Katsuya Mutoh

Comprehensive understanding of structure- photosensitivity relationships of photochromic [2.2]paracyclophane-bridged imidazole dimers.

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.

Enhancing the Versatility and Functionality of Fast Photochromic Bridged Imidazole Dimers by Flipping Imidazole Rings

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.

Photochromism of a water-soluble vesicular [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.

Self-Contained Photoacid Generator Triggered by Photocyclization of Triangle Terarylene Backbone

We herein propose a new type of efficient neutral photoacid generator. A photoinduced 6π-electrocyclization reaction of photochromic triangle terarylenes triggers subsequent release of a Brønsted acid, which took place from the photocyclized form. A H-atom and its conjugate base were introduced at both sides of a 6π-system to form the self-contained photoacid generator. UV irradiation to the 6π-system produces a cyclohexa-1,3-diene part with a H-atom and a conjugate base on the sp(3) C-atoms at 5- and 6-positions, respectively, which spontaneously release an acid molecule quantitatively forming a polyaromatic compound. A net quantum yield of photoacid generation as high as 0.52 under ambient conditions and a photoinitiated cationic polymerization of an epoxy monomer are demonstrated.

Spectroelectrochemistry of a Photochromic [2.2]Paracyclophane-Bridged Imidazole Dimer: Clarification of the Electrochemical Behavior of HABI

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.

Stepwise Two-Photon-Gated Photochemical Reaction in Photochromic [2.2]Paracyclophane-Bridged Bis(imidazole dimer)

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.

Electrochemistry of photochromic [2.2]paracyclophane-bridged imidazole dimers: rational understanding of the electronic structures.

[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.

Turn-on mode fluorescence photoswitching of diarylethene single crystals

The photochromic and fluorescence properties of two diarylethene derivatives having benzo[b]thiophene S,S-dioxide groups were studied in the single-crystalline phase. The derivatives showed reversible photochromic reactions and turn-on mode photoswitching of fluorescence in solution as well as in the single-crystalline phase. Upon irradiation with UV light, the open-ring isomers in the crystals underwent cyclization reactions to produce fluorescent closed-ring isomers. The UV-irradiated crystals emitted green or yellow-green fluorescence and exhibited dichroism of their absorption and fluorescence spectra under linearly polarized light. Upon irradiation with visible light, the closed-ring isomers reverted to the open-ring isomers and the fluorescence of the crystals disappeared. The switching cycles could be repeated more than 50 times without detectable deterioration. Area-selective switching of the fluorescence in the single crystal was demonstrated by patterned light irradiation, suggesting potential applications of the photoswitchable fluorescent molecular crystals in optical memory and display devices.

Intensity-Dependent Photoresponse of Biphotochromic Molecule Composed of a Negative and a Positive Photochromic Unit

Light-selective multiple photochromic systems are important for advanced photoswitching of chemical reactions and biological activities. While UV light has been frequently utilized to induce photochromic reactions, visible light is energetically acceptable to avoid undesired reactions. However, many of the reported multiphotochromic systems still rely on UV light to induce at least a part of photochromic reactions. In this work, we designed a biphotochromic molecule showing intensity-dependent multiple coloration with a visible-light source by incorporating two T-type photochromic units; a colorless positive photochromophore and a colored negative photochromophore in a molecule. The negative photochromophore acts as a visible-light sensitizer for the positive photochromic reaction. The compound shows an intensity-dependent color change under visible-light irradiation. The weak visible-light excitation leads to gradual decoloration from orange to yellow, whereas intense laser excitation clearly changes the color to green. This characteristic photochromism can be achieved by control of the photochromic reaction rates of the negative and positive photochromic reactions. The combination of negative and positive photochromic reactions gives attractive important insight into the development of multiresponsive optical materials.

Fast Photochromism Involving Thermally-Activated Valence Isomerization of Phenoxyl-Imidazolyl Radical Complex Derivatives.

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.