Benzo-annulated chromenes, i.e., naphthopyrans, are well-known photochromic molecules that undergo photochemical ring-opening reactions to form two colored open-ring isomers, the transoid-cis and transoid-trans forms, upon light irradiation. Though the transoid-cis form returns thermally to the uncolored closed form, the fading rate of the transoid-trans form is extremely slow because of its higher thermal stability. This slow fading behavior of the transoid-trans form is responsible for the persistence of residual color for several minutes to hours, and prevents the application of such molecules to fast photoswitching materials. We have found a new simple and versatile strategy to substantially reduce the amount of the undesirable long-lived colored transoid-trans form by introducing an alkoxy group at the 1-position of azino-fused chromenes, i.e., 8H-pyranoquinazolines. The alkoxy group effectively reduces the formation of the transoid-trans form due to C–H···O intramolecular hydrogen bonding in the transoid-cis form. Moreover, the introduction of a condensed aromatic ring at the 3-position was found to be effective to increase the photosensitivity of the ring-opening reaction. This strategy can also be applied for naphthopyran derivatives and is useful for the development of fast photoresponsive photochromic lenses and fast photoswitching applications such as dynamic holographic materials and molecular actuators.

Delocalized biradicals have been extensively studied because of fundamental interests to singlet biradicals and several potential applications such as to two-photon absorption materials. However, many of the biradical studies only focus on the static properties of the rigid molecular structures. It is expected that the biradical properties of the delocalized biradicals are sensitive to the subtle changes of the molecular structures and their local environments. Therefore, the studies of the dynamic properties of the system will give further insight into stable radical chemistry. In this study, we directly probe the ultrafast dynamics of the delocalized biradical of a photochromic radical dimer, pentaarylbiimidazole (PABI), by time-resolved visible and infrared spectroscopies and quantum chemical calculations with the extended multistate complete active space second-order perturbation theory (XMS-CASPT2). While the photogenerated transient species was considered to be a single species of the biradical, the present ultrafast spectroscopic study revealed the existence of two transient isomers differing in the contributions of biradical character. The origin of the two metastable isomers is most probably due to the substantial van der Waals interaction between the phenyl rings substituted at the imidazole rings. Unraveling the temporal evolution of the biradical contribution will stimulate to explore novel delocalized biradicals and to develop biradical-based photofunctional materials utilizing the dynamic properties.

•We developed fast switchable photochromic 3H–naphthopyrans with bulky substituents.•We investigated the thermal decoloration behavior in the PMMA-b-PBA copolymer.•The PMMA-b-PBA copolymer is attractive for fast switchable photochromic molecules.Fast photochromic molecules have attractive potential to apply to the ophthalmic lenses and real-time dynamic holography. However, the thermal decoloration reaction rate and the photoconversion efficiency of the photochromic molecules largely depend on their surrounding medium. Herein, we investigated the thermal decoloration behavior of fast photochromic 3H–naphthopyrans in poly(methyl methacrylate)-b-poly(n-butylacrylate) (PMMA-b-PBA) block copolymer. The rapid photoswitching properties of the fast photochromic molecules were achieved by using the block copolymer as a host polymer matrix.

Steric protection groups are usually necessary for stable radicals. However, here, we developed novel photochromic radical complexes which generate sterically unprotected imidazolyl and phenoxyl radicals upon UV light irradiation based on the phenoxyl–imidazolyl radical complex (PIC) framework. These photochromic compounds show excellent durability against repeated irradiation of intense nanosecond laser pulses even in polar protic solvents, such as ethanol.

Instantaneous coloration with large absorbance and quick color fading in the dark are desired properties for thermally reversible photochromic compounds. In the case of naphthopyran derivatives, which have been employed to commercial ophthalmic lenses, the quick color fading has been recently achieved by suppression of the generation of the transoid-trans (TT) form by steric hindrance of bulky substituents. However, there are still open questions whether the steric hindrance decreases the photochromic reaction efficiency, which is a crucial problem for industrial applications. Herein, we apply a wide range of electronic and vibrational time-resolved spectroscopies and reveal that the photochromic reaction yields of the naphthopyrans with bulky substituents are almost comparable (∼0.7) to that of nonsubstituted naphthopyran. The suppression of the formation of the TT form and the effect of solvent polarity on the photodynamics are systematically investigated. These findings are important for fundamental photochemistry and developing naphthopyran-based optimal photofunctional materials.

Stepwise two-photon excitations have been attracting much interest because of their much lower power thresholds compared with simultaneous two-photon processes and because some stepwise two-photon processes can be initiated by a weak incoherent excitation light source. Here we apply stepwise two-photon optical processes to the photochromic bridged imidazole dimer, whose solution instantly changes color upon UV irradiation and quickly reverts to the initial color thermally at room temperature. We synthesized a zinc tetraphenylporphyrin (ZnTPP)-substituted bridged imidazole dimer, and wide ranges of time-resolved spectroscopic studies revealed that a ZnTPP-linked bridged imidazole dimer shows efficient visible stepwise two-photon-induced photochromic reactions upon excitation at the porphyrin moiety. The fast photoswitching property combined with stepwise two-photon processes is important not only for the potential for novel photochromic materials that are sensitive to the incident light intensity but also for fundamental photochemistry using higher excited states.

Naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) is a novel fast switchable negative photochromic compound, which shows the thermal back reaction in the millisecond time scale. Upon UV light irradiation, Np-PIC shows the hypochromic effect in the UVA region due to there being less conjugation in the transient isomer. By replacing the phenoxyl unit with a naphthoxyl unit, the molecular structure has an asymmetric carbon, leading to fast chiroptical switching. This simple molecular design will be a good candidate for the future development of negative photochromic compounds.

Phenoxyl-imidazolyl radical complex (PIC) is a novel photochromic compound, which generates two nonequivalent radicals upon UV light irradiation. While the color fading speed of PIC can be tuned from tens of nanoseconds to seconds, the low visible sensitivity has limited their potential applications. Herein, we developed novel thiophene-substituted PIC derivatives. The visible sensitivity and the increased absorption coefficient were achieved with keeping the moderate color fading speed by introducing a phenyl group at the 5-position of the thiophene ring.

Open-shell biradicals have received considerable attention in material science because of their high two-photon absorption cross sections and broad and high absorptive features over the visible region. However, the instability of the biradical caused by the open-shell nature was one of the drawbacks; therefore, novel radical compounds which can suppress unwanted reactions by tuning the open-shell features are desired to expand the versatility of the radical compounds. Here, we report a novel radical-dissociation-type photochromic compound whose photochromic reaction involves a valence isomerization from the open-shell biradical to closed-shell quinoidal forms by using a phenoxyl-imidazolyl radical complex framework. The valence isomerization from the biradical to quinoid forms effectively tunes the open-shell feature in time and drastically changes the spectral features, which were revealed by time-resolved Fourier transform infrared spectroscopy. This novel fast photochromic property not only is important for fundamental spin chemistry but also expands the versatility of the radical compounds for novel advanced photofunctional materials.

Stepwise two-photon processes not only have great potential for efficient light harvesting but also can provide valuable insights into novel photochemical sciences. Here we have designed a [2.2]paracyclophane-bridged bis(imidazole dimer), a molecule that is composed of two photochromic units and absorbs two photons in a stepwise manner. The absorption of the first photon leads to the formation of a short-lived biradical species (half-life = 88 ms at 298 K), while the absorption of the additional photon by the biradical species triggers a subsequent photochromic reaction to afford a long-lived quinoid species. The short-lived biradical species and the long-lived quinoid species display significantly different absorption spectra and rates of the thermal back-reaction. The stepwise two-photon excitation process in this photochromic system can be initiated even by incoherent continuous-wave light irradiation, indicating that this two-photon reaction is highly efficient. Our molecule based on the bridged bis(imidazole dimer) unit should be a good candidate for multiphoton-gated optical materials.

We report a novel photochromic molecular system, phenoxyl-imidazolyl radical complex (PIC), in which both a phenoxyl radical site and an imidazolyl radical site are reversibly and simultaneously generated upon UV light irradiation. PIC consists of the three parts: an aromatic linker, a diarylimidazole moiety, and a 4H-cyclohexadienone ring. Upon UV light irradiation, the C–N bond between the 4H-cyclohexadienone ring and the imidazole ring in the colorless closed-ring isomer of PIC undergoes a homolytic cleavage, leading to the formation of the transient colored open-ring isomer. Based on the substituents on the imidazoyl/4H-cyclohexadienone rings and the nature of the aromatic linker, the half-life of the colored open-ring isomer can be varied between tens of nanoseconds and seconds. PIC derivatives containing a 1,2-phenylene linker exhibit high fatigue resistance toward repeated photochromic reactions. Analysis using laser flash photolysis reveals that the absorption spectra of the open-ring isomers are not readily rationalized by a straightforward superposition of the spectra of the two component radical fragments and the photogenerated radicals are electronically coupled through the aromatic linker. Furthermore, the open-ring isomer can be treated as a hybrid of the pure open-shell biradical and closed-shell quinoid resonance structures.

Negative photochromism, in which a thermally stable colored form isomerizes to the transient colorless form by light irradiation and the back reaction occurs thermally, is advantageous in its applications for photoswitching materials because visible light can cause the photochromic color change of the materials. Moreover, the photochromic color change can be induced even on the inside of the materials due to the absence of the reabsorption of the visible excitation light by the photogenerated colorless species. While several negative photochromic compounds have been reported, the time scales of the back reaction are still slower than minutes, and no available fast responsive negative photochromic compounds have been reported. Here, we developed a negative photochromic 1,1′-binaphthyl-bridged phenoxyl–imidazolyl radical complex (BN-PIC) which enables fast photoswitching by visible light. The stable colored BN-PIC shows instantaneous decoloration by visible light irradiation, and the photogenerated colorless form thermally reverts to the initial colored form with a half-life of 1.9 s at room temperature. BN-PIC can also cause the drastic change in the chiroptical properties by the photochromic reaction, and the rate of the thermal back reaction is affected by the chirality of the solvent. Since the negative photochromic reaction can occur on the inside of the materials, the fast negative photochromism is expected to have an impact in the fields of photoresponsive materials of solid states and molecular aggregates.

We report rational molecular designs for acceleration of the color-fading speed of photochromic 3H-naphthopyrans. By using steric and electrostatic repulsions induced by substituents at the 2- and 10-positions of 3H-naphthopyrans, the color-fading speed accelerates from tens of minutes to microsecond time scales. The long-lived residual color, which is an important problem to be solved for industrial applications, can also be suppressed by these strategies.

We demonstrate that a biphenyl-bridged imidazole dimer exhibits fast photochromism with a thermal recovery time constant of ∼100 ns, which is the fastest thermal back reaction in all reported imidazole dimers. Sub-ps transient absorption spectroscopy reveals that the generation process of the colored species occurs within 1 ps.

Biradicaloid species have been extensively studied for their characteristic features in electric conductivity, magnetism, and optical nonlinearity. Theoretical investigations of rigid biradicaloid species have been suggesting that they are represented as a resonance hybrid of open-shell biradical and closed-shell quinoid structures. However, much is still unknown about flexible biradicaloid species whether the activation free energy barrier between these states exists or not. Herein, we investigated the thermal isomerization from the photogenerated unstable biradical to the stable quinoid species observed for the photochromic dimer of a bisimidazolyl radical and found that the large negative activation entropy for the valence isomerization causes the activation free energy barrier between these two states.

Photochromic compounds have attracted attention as ophthalmic lenses because of their reversible color modulation upon irradiation with light. However, the efficiency of the photochromism is strongly affected by their surrounding because of the structural changes concomitant with the photochromism, which causes the decrease in the photochromic performance in the polymer matrix. Therefore, the clarification of the degree of the structural changes is necessary to apply to the ophthalmic lenses. Bridged imidazole dimers are one of the fast photoswitch molecules possessing high photochromic quantum yield and durability. Although the enhancement of the photochromic properties of bridged imidazole dimers has been vigorously studied, the quantitative information about the structural changes has not been revealed in detail. In this study, we investigated the pressure effects on the photochromic properties of bridged imidazole dimers. The activation volume for the thermal back-reaction of the photogenerated biradical species becomes an effective measure to predict the degree of the structural change during the photochromic reaction. We revealed that the smaller activation volume is suitable for keeping the efficient photochromic reaction in the polymer matrix because the photochromic reaction is not affected by the surroundings. These fundamental insights into the molecular dynamics provide valuable information to develop fast photochromic compounds that are suitable for the use in the polymer matrix and pressure sensitive photochromic materials.

The widely tunable optical properties and the visible sensitivity have been required for fast photochromic molecules whose coloration–decoloration cycle completes in μs to ms time scale not only for practical applications such as full-color holographic displays but also for fundamental researches in biochemistry. However, the so far developed [2.2]paracyclophane-bridged imidazole dimers, which are one of the best candidates for fast photochromic molecules, have their weaknesses for these requirements. Herein, we overcome the issues with sustaining fast photochromism and high durability by flipping the two imidazole rings (the head-to-tail and tail-to-tail forms). The alteration in the relative configuration of the imidazole rings suppresses the broad absorption band resulting from the radical–radical interaction. The substitution to the 2-position of the imidazole ring of the tail-to-tail form gives the drastic changes in the steady-state and the transient absorption spectra. The pyrene-substituted tail-to-tail form demonstrates that the transient absorption spectrum is featured by the inherent spectrum of the imidazolyl radical. This molecular framework is easy to functionalize fast photochromic molecules such as sensitizations to the red light, chirality, and biological tagging, and therefore it is versatile for various fast photochromic applications.

We developed a chiral 1,1′-bi-2-naphthol-bridged imidazole dimer possessing 100 μs fast photochromism and high fatigue resistance. It offers great opportunities for the practical applications to fast photoresponsive chiral dopants, invisible security materials and optical trigger molecules to induce the dynamic structural changes in biological matters.

