TRIPLET-STATE Z/E ISOMERIZATION OF A P-STYRYLSTILBENE, A PARTLY ADIABATIC PROCESS

The quantum yields for biacetyl-sensitized Z/E photoisomerization of the isomers of 3,3′′,5,5′-tetra-tert-butyl-4′-styrylstilbene, ZZ, ZE, and EE, have been determined. ZZ and ZE showed increased isomerization quantum yields with increased concentration due to a quantum chain process with 3EE* as the chain-carrying species. The photostationary state consists of a mixture of ZE and EE with more of the latter at higher concentration. Addition of azulene to the photoisomerization reaction drastically increases the proportion of EE at the photostationary state from ca. 50% to 99%. The T1-Tn absorption spectra for all three isomers are identical as is the triplet lifetime (τ = 3.5 μs at 298 K). From a temperature study of the triplet lifetime (τ = 120 μs at 193 K) we conclude that there are two important triplet-state conformations: one with a planar E,E structure and the other where one double bond is twisted close to 90°. The energy difference between the two conformations was determined to be 4.4 ± 0.5 kcal mol-1, with the planar structure being the more stable. From quenching experiments the triplet energy of EE was determined to be 41 kcal mol-1. © 1990 American Chemical Society.