Hirochika Sakuragi

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.

“One-way” photoisomerization between cis- and trans-olefin. A novel adiabatic process in the excited state

Abstract Triplet sensitized and unsensitized irradiation of 1-(2-anthryl)-3,3-dimethyl-1-butene in benzene resulted in “one-way” isomerization from the cis- to trans-isomer giving almost 100% of the trans-isomer through a quantum chain process. No “return” isomerization from the trans- to cis- isomer occurred. The isomerization can be interpreted in terms of an adiabatic process in the triplet excited state.

Semiconductor-catalyzed photooxygenation of aromatic olefins

Abstract Irradiation of semiconductors (TiO 2 and CdS) suspended in organic solvents containing 1,1-diphenylethylene and its derivatives under oxygen afforded the corresponding epoxides and benzophenone. A possible mechanism is discussed.

New insight into “one-way” photoisomerization of anthrylethylenes: absorption spectra of the trans triplets as intermediates of the isomerization

The quantum yield for the isomerization and its concentration dependence were determined for 1-(2-anthryl)-3,3-dimethyl-1-butene, and the T1 → Tn absorption spectra were measured for the cis and trans isomers of several 2-anthrylethylenes. The spectra were assigned to their planar triplets, the intermediate of the “one-way” isomerization proceeding through an adiabatic quantum chain process on the triplet manifold.

Interaction of hydrophobic molecules with water influenced by the clustering conditions of acetonitrile–water mixtures

In acetonitrile–water mixtures, dynamical processes of hydrophobic substrates, such as deprotonation of excited-state 2-naphthol and hydrolysis of tert-butyl chloride, hardly occur at water mole fractions (xw) less than 0.8. In contrast, the reaction rates increased markedly with increasing xw for xw > 0.8. These results were correlated with properties which reflect the solvent clustering conditions, such as partial vapour pressure and IR absorption of acetonitrile–water mixtures. IR absorption of water molecules due to the antisymmetric O—H stretching vibration showed a step-wise change of the absorption peak position suggesting the presence of four different solvent structures in the regions 0 < xw < 0.2, 0.2 < xw < 0.5, 0.5 < xw < 0.8 and 0.8 < xw < 1.0. Mass spectrometric analyses of liquid fragments of acetonitrile–water mixtures and the solutions with phenol, benzyl alcohol and 2-n-butoxyethanol showed clustering conditions characteristic of the water mole fractions in the mixtures. In 0.8 < xw < 1.0 the hydrate clusters of phenol and benzyl alcohol increased very quickly with increasing xw. In xw < 0.8, the signals of the hydrate clusters of the solutes were very weak, while the acetonitrile hydrates increased with decreasing xw. The observed mass spectral change with varying xw is discussed in relation to the structure of acetonitrile–water solutions and its effect on the dynamical processes of hydrophobic substrates.

Mechanistic considerations on photoreaction of organic compounds via excitation of contact charge transfer complexes with oxygen

Abstract Reactions of various organic compounds induced by excitation of their contact charge transfer (CCT) pairs with oxygen were classified into three types of behaviour on the basis of reaction products. It is proposed that the excited state of the CCT pairs gives the products through the most isoenergetic course along the plausible exothermic pathway.

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.

Violet emission observed from phthalocyanines

Spectral features of violet emission observed from phthalocyanines in addition to the well-known strong red emission are presented. The origin of the violet emission from the present and reported phthalocyanines is argued based upon the observed and estimated features.

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.

Spectroscopic evidence for an energy minimum at the cis geometry of the triplet state of a butenylanthracene. Further insight into across-a-ridge isomerization of anthrylethylenes

The triplet energy surface of 2-(3,3-dimethyl-1-butenyl)anthracene (1) was investigated in detail on the basis of isomerization quantum yields, phosphorescence and transient absorption spectra, and emission-absorption study. There exists an energy minimum at the cis geometry as well as at the trans geometry of the triplet state of 1; this olefin undergoes “across-a-ridge” one-way isomerization from the cis to the trans isomer.