We report a new type of fast photochromic imidazole dimer, pentaarylbiimidazole (PABI), which shows a few μs fast photochromism with high fatigue resistance against light irradiation. PABI has an unusual spiroconjugated imidazoisoindole skeleton and its derivatives can be prepared by simple synthetic procedures.

The bridged imidazole dimers are some of the attractive fast photochromic compounds which have potential applications to the ophthalmic lenses, real-time hologram and molecular machines. The strategy for expanding their photochromic properties such as the colour variation and tuning the decolouration rates has been vigorously investigated, but the insight into the structural changes along the photochromic reactions has not been demonstrated in detail. Here, we demonstrated the pressure dependence of the radical–radical recombination reaction of the bridged imidazole dimers. The radical–radical interaction can be controlled by applying high pressure. Our results give fundamental information about the molecular dynamics of the bridged imidazole dimers, leading to the development of new functional photochromic machines and pressure-sensitive photochromic materials.

A new type of the bridged imidazole dimer with a 1-phenylnaphthalene moiety that bridges two diphenylimidazole units at the 2- and 2′-positions was synthesized and the photochemical and thermochemical properties were investigated. This molecule shows unique multistate photochromism, in which the stable colorless 1,2′-isomers A and B photochemically isomerize to the colorless 2,2′-isomer through the short-lived biradical with a half-life of 180 ns at 25 °C. The 2,2′-isomer thermally returns to the 1,2′-isomers A and B through the colored isomer at elevated temperatures. The 1,2′-isomers A and B, the 2,2′-isomer, and the colored isomer were isolated, and their molecular structures were determined by X-ray crystallographic analysis. These isomers are stable at room temperature and can be almost fully converted to the 2,2′-isomer by light irradiation. This study serves the useful strategy for the molecular design of a new type of negative photochromic molecules applicable to switch molecular properties by visible light irradiation.

A unique solvatochromic 2-phenyl-4,5-diarylimidazole derivative linked with a phenol moiety and a p-quinonemethide moiety at the 4- and 5-positions of a imidazole ring, which shows remarkable color change via proton tautomerism, was synthesized and the mechanism of the solvatochromic color change was investigated. The yellow-colored OH tautomeric form (1_OH) exists as a dominant species in nonpolar solvents, whereas the blue-colored NH tautomeric form (1_NH) is stabilized in polar solvents. The molecular structures of these tautomers were determined by X-ray crystallographic analysis. The p-quinonemethide moiety and the imidazole ring of 1_OH are coplanar to one another and possess a planar quinoidal structure. On the other hand, 1_NH has a nonplanar twisted quinoidal structure causing large bathochromic shift in the visible absorption spectrum. Moreover, the X-ray crystallographic analysis and the DFT calculations support the closed-shell singlet character of 1_OH. In contrast 1_NH possesses partial single bond character that leads to the open-shell singlet biradical character and the decrease in the singlet–triplet energy gap. The twisting of the π-conjugated electron system induced by the proton tautomerization was found to be the origin of the open-shell biradical character of 1_NH and the enhanced solvatochromic color change.

[2.2]Paracyclophane-bridged imidazole dimers, which show unique fast photochromism, have various practical applications in industry. To put them to practical use, it is necessary to prepare various types of the imidazole dimers which have different color, reaction rate, sensitivity, etc. One of the simple methods for modulating the optical properties is to add substituents and sensitizers. However, it is difficult to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the imidazole dimers by optical spectroscopy because the LUMO of the imidazole dimers are optically inactive. In the present study, we applied electrochemistry and density functional theory to reveal the effect of substituents on the electronic states of the imidazole dimers. We revealed that the HOMO and LUMO of the imidazole dimers are localized over only one of the imidazole rings of the imidazole dimer. By comparing the measured LUMO energies of the imidazole dimers and calculated LUMO energies of several visible sensitizers, we found which visible sensitizers work in the imidazole dimer systems. These fundamental insights provide useful information for understanding the electronic structures of the imidazole dimers and give a strategy for designing novel fast photochromic molecules whose photochromism is triggered by visible light.

We have synthesized a new photochromic compound that exhibits unusual negative photochromism, in which the stable colored species photochemically converts into the metastable colorless species via a short-lived radical. This compound has a 1,1′-binaphthyl moiety bridging the two diphenylimidazole units. Its photochemical properties were investigated by nanosecond laser flash photolysis. The colored species isomerizes to the colorless species upon exposure to visible light and thermally returns to the original colored species within 20 min at room temperature. Moreover, the photodecoloration reaction proceeds via a short-lived radical with a half-life of 9.4 μs in benzene at room temperature. Both the colored and colorless species show the photoinduced homolytic bond cleavage reaction of the C–N bond between the nitrogen atom of the imidazole ring and the carbon atom of the 1-position of the 1,1′-binaphthyl moiety and that of the C–C bond between each of the carbon atoms of the 2-position of the imidazole ring, respectively, followed by their formation by rapid radical coupling.

Anti-1,8-bisTPI-naphthalene in which two imidazole rings are constrained to an anti-conformation leading to the first-formed 1,4′-isomer of the bridged imidazole dimer has been synthesized. The color of the radicals is different from that of the previously reported bridged-imidazolyl radicals because the intramolecular interaction between the radicals becomes weak due to the anti-conformation. This molecular design would be a profitable strategy to control the color of the radicals of the bridged imidazole dimer for application in ophthalmic lenses.

Recently, we have developed a series of fast photochromic imidazole dimers with a [2.2]paracyclophane ([2.2]PC) moiety that bridge diphenylimidazole units and succeeded the acceleration of the thermal decoloration rate. The colorless [2.2]PC-bridged imidazole dimers show a photoinduced homolytic bond cleavage of the C–N bond between the imidazole rings to give a pair of colored imidazolyl radicals upon UV light irradiation, followed by the radical–radical coupling reaction to form the initial C–N bond between the imidazole rings. The decoloration reaction to give the initial imidazole dimer proceeds only thermally. The high quantum yield close to unity of the photochromic reaction and the large extinction coefficient of the radical achieve both high optical density at the photostationary state and rapid switching speed. The application to rapid fluorescence switching has been investigated to develop a new type of photochromic fluorescence switching molecule applicable to super-resolution microscopy. The widespread absorption of the colored radical lying between 500 and 900 nm enables the efficient quenching of the excited electronic state of the fluorophores by Förster resonance energy transfer (FRET) from the fluorophores to the radical moiety. We successfully developed a [2.2]PC-bridged imidazole dimer possessing a fluorescein moiety as a fluorescence unit. This photochromic dye shows fast photochromism to give a pair of imidazolyl radicals that quench the fluorescence from the fluorescent unit by the FRET mechanism. The fluorescence intensity can be switched rapidly with the fast photochromism.

Graphite has been known as a typical diamagnetic material and can be levitated in the strong magnetic field. Here we show that the magnetically levitating pyrolytic graphite can be moved in the arbitrary place by simple photoirradiation. It is notable that the optical motion control system described in this paper requires only NdFeB permanent magnets and light source. The optical movement is driven by photothermally induced changes in the magnetic susceptibility of the graphite. Moreover, we demonstrate that light energy can be converted into rotational kinetic energy by means of the photothermal property. We find that the levitating graphite disk rotates at over 200 rpm under the sunlight, making it possible to develop a new class of light energy conversion system.

A [2.2]paracyclophane-bridged imidazole dimer having bulky 3′,4′,5′-triphenyl-1,1′:2′,1″-terphenyl substituents was synthesized and the photochromic properties were investigated. The half-life of the colored species generated by the UV irradiation of the parent imidazole dimer was 1.0 ms at 298 K in benzene. Although such bulky substituents are introduced on the imidazole dimer, the fast thermal back-reaction can be observed as found in other [2.2]paracyclophane-bridged imidazole dimers. Herein, enthalpy and entropy of the activation energies (∆H‡ and ∆S‡, respectively) for the thermal back-reaction of the present molecule were compared with other [2.2]paracyclophane-bridged imidazole dimers in order to investigate the generality and the limitation of the molecular design of the photochromic imidazole dimer along with the correlation between bulkiness of the substituents and the rate constant of the thermal back-reaction. Finally, we referred to a new method for controlling the rate constant of the thermal back-reaction of [2.2]paracyclophane-bridged imidazole dimers.Highlights► We designed thermally reversible photochromic molecules bearing bulky substituents. ► The photochromic molecules showed efficient photo-reaction even in the solid state. ► We investigated the correlation between the bulkiness and the thermal back-reaction. ► The bulky substituents decreased the rate of the thermal back-reaction entropically. ► We showed that the thermal back-reaction could be controlled entropically.

2,4,5-Triphenylimidazole (lophine) is known as the first chemiluminescence substrate, and its oxidized derivative, the 2,4,5-triphenylimidazolyl radical, corresponds to the coloured species in the photochromic reaction of hexaarylbiimidazole (HABI). We report the first direct observation of the O2 adduct of the imidazolyl radical that forms the end-on peroxide-bridged imidazole dimer. The ring-opening reaction of the peroxide-bridged imidazole dimer leading to the formation of an N-benzoylbenzamidine derivative supports the presence of the 4,5-epidioxide of lophine as a reaction intermediate of its chemiluminescence.

