Aza-diarylethenes have emerged as a unique class of photochromic molecules exhibiting reaction pathways that are inaccessible to conventional diarylethenes. In particular, certain azadiarylethenes generate zwitterionic closed-ring photoproducts, giving rise to unusual photochromic behavior. However, the elementary processes responsible for the formation of zwitterionic photoproducts remain unclear, hindering mechanistic understanding of this unusual class of photochromic reactions. Here, we investigate the photochromic reaction mechanism of a benzofuran-fused aza-diarylethene (BF-ADE) using steady-state spectroscopy, thermal kinetic analysis, single-crystal X-ray diffraction, visible-to-near-infrared transient absorption spectroscopy, time-resolved infrared spectroscopy, oxygen-quenching experiments, and density functional theory (DFT) calculations. Spectroscopic, computational, and crystallographic analyses consistently demonstrate that photoirradiation of BF-ADE ultimately generates a zwitterionic closed-ring isomer, BF-ADE(c-Z), while thermal bleaching proceeds through the neutral closedring isomer, BF-ADE(c). Femtosecond transient absorption spectroscopy revealed ultrafast excited-state dynamics with a characteristic time constant of 1.6 ps. More importantly, timeresolved infrared spectroscopy provided direct spectroscopic evidence for the formation of the conventional closed-ring intermediate BF-ADE(c) within the first few picoseconds after photoexcitation. Oxygen-quenching experiments combined with DFT calculations further demonstrated that BF-ADE(c-Z) is generated predominantly from BF-ADE(c) on the ground-state potential energy surface rather than through a triplet-mediated pathway. These findings provide a unified mechanistic framework for zwitterionic photochromism in aza-diarylethenes and reveal the pivotal role of a conventional closed-ring intermediate in generating zwitterionic photoproducts.

Mechanochromic luminescence (MCL), defined as a fluorescence color change induced by mechanical stimulation, has recently attracted considerable attention due to its sensitivity to molecular packing and intermolecular interactions. In this study, we synthesized three regioisomers of dibenzoylmethanatoboron difluoride (BF2DBM) with a methoxy substituent at the ortho, meta, or para position of one phenyl ring ( o -, m -, and p -a2bBF 2 ), while the other ring was substituted with di-tert-butyl groups. In addition, two crystal polymorphs of p -a2bBF 2 (Form I and Form II) were investigated. Clear MCL behavior was observed for o -a2bBF 2 and for Form I of p -a2bBF 2 . In contrast, m -a2bBF 2 exhibited only a minor spectral change without an apparent fluorescent color change, and Form II showed virtually no color change. Spectroscopic and crystallographic analyses revealed that these contrasting responses originate from differences in molecular packing. In Form I and o -a2bBF 2 , smearing induces a transition from monomer-like to excimer-like emission, whereas Form II and m -a2bBF 2 retain excimer-like emission sites that are largely insensitive to mechanical perturbation. These findings demonstrate that both substitution position and crystal polymorphism critically govern MCL activity by modulating the balance between weak CH-pi and strong pi-pi intermolecular interactions.

Photochromic molecules capable of reversible structural transformation upon light irradiation have attracted considerable attention for applications in molecular switches, optical memories, and sensing systems. In such applications, controlling the color of the photogenerated isomer is a key challenge. Here we report the systematic control of photochromic colors in aza-diarylethenes through molecular design of nitrogen-containing aryl units. A series of aza-diarylethenes incorporating pyrimidine-based aryl groups was synthesized, and their photochromic properties were investigated in solution. Upon UV irradiation, the compounds undergo reversible electrocyclic reactions to generate closed-ring isomers exhibiting distinct visible absorption bands. TD-DFT calculations revealed that the visible absorption originates predominantly from the HOMO→LUMO and HOMO→LUMO+1 transitions. Extension of π-conjugation through phenyl-substituted pyrimidine units stabilizes the LUMO levels, resulting in a redshift of the absorption bands and enabling systematic color tuning. Furthermore, the perceived colors predicted from the calculated absorption spectra capture the overall trends of the experimentally observed colors. These results demonstrate that the combination of rational molecular design and computational prediction provides an effective strategy for controlling photochromic colors in aza-diarylethenes. The present work establishes design guidelines for color-tunable photochromic molecules and provides a framework for predictive color design in molecular photoswitches.

