TRANS-CIS PHOTOISOMERIZATION OF THE STILBENES AND A RE-EXAMINATION OF THE POSITