Hikaru Sotome

Fluorescent Photochromic Diarylethene That Turns on with Visible Light.

A new fluorescent photochromic diarylethene that can be activated by irradiation with 405 nm light was synthesized. The turn-on mode switching of fluorescence with visible light is favorable for application to biological systems. The fluorescence quantum yield of the photogenerated closed-ring isomer was as high as 0.8 in less or medium polar solvents, and even in polar acetonitrile the yield was higher than 0.6.

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.

Flapping viscosity probe that shows polarity-independent ratiometric fluorescence

A variety of fluorescent molecular viscosity probes have been widely used for mapping the local viscosity in cells and for monitoring the microenvironments in materials. However, their viscosity-sensing structural design still relies strongly on molecular rotors featuring intramolecular rotational dynamics. Here we report flapping molecules (FLAP) as a ratiometric viscosity-sensing fluorophore that shows polarity-independent dual fluorescence. Viscosity-sensing mechanism is based on a unique V-shaped-to-planar conformational change in the singlet excited state (S1), in which the flexible motion of an eight-membered ring plays an important role. Fast conformational dynamics have been studied by time-resolved spectroscopies, and the viscochromic properties have been quantitatively analyzed. Application of FLAP to monitoring the curing process of epoxy resins has also been demonstrated, in which other typical environment-sensitive dyes did not work as a local viscosity probe.

Hexa-peri-hexabenzo[7]helicene: Homogeneously π-Extended Helicene as a Primary Substructure of Helically Twisted Chiral Graphenes

Helically twisted graphenes can be considered as a promising candidate for the nanometer-sized molecular inductors in molecular electronics and molecular spring materials in nanomechanics. Here, we report the synthesis of hexa- peri-hexabenzo[7]helicene, which represents a primary substructure of the helical graphenes. The helically twisted polycyclic aromatic hydrocarbon was synthesized by a tetrasubstituted alkene formation using McMurry coupling followed by stepwise photocyclodehydrogenation and aromatization reactions. The π-extended helicoid structure with a noticeable intramolecular π-π interaction was unambiguously determined by X-ray crystallography. The primary helical nanographene molecule has a small HOMO-LUMO band gap evidenced by the absorption edge that appeared at ca. 800 nm, which exhibits an excellent chiroptical property with a dissymmetry factor of circular dichroism of | gCD| = 0.016 at 680 nm. The femtosecond transient absorption spectroscopy revealed the ultrafast excited-state dynamics of the helical nanographene molecule, with a lifetime of only few picoseconds in the lowest-energy excited (S1) state.

Efficient Cycloreversion Reaction of a Diarylethene Derivative in Higher Excited States Attained by Off-Resonant Simultaneous Two-Photon Absorption

Off-resonant excitation of the closed-ring isomer of a photochromic diarylethene derivative at 730 nm induced the efficient cycloreversion reaction with a yield of ∼20%, while the reaction yield was only 2% under one-photon excitation at 365 nm. Excitation wavelength dependence of the one-photon cycloreversion reaction yield under steady-state irradiation in a wide wavelength range showed that the specific electronic state leading to the large cycloreversion reaction yield, which is originally forbidden in the optical transition but partially allowed owing to the low symmetry of the molecule, is spectrally overlapped with the electronic state accessible by the allowed one-photon optical transition in the UV region. Femtosecond transient absorption spectroscopy also revealed that the off-resonant two-photon excitation preferentially pumped the molecule into the specific state, leading to the 10-fold enhancement of the cycloreversion reaction.

Optical properties and solvatofluorochromism of fluorene derivatives bearing S,S-dioxidized thiophene

We synthesized fluorene derivatives having phenylthiophene (FPT) or benzothiophene (FBT), and their S,S-dioxidized compounds (FPTO2, FPTO4, FBTO2 and FBTO4), which are prepared by oxidation of the thiophene rings in FPT and FBT with m-chloroperoxybenzoic acid. FPT and FBT exhibited similar optical properties for absorption maximum wavelength, fluorescence maximum wavelength and fluorescence quantum yield. However, the absorption and fluorescence spectra of FPTO2 were largely shifted toward a longer wavelength in comparison with those of FPT, and their fluorescence quantum yields dramatically decreased with oxidation. In contrast, the absorption and fluorescence spectra and the fluorescence quantum yields of FBTO2 and FBTO4 were similar to those of FBT. Moreover, FPTO2 and FBTO2 showed strong solvatofluorochromism. Such solvent dependent properties are ascribed to the charge transfer character of the molecule.

Fluorescence On/Off Switching in Nanoparticles Consisting of Two Types of Diarylethenes

Single- and double-component nanoparticles consisting of two types of diarylethenes, 1,2-bis(3-methyl-5-phenyl-2-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were fabricated by a reprecipitation method. Nanoparticles consisting of 1a exhibited orange or green fluorescence depending on the fabrication condition and did not undergo any photocyclization reaction. On the other hand, nanoparticles consisting of 2a underwent photoreversible photochromic reactions upon alternating irradiation with ultraviolet and visible light. Nanoparticles consisting of 1a and 2a exhibited fluorescence on/off switching with rapid switching speed and high on/off contrast, accompanying the photochromic reactions of 2a. The dependence of fluorescence on/off switching properties on Förster distance and molar fraction was observed and quantitatively evaluated by a simplified model.

Direct Observation of the Ultrafast Evolution of Open-Shell Biradical in Photochromic Radical Dimer

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.