Herein, we report the synthesis of a novel diarylethene derivative, which has benzo[b]selenophene groups as the aryl units, and its photochromic properties. The derivative exhibited a unique turn-on mode fluorescence photoswitching along with photochromic reactions in solution as well as in solid powder states.

Aza-diarylethene has been developed as a new family of photochromic compounds. This study explores the photochromic properties and thermal back reactivities of various aza-diarylethene regioisomers (N1–N4 and I1–I4) in n-hexane. These molecules exhibit fast thermally reversible photochromic reactions driven by 6π aza-electrocyclization. Kinetic analysis of the thermal back reaction revealed activation parameters, highlighting how the substitution position of the aryl group affects the thermal stability. Additionally, density functional theory calculations identified M06 and MPW1PW91 as the most accurate functionals for predicting the thermal back reactivity, closely matching the experimental data. These findings offer valuable insights for the design of advanced photochromic materials with tailored thermal and photophysical characteristics.

Dibenzoylmethanatoboron difluoride complexes (BF2DBM) exhibit aggregation-induced emission (AIE) properties by introducing a methyl group at the alpha-position of the dioxaboroline ring. In this study, we synthesized alpha-substituted BF2DBM derivatives with alkyl groups and evaluated their photophysical and AIE properties as a function of water fraction (fw) in water/acetone mixtures. The aggregation behavior of the BF2DBMs differed depending on fw, although they exhibited AIE independently of the alkyl groups. The fw dependence suggested that the alkyl groups exhibit hierarchical aggregation behavior, indicating that the alkyl groups affect the intermolecular stacking pattern based on the X-ray crystallography, leading to the difference in the fluorescence properties of the crystals.

Diarylperfluorocyclopentenes bearing thiophene rings as aryl groups are thermally stable and exhibit photochemically reversible photochromism (P-type photochromic behavior). However, we found that the addition of acid such as trifluoroacetic acid (TFA) converts it into T-type photochromism that exhibits a thermal ring-opening reaction at room temperature. Remarkably, the rate constant for the thermal ring-opening reaction increases by a factor of 105 to 107 compared to the reaction in the absence of acid, indicating an extraordinary acceleration effect. The rate constant rises exponentially with increasing amounts of TFA. Solvent effects were also pronounced; among n-hexane, toluene, and dichloromethane, the reaction proceeded fastest in dichloromethane. The absorption spectrum of the intermediate in dichloromethane appeared at a longer wavelength than that of the closed-ring isomer. Time-dependent density functional theory (TD-DFT) calculations suggested the formation of an intermediate cation, based on the predicted absorption spectrum. Herein, we propose a mechanism for the acid-assisted thermal ring-opening reaction.

Photoreactivity in crystal is one of the essential properties for creating photo-functional crystalline materials. This study explores the impact of the dihedral angle in aryl groups on the photocyclization reactivity...

Aryl twisting in diarylbenzene photoswitches was found to control their thermal back reactivity. Modulation of the dihedral angles altered π-conjugation in the transition state, thereby tuning the activation free energy and thermal half-life.

A patterned growth of crystals of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (1a) on the glass substrate with convex guides is reported by sublimation methods. The lower supersaturation of substrate surfaces with higher temperatures can facilitate the vapor-to-liquid process rather than the vapor-to-crystal process in the early stage of the sublimation. Micro-droplets of melts of 1a are generated on the sidewalls of the convex guides, then crystallized into the microcrystals, accompanied by the rearrangements of the crystallographic in-plane orientations. Moreover, the crystalline patterns fringed with the rod crystals are colored red upon irradiation with ultraviolet light. This well-controllability of crystal morphologies in a simple use of sublimation methods will pave the way for large-sized photomechanical materials with the desired morphologies.

