Novel insights into photoisomerization of olefins

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Abstract
Aromatic olefins like anthrylethylenes (1a-1d) 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 stilbenses and other olefins. The resulting 3t*''s have extraordinarily long lifetimes in the or of 102 .mu.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 (1e-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.

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