Yasunao Kuriyama

Novel insights into photoisomerization of olefins

(le-d) in which there is an energy barrier of 11 kcal/mol between 3c* and 3tk, The mode of isomerization of aromatic olefins, either one-way or two-way, is revealed to be governed by the triplet energy of the aromatic group on one ethylenic carbon as well as by the substituent on the other ethylenic carbon; decrease of the triplet energy of the aromatic group favors the one-way mode, and most olefins in the series of ArCH=CHPh prefer the two-way mode compared to the corresponding olefins in the series of ArCH=CHtBu.

Factors distinguishing one-way and two-way photoisomerization of aromatic olefins. Effects of substituents on triplet energy surfaces of 8-fluoranthenylethylenes

Abstract 8-fluoranthenylethylenes (8-FlCHCHR) behave as a boundary case distinguishing between one-way and two-way isomerizations. Introduction of an alkyl group (R = t -Bu) and an aryl group (R = Ph) on the β-ethylenic carbon leads to the one-way and two-way isomerization, respectively. Their triplet energy surfaces are proposed on the basis of measurements of the quantum yields, photostationary isomer ratios, and T-T absorption spectra.

Direct observation of cis-to-trans conversion of olefin radical cations. Electron transfer-induced isomerization of stilbene derivatives

Abstract The efficiency of unimolecular cis → trans conversion of radical cations of stilbene derivatives depends on the substituents on the phenyl rings, as observed by transient spectroscopy and on steady irradiation. Among the stilbenes employed, the radical cations of cis-4,4′-dibromo- and cis-4,4′-dimethylstilbene were, when generated by secondary electron transfer, converted unimolecularly to those of the corresponding trans-stilbenes in high quantum yields at ambient temperature.

Photooxygenation of stilbenes in zeolite by excitation of their contact charge transfer complexes with oxygen

Abstract Contact charge transfer (CCT) complexes of trans -stilbenes with oxygen molecules were formed in zeolite NaY. Excitation of the CCT bands by a 313-nm wavelength light produced the corresponding benzaldehydes through an electron transfer reaction to generate stilbene cation radicals and superoxide anion radical. Irradiation of the CCT complexes by a 254-nm wavelength light, by contrast, gave rise to isomerization and formation of phenanthrene without yielding any oxygenation products, as in the photoreaction under oxygen in solution.

Photosensitized cis—trans isomerization of stilbene via cation radical mechanism in micellar solution

Abstract 9,10-Dicyanoanthracene (DCA) and 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+) were used to sensitize the isomerization of cis-stilbene in sodium dodecylsulphate (SDS) micellar solutions with steady state and pulsed laser excitation. During TPP+ triplet sensitization, the isomerization quantum yield was nearly four times higher than that in homogeneous solution; however, the DCA excited singlet was not effective in isomerization.

Polarization Dependence of Absorption and Luminescence Spectra of a 1,5-Dihydroxyanthraquinone Single Crystal

A study has been made on the polarization dependence of absorption and luminescence spectra of a 1,5-dihydroxyanthraquinone single crystal with the thickness of 0.5 mm, which was prepared by gas ph...

KINETICS AND MECHANISM OF PHOTOCYCLOADDITION OF DEOXYURIDINES TO 2,3-DIMETHYL-2-BUTENE

The mechanism of photocycloaddition of 2′‐deoxyuridine (1a) and thymidine (1b) to 2,3‐dimethyl‐2‐butene (Bu) in acetonitrile by UV irradiation has been studied. The reciprocal quantum yield for the cycloaddition increased linearly with reciprocal concentrations of Bu in acetonitrile to give limiting quantum yields at infinite concentration of Bu as 0.030 and 0.0096 for 1a and 1b, respectively. This shows that the cycloaddition proceeds in a two‐step mechanism between the triplet state of 1 and Bu through biradical intermediates. Addition of cis‐1,3‐pentadiene quenched the reaction obeying the Stern–Volmer equation. The above quenching experiments and laser transient spectroscopy revealed that the triplet state of 1a reacts with Bu with much larger rate constant (1.3–1.6 × 109 M−1 s−1) than that of 1b (4–5 × 107 M−1 s−1) reflecting larger steric hindrance exerted in the reaction of 1b than that of 1a.

HIGHLY SPECIFIC ISOMERIZATION OF AROMATIC OLEFIN RADICAL CATIONS PRODUCED BY PHOTOSENSITIZED ELECTRON TRANSFER

The efficiency of unimolecular cis→trans conversion of the stilbene radical cations is enhanced by introduction of two bromine atoms, two methyl or methoxy groups at the 4- and 4′-positions and by addition of salts like lithium or magnesium perchlorate. On 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+ BF 4 -) sensitization, only the triplet radical pair is effective for the isomerization though both of the singlet and triplet excited states of the sensitizer are efficiently quenched. The isomerization efficiency increases in a sodium dodecyl sulfate (SDS) micellar solution.

Electron transfer between neutral radicals generated from photolysis of diphenylmethyl 1-naphthalenecarboxylate in polar solvents

Abstract From the fact that photolysis of diphenylmethyl 1-naphthalenecarboxylate in methanol, ethanol, and 2-propanol afforded the corresponding alkoxydiphenylmethanes in addition to 1,1,2,2,-tetraphenylethane and other products, it is proposed that the radical pairs of Ph 2 CH· and ArCO 2 · resulting from the photolysis undergo electron transfer to give Ph 2 CH + and ArCO 2 − with a rate constant comparable to radical recombination in the solvent cage.

TIME‐RESOLVED SPECTROSCOPIC STUDIES ON PHOTOPHYSICAL PROPERTIES OF BENOXAPROFEN IN MICELLAR SOLUTIONS: AN INTRAMICELLAR FLUORESCENCE QUENCHING

The micellar dependencies of the photophysical properties of benoxaprofen (BXP), a 2‐phenyl benzoxazole derivative, have been investigated using fluorescence spectroscopy and laser flash photolysis techniques. The fluorescence of BXP in aqueous solution has been observed to be remarkably quenched upon addition of a surfactant, cetyltrimethyl ammonium bromide (CTAB) or Triton X‐100, in contrast to its enhancement in sodium dodecyl sulfate (SDS) micellar solution. Time‐resolved fluorescence measurements show that the fluorescence decays biexponentially in the micellar solution, indicating the relaxation of micellar environments surrounding the excited BXP. The major component of fluorescence lifetimes in CTAB or Triton X‐100 micellar phase is even shorter (330–427ps) than in SDS micellar phase (731 ps). The nonradiative decay constants are significantly larger (ca 3.0 times 109 s−1) in the CTAB or Triton X‐100 micellar phase than in SDS micelles by a factor of ca 10. The major nonradiative decay is interpreted to be the internal conversion due to nuclear geometric change of BXP in the first excited singlet state. This is consistent with the observation that the quantum yields of intersystem crossing are very low (less than 0.01) in the micellar solutions as determined by the laser flash photolysis technique. The laser‐induced transient absorption spectrum of BXP in CTAB or Triton X‐100 micellar solution shows that the decay kinetics of the transients in CTAB or Triton X‐100 are significantly different from first order kinetics in SDS.