Molecular photoswitches have attracted significant attention due to their potential applications in materials science and life science. In this study, we synthesize a series of aza‐diarylethenes incorporating benzazole groups as the nitrogen atom source for 6π azaelectrocyclization and investigate their photochromic behaviors. Compounds 1–3, bearing benzothiazole, benzoxazole, and benzimidazole groups, exhibit thermally reversible type photochromism with thermal back reaction half‐lives (t1/2) ranging from milliseconds to seconds at 293 K. Additional derivatives 4–6 with benzothiophene‐S,S‐dioxide at the ethene bridge moiety, further expand the range of thermal back reactivity, with t1/2 values extending up to tens of minutes. Quantum chemical calculations confirm that the variation in thermal back reactivity is governed by the energy difference between open‐ and closed‐ring isomers, in accordance with the Bell−Evans−Polanyi principle. These findings provide a molecular design strategy for controlling the thermal back reactivity of aza‐diarylethenes, enabling further development of functional photochromic materials.

Both the photochemical kinetics and the spatial reaction dynamics in single crystals could be rationalized in terms of the difference in the cooperativity of the reactions that relates the magnitude of the conformational change required for reaction.

Exploring novel molecular photoswitches plays a crucial role in the field of photo-functional materials chemistry. In this study, we synthesized aza-diarylethenes with benzothiophene-S,S-dioxide as a part of the hexatriene structure and investigated their photochromic properties. Unlike previously reported aza-diarylethenes, which exhibit fast thermally reversible photochromism, the compounds synthesized here exhibited pseudo-photochemically reversible photochromism. Due to their thermal stability, we successfully isolated the colored isomer. X-ray crystallographic analysis revealed for the first time that the colored isomer adopts a closed-ring structure with a bond between carbon and nitrogen atoms. Remarkably, these aza-diarylethenes exhibited not only photochemical ring-closing and ring-opening reactions but also thermal ring-closing and ring-opening reactions, driven by a thermal equilibrium between the open- and closed-ring isomers. This behavior, unprecedented for common diarylethenes, was elucidated through kinetic analysis, revealing an energy-level diagram for the thermal equilibrium between these isomers. Furthermore, 1H NMR spectroscopy revealed that both photochemically and thermally generated closed-ring isomers adopt the same molecular structure, which was well explained based on the reaction mechanism of photochemical and thermal ring-closing reactions. These findings not only advance the field of aza-diarylethenes but also inspire future research in the development of new photoswitches.

Organic crystals dominated by weak intermolecular interactions are formed in various molecular packings with different physical properties, especially fluorescence properties, also known as mechanochromic luminescence. In this study, evaporative crystallization was investigated using aggregation-dependent fluorescence properties. Dibenzoylmethanatoboron difluoride derivatives (BF2DBM) exhibited large Stokes shifts (orange emission) both in the intermediate liquid-like cluster state during solvent evaporation and in the amorphous state due to smearing. This behavior is most likely related to specific intermolecular interactions. Herein, we investigate the alpha-substitute dependence of BF2DBM with methoxy substituents at the dibenzoyl parts (2a) to clarify the intermolecular interaction. The large Stokes shift was observed only for 1,3-bis-(4-methoxyphenyl)-methanatoboron difluoride (2aBF (2) ). The methyl-substituted 2a at the alpha-position (2amBF (2) ) exhibited aggregation-induced fluorescence and weak emission in the intermediates during solvent evaporation without a large Stokes shift. The difluoroboron 2-benzoyl-1-tetralone derivative (2atBF (2) ) also showed a small Stokes shift compared to 2aBF (2 ). These findings suggest that the hydrogen atom at the alpha-position acts as an intermolecular interaction with another F atom by hydrogen bonding, leading to a larger Stokes shift originating from dimeric aggregation, such as J-aggregates. Apparently, both 2atBF( 2) and 2amBF (2) cannot form the J-aggregates mediated by the H center dot center dot center dot F hydrogen bonding because these two molecules do not consist of an H atom at the alpha-position. The strength of the intermolecular interactions affects the sequential and stepwise aggregation, as revealed by the phase- and aggregation-dependent fluorescence properties.

Understanding photoreaction dynamics in crystals is important for predicting the dynamic property changes accompanying these photoreactions. In this work, we investigate the photoreaction dynamics of p-phenylenediacrylic acid dimethyl ester (p-PDAMe) in single crystals that show reaction front propagation, in which the photoreaction proceeds heterogeneously from the edge to the center of the crystal. Moreover, we find that p-PDAMe single crystals exhibit a distinctive crystal shape change from a parallelogram to a distorted shape resembling a fluttering flag, then to a rectangle as the photoreaction proceeds. Density functional theory calculations predict the crystal structure after the photoreaction, providing a reasonable explanation of the distinctive crystal shape change that results from the spatially heterogeneous photoreaction. These results prove that the spatially heterogeneous photoreaction dynamics have the ability to induce novel crystal shape changes beyond what would be expected based on the equilibrium reactant and product crystal shapes.

