Novel insights into photoisomerization of olefins

Abstract

Aromatic olefins like anthrylethylenes (la-ld) undergo one-way isomerization, which proceeds solely from the cis to trans isomer in very high quantum yields far exceeding unity through adiabatic conversion of initially produced cis triplet (3c) to trans triplet (3t) passing through a perpendicularly twisted conformation. This is in remarkable contrast with the well accepted behavior of stilbenes and other olefins. The resulting 3t's have extraordinarily long lifetimes in the order of 102 s, and therefore can, in competition with their relatively slow decay, undergo energy transfer to the cis isomer to regenerate 3c, thus accomplishing a quantum chain process. The characteristic feature of the potential energy surface was clarified with 2-(ethenyl~2-d)anthracene (le-d) in which there is an energy barrier of 11 kcal/mol between 3c and 3t. 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. © 1988 IUPAC