new benzoxazinone spiropyran containing a crown ether fragment have been synthesized. This compound was studied by both experimental and theoretical methods. In contrast to previously studied analogs, photoinduced coloration is not observed. By irradiating this crown-containing spiropyran, only an E-Z isomerization reaction proceeds. Complexation of the closed spiropyran form runs in a solution containing a calcium salt. A hypsochromic shift of the long-wavelength maximum in the absorption spectrum is observed.