Sublimation methods utilizing the surface properties of substrates can address the challenge of controlling hollow morphologies in rod crystals. Spherulites were formed on the hydrophilic surface of the (0001) planes of α-quartz and sapphire substrates by sublimation of 1,2-bis(3,5-dimethyl-2-thienyl)perfluorocyclopentene (1a). Various types of hollow morphologies, distinguished by the size and shape of their cross sections and by the presence or absence of branching structures, were formed separately on α-quartz and sapphire substrates. Such precise control of the hollow morphologies was attributed to the wettability of each substrate, leading to the formation of spherulites of 1a. In addition, it was indicated that the formation process of the surface morphologies of spherulites was associated with the hollow morphologies of rod crystals of 1a.

In this study, we investigated the solvent dependence on the thermal cycloreversion reaction of photochromic diarylbenzene derivatives. Interestingly, only in the case of a diarylbenzene derivative with methoxy groups, the thermal cycloreversion reaction was accelerated when protic solvents such as ethanol and methanol were used. Moreover, it was found that the thermal cycloreversion reaction was drastically accelerated by acid addition. Graphical Abstract The solvent dependence on the thermal cycloreversion reaction of photochromic diarylbenzene derivatives was investigated. It was found that protic solvents accelerate the thermal cycloreversion reaction of a diarylbenzene derivative with methoxy groups. Quantum chemical calculations revealed that the acceleration is due to a decrease in electron-donating effects of methoxy groups through the coordination of protic solvents.

Organic luminescent solid materials have attracted much attention due to practical applications such as sensor materials and optical waveguides. We have previously reported that inverse type diarylethenes exhibit strong emission in crystal without causing aggregation‐caused quenching. However, the emission color was limited to mainly green. To tune the emission color, in this work, we newly synthesized inverse type diarylethenes having a shortened π‐conjugation length or a polar substituent and investigated their fluorescence properties in solutions and crystals. The crystals exhibited various emission colors from blue, green, yellow to red depending on the molecular structure. The emission color changes of the crystals were induced by the intermolecular interactions such as CH‐π interactions in addition to the shortened π‐conjugation length and the intramolecular charge transfer character.

Diarylethene derivatives are one of the promising compounds for practical applications including optical memory, display, sensor, and photoactuator. In this work, we newly designed and synthesized an inverse type diarylethene derivative bearing sodium carbonate groups at the p-position of the lateral phenyl ring and investigated the photochromic behavior in the presence and absence of cyclodextrins (CDs) (alpha CD, beta CD, and gamma CD). Interestingly, only in the presence of beta CD, the photocyclization reactivity decreased. From the results of job-plots, NOESY spectra, and quantum chemical calculations, it was suggested that the distinctive interaction between the diarylethene and beta CD leads to the restriction of molecular geometrical change, resulting in the suppression of the photocyclization reactivity. These results provide information on the rational designing of inverse type diarylethenes with advanced properties. The photocyclization reactivity of an inverse type diarylethene derivative decreased by inclusion into beta-cyclodextrin. From the detailed investigation using job-plots, NOESY spectra and DFT calculations, it was elucidated that this is ascribed to the restriction of the rotational motion between thiophene and phenyl rings. These results would provide useful information for the photochromic reaction dynamics of inverse type diarylethenes. image

Gaining insight into the dynamics of electrocyclic reactions is very important from both fundamental and application perspectives. In this study, we developed novel diarylethene photoswitches that undergo 6π azaelectrocyclic reaction. We found that they exhibit fast thermally reversible type (T-type) photochromism, in contrast to the fact that common diarylethenes exhibit photochemically reversible type (P-type) photochromism. The quantum chemical calculations revealed that the fast T-type photochromism originates from the unprecedented disrotatory thermal cycloreversion of the closed-ring isomer. Our results provide useful information not only for the dynamics of the 6π azaelectrocyclic reaction but also for the further development of diarylethene photoswitches utilizing the 6π azaelectrocyclic reaction.