Takashi Karatsu

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.

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.

Triplet energy surfaces of one-way isomerizing olefins as studied by singlet oxygen luminescence technique

Abstract Quantum yields for singlet oxygen [O 2 ( 1 Δ g )] production (Φ Δ ) were measured on quenching of the triplet states of one-way isomerizing 2-anthrylethylenes by means of the singlet oxygen luminescence technique. The trans triplets of these olefins yielded O 2 ( 1 Δ g ) efficiently through energy transfer. The triplet energy surfaces of the olefins, particularly those at the cis to twisted geometries, are discussed in terms of the measured Φ Δ values.

Highly selective rotational isomerization of 2-vinylanthracene in the excited singlet state. Picosecond time-resolved fluorescence study

Abstract Time-resolved fluorescence spectra and fluorescence kinetics of 2-vinylanthracene in benzene were measured by the picosecond time-correlated single-photon counting technique. Upon 316 nm laser excitation, one rotamer was selectively excited and underwent one-way isomerization to the other rotamer within their lifetimes in the excited singlet state.

A novel “across-a-ridge” isomerization of olefins. The energy surface of 2-ethenylanthracene in the excited triplet state

Abstract The temperature dependence of the quantum yield and the quenching of T-T absorption have revealed that 2-(ethenyl-2- d )anthraceneisomerizes adiabatically from the E to the Z isomer by passing over an activation barrier (11.1 kcal/mol) in the excited triplet state. The perpendicularly twisted geometry on the triplet energy surface is a transition state of interconversion between the E and Z isomers.

Effects of β-substituents on internal rotation of the anthryl group of 2-anthrylethylenes in the excited singlet state

Abstract 2-Vinylanthracene and its β-alkyl derivatives undergo, in the excited singlet state, almost one-way rotational isomerization around the single bond connecting the anthracene nucleus and Cue5fbC double bond, while no rotational isomerization takes place for 2-anthrylethylenes with a phenyl or 2-naphthyl group in the β-position.

Photoinduced isomerization and charge transfer in trans-1,2-bis(1-methyl-4-pyridinio)ethylene salts

Abstract The fluorescence quantum yield and lifetime of trans-1,2-bis-(1-methyl-4-pyridinio)ethylenediperchlorate. t -BPE 2+ (ClO 4 − ) 2 , are respectively 0.01 and 3 CN. The quantum yield of trans to cis photoisomerization is 0.35 ± 0.07. Iodide complexes the olefin in the ground state with an equilibrium constant of 4.6 M −1 and the charge transfer absorption maximum is at 420 nm. Iodide quenches the olefin fluorescence both by diffusional encounter ( K sv = 10 M −1 ) and through complexation, leading to the formation of t -BPE + and radicals with a quantum yield of 0.10 ± 0.03.

General Aspects of Photochemistry. Search for New Basic Types of Photochemical Reactions

Publisher Summary This chapter discusses an approach to find out new types of basic photochemical reactions, particularly, the isomerization of olefins. Currently, many photochemical reactions of organic molecules have extensively been investigated particularly in solution, and their essential mechanisms are well established. As one of the key types of photochemical reactions, cis -trans isomerization of olefins has well been investigated from mechanistic, as well as synthetic aspects of the reaction. Many olefins isomerize mutually between their cis - and trans -isomers. Introduction of aromatic groups with low triplet energies drastically changes behavior of the olefins, and the novel one-way and the traditional two-way can be understood by a new wide concept. The mode of isomerization gradually changes from two-way to one-way by reduction of the triplet energies of the aromatic groups. Irradiation of particles of semiconductors like those that TiO 2 or CdS suspended in a solution of stilbene led to isomerization of its cis - to trans -isomer. The reaction is initiated by electron transfer from cis -stilbene to the positive hole generated in the semiconductor by irradiation.