The photochromic behavior of the imidazole dimers can be attributable to the photoinduced homolytic cleavage of the C–N bond between the two imidazole rings. On the other hand, although the simultaneous formation of the imidazolyl radical and imidazole anion by the one-electron reduction of an imidazole dimer was reported, no definitive evidence for this electrochemical reaction has been demonstrated. We report the first direct evidence for the electrochemical generation of the imidazolyl radical from the radical anion of the imidazole dimer by conducting the UV–vis–NIR spectroelectrochemical analysis of the [2.2]paracyclophane-bridged imidazole dimer.

Here we report the first photochromism of a newly designed [2.2]paracyclophane-bridged imidazole dimer in water. The photochromic dye with a hydrophilic and a hydrophobic substituent forms vesicles in water and shows instantaneous colouration upon UV light irradiation and successive rapid fading in the dark.

A novel photochromic organogelator possessing [2.2]paracyclophane-bridged imidazole dimer unit with four hydrogen bonding urea arms was obtained. Irradiating UV light to benzene solution of the photochromic organogelator, rapid photochromism from colorless to green is observed. From the transient vis–NIR absorption measurement, a characteristic absorption around 400 nm and a broad absorption from 500 to 1000 nm can be attributed to the colored species. The half-life of the colored species is 243 ms at 298 K. In cyclohexane, the organogel was successfully formed, which was characterized by SEM observation and IR spectroscopy and showed fast photochromism even in the gel phase.

The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. Experimental details for the enhancement of the photosensitivity and the unique photoisomerization of newly designed [2.2]paracyclophane-bridged imidazole dimers are demonstrated. We explored the structure–property relationships and demonstrated an efficient strategy for designing high-performance fast-photochromic molecules with increased photosensitivity to solar UVA radiation. The [2.2]paracyclophane-bridged imidazole dimer consists of two types of imidazole rings, Im1 and Im2. Im1 is characterized by a 6π electron system with an electron-donating characteristic, whereas Im2 is distinguished by a 4π electron system with an electron-withdrawing characteristic. The introduction of electron-donating substituents into the phenyl rings attached to the electron-withdrawing Im2 was proved to enhance the photosensitivity with the aid of the intramolecular charge transfer transitions. The unique photoisomerization resulting from the changes in the bonding manner between two imidazole rings was also investigated in detail.

The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. A new [2.2]paracyclophane-bridged imidazole dimer, pseudogem-PPI-DPI[2.2]PC, with high photosensitivity to UVA radiation was developed. To enhance the photosensitivity, we introduced pyrenyl moieties to the [2.2]paracyclophane-bridged imidazole dimer. The localized π–π* transition of pyrenyl moieties appears in the UVA radiation region by introducing a pyrenyl moietiy on the 4-position of the imidazole rings. The expansion of the π-electron system also affects the absorption spectrum of the colored species. The broad absorption band of the colored species covers the whole range of visible light region and its absorbance is approximately equal throughout the visible light region. Thus, pseudogem-PPI-DPI[2.2]PC shows the photochromic reaction coloring black upon light irradiation and successive fast thermal bleaching following the monoexponential kinetics with a time constant of 12 ms at room temperature.

Pseudogem-DPIM-DPI[2.2]PC dimer (3), with a C−N bond between the [2.2]paracyclophane ([2.2]PC) moiety and the imidazole ring, was synthesized. This is the first crystallographic observation of a highly sterically constrained 1-ene-2,5-cyclohexadiene structure for a [2.2]PC derivative. Compound 3 shows a photochromic behavior, exhibiting a color change from pale yellow to green upon either UV or visible light irradiation, both in the solid state and in solution at room temperature.

The synthesis and photochromic behavior of the fast photochromic polymers carrying [2.2]paracyclophane-bridged imidazole dimer are demonstrated. A significant feature of this synthetic strategy is that we can modify the photochromic properties such as coloration/decoloration rate, coloring, and photosensitivity via the stepwise synthetic approach of the imidazole dimer system. Notably, the photochromic behavior of the polymers is not affected by the environment around the photochromes and copolymerization with other monomers in both solution and film, which cannot be realized in any other conventional photochromic systems. The comparable photochromic behavior of the homopolymers and copolymers in solution and film indicates that the photochromic unit is independent from the local environment, which allows effective molecular design of the photochromic monomer unit to accomplish desired photochromic properties of the polymer.

We demonstrate that photochromism based on light-driven reversible C−N single bond cleavage can enable rapid coloration upon UV light irradiation and successive fast thermal back-reaction within tens of microseconds at room temperature. According to Marcus theory, the thermal back-reaction would be accelerated with increasing ΔG0, which is closely linked to the decrease in ΔG‡. We have considered that the ΔG0 of the thermal back-reaction could be enlarged by destabilizing the colored species and designed pseudogem-DPI-PI[2.2]PC, with a [2.2]paracyclophane moiety that couples diphenylimidazole and phenanthroimidazole groups. The present study demonstrates that controlling the stability of the biradical state is effective in accelerating the thermal back-reaction for the photochromic [2.2]paracyclophane-bridged imidazole dimer.Keywords (keywords): biradical; DFT; imidazole; Marcus theory; photochromism; [2.2]paracyclophane;

The photodissociation dynamics of a hexaarylbiimidazole (HABI) derivative with two pyrenyl groups was investigated by time-resolved transient absorption spectroscopy and fluorescence measurements. Transient absorption spectroscopy revealed that photodissociation took place in the wide time region of <100 fs to 10 ns. On the other hand, fluorescence time profiles showed the dynamic red shift in the time region <100 ps. The apparent dispersive photodissociation process was attributed to the increase in the interaction between the pyrenyl moiety in the excited state and the other moiety in the ground state, resulting in the gradual increase of the activation energy for the crossing between the attractive potential surface of an excited pyrenyl unit and the repulsive potential surface.

The formation of hexaarylbiimidazole heterodimers has been investigated in detail by X-ray crystallography and 1H NMR spectroscopy.

The synthesis and photochemical properties of poly[1,3-bis(dithienyl)benzene] having a porphyrin group as a side-chain were studied. As a target monomer, 1,3-bis(dithienyl)benzene with an ethynylporphyrin substituent was synthesized by Pd-catalyzed cross-coupling reactions. The electrochemical polymerization of the monomer afforded a conductive polymer film on the ITO-coated glass. The spectroelectrochemistry of the polymer was investigated to clarify the electrochemical oxidation behavior. The current density of the target polymer showed a good photoresponse and the much larger photocurrent was observed as compared with the dark current.

Activation parameters were determined for the recombination of radical pairs arising from newly designed, photochromic, radical diffusion-restricted hexaarylbiimidazole (HABI) derivative. We have developed a new type of radical diffusion-inhibited HABI derivative, which contains two equivalent HABI units and yields a tetraradical with four equivalent 2,4,5-triphenylimidazolyl radical (TPIR) units by photoirradiation. This radical dimerization proceeds by a successive first-order reaction from the tetraradical to the parent molecule via a diradical. The rate constants of each reaction were determined from the decay profile of EPR signal intensities. The entropies of activation (Δ S ‡) for the first and the successive dimerization steps were estimated to be −178.5 and −205.5 J K −1 mol −1, respectively. Within the experimental temperature range, the radical dimerizations are entropy-controlled (− TΔ S ‡ > Δ H ‡). The large negative Δ S ‡ values imply a highly ordered transition state, indicating that the radical dimerizations occur when the TPIR units interact at a specific orientation. The present study demonstrates the availability of radical diffusion-inihibited HABI for the kinetic study of radical−radical reaction.

Dynamics of the photodissociation process of a hexaarylbiimidazole derivative, 2,2′-di(ortho-chlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole (o-Cl-HABI), in benzene solution was investigated by means of the femtosecond–nanosecond laser spectroscopy. The time evolution of the transient absorption of the lophyl radical, the product of the photodissociation, was biphasic with the faster and the slower time constants of 80 fs and 1.9 ps, respectively. The faster time constant was ascribable to the formation of the lophyl radical through the bond fission after the excitation, and the slower time constant may be attributed to the conformational change and/or vibrational cooling of nascent radicals.Photodissociation of HABI to radicals.

We have designed and synthesised a new carborane derivative containing two nitronyl nitroxides. This molecule can be considered as a prototype of a three-dimensional analogue of o-benzoquinodimethane. The magnetic susceptibility of a crystalline sample revealed that this biradical possesses a weak antiferromagnetic interaction (θ = −1.05(4) K) and an ESR study in frozen matrix in the 5.4–104 K range gave an intramolecular antiferromagnetic interaction of −27(2) K. The synthesis, physical properties and DFT calculation result are also reported.

The molecular alignment and thermal stability of the mixtures DA composed of two different kinds of Schiff base liquid-crystalline materials, D with an electron-donating –N(CH3)2 unit and A with an electron-withdrawing–NO2 unit, were investigated by using DSC, POM, XRD, and FT-IR measurements. The mesophases of the mixtures DA were more stable and had wider temperature ranges than those of the individual compounds D and A. The mixture DA55 (D:A=50:50 mol%) on cooling exhibited the most stable liquid-crystalline phases affording the highest TSI (119 °C), the widest temperature range of the smectic phase (46.4 °C) and the highest values of ΔSSI (26.1 J K−1 mol−1) among the mixtures DA examined. Based on the analysis of the XRD measurements and the temperature dependence on the FT-IR spectra, the molecular packing of the mixture DA55 was proposed to form a bilayer structure in the smectic A phase, in which the heading group of the compounds D and A overlapped partially. The induction of the smectic phase and the enhanced thermal stability for the mixtures DA would be caused by the intermolecular interaction between the electron donor and acceptor mesogens.

2,4,5-Triphenylimidazole (lophine) is known as the first chemiluminescence substrate, and its oxidized derivative, the 2,4,5-triphenylimidazolyl radical, corresponds to the coloured species in the photochromic reaction of hexaarylbiimidazole (HABI). We report the first direct observation of the O2 adduct of the imidazolyl radical that forms the end-on peroxide-bridged imidazole dimer. The ring-opening reaction of the peroxide-bridged imidazole dimer leading to the formation of an N-benzoylbenzamidine derivative supports the presence of the 4,5-epidioxide of lophine as a reaction intermediate of its chemiluminescence.

We demonstrate that a biphenyl-bridged imidazole dimer exhibits fast photochromism with a thermal recovery time constant of ∼100 ns, which is the fastest thermal back reaction in all reported imidazole dimers. Sub-ps transient absorption spectroscopy reveals that the generation process of the colored species occurs within 1 ps.

Phenoxyl-imidazolyl radical complex (PIC) is a novel photochromic compound, which generates two nonequivalent radicals upon UV light irradiation. While the color fading speed of PIC can be tuned from tens of nanoseconds to seconds, the low visible sensitivity has limited their potential applications. Herein, we developed novel thiophene-substituted PIC derivatives. The visible sensitivity and the increased absorption coefficient were achieved with keeping the moderate color fading speed by introducing a phenyl group at the 5-position of the thiophene ring.

We report rational molecular designs for acceleration of the color-fading speed of photochromic 3H-naphthopyrans. By using steric and electrostatic repulsions induced by substituents at the 2- and 10-positions of 3H-naphthopyrans, the color-fading speed accelerates from tens of minutes to microsecond time scales. The long-lived residual color, which is an important problem to be solved for industrial applications, can also be suppressed by these strategies.

Here we report the first photochromism of a newly designed [2.2]paracyclophane-bridged imidazole dimer in water. The photochromic dye with a hydrophilic and a hydrophobic substituent forms vesicles in water and shows instantaneous colouration upon UV light irradiation and successive rapid fading in the dark.

Steric protection groups are usually necessary for stable radicals. However, here, we developed novel photochromic radical complexes which generate sterically unprotected imidazolyl and phenoxyl radicals upon UV light irradiation based on the phenoxyl–imidazolyl radical complex (PIC) framework. These photochromic compounds show excellent durability against repeated irradiation of intense nanosecond laser pulses even in polar protic solvents, such as ethanol.

We report a new type of fast photochromic imidazole dimer, pentaarylbiimidazole (PABI), which shows a few μs fast photochromism with high fatigue resistance against light irradiation. PABI has an unusual spiroconjugated imidazoisoindole skeleton and its derivatives can be prepared by simple synthetic procedures.

Naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) is a novel fast switchable negative photochromic compound, which shows the thermal back reaction in the millisecond time scale. Upon UV light irradiation, Np-PIC shows the hypochromic effect in the UVA region due to there being less conjugation in the transient isomer. By replacing the phenoxyl unit with a naphthoxyl unit, the molecular structure has an asymmetric carbon, leading to fast chiroptical switching. This simple molecular design will be a good candidate for the future development of negative photochromic compounds.

The bridged imidazole dimers are some of the attractive fast photochromic compounds which have potential applications to the ophthalmic lenses, real-time hologram and molecular machines. The strategy for expanding their photochromic properties such as the colour variation and tuning the decolouration rates has been vigorously investigated, but the insight into the structural changes along the photochromic reactions has not been demonstrated in detail. Here, we demonstrated the pressure dependence of the radical–radical recombination reaction of the bridged imidazole dimers. The radical–radical interaction can be controlled by applying high pressure. Our results give fundamental information about the molecular dynamics of the bridged imidazole dimers, leading to the development of new functional photochromic machines and pressure-sensitive photochromic materials.

We developed a chiral 1,1′-bi-2-naphthol-bridged imidazole dimer possessing 100 μs fast photochromism and high fatigue resistance. It offers great opportunities for the practical applications to fast photoresponsive chiral dopants, invisible security materials and optical trigger molecules to induce the dynamic structural changes in biological matters.

Biradicaloid species have been extensively studied for their characteristic features in electric conductivity, magnetism, and optical nonlinearity. Theoretical investigations of rigid biradicaloid species have been suggesting that they are represented as a resonance hybrid of open-shell biradical and closed-shell quinoid structures. However, much is still unknown about flexible biradicaloid species whether the activation free energy barrier between these states exists or not. Herein, we investigated the thermal isomerization from the photogenerated unstable biradical to the stable quinoid species observed for the photochromic dimer of a bisimidazolyl radical and found that the large negative activation entropy for the valence isomerization causes the activation free energy barrier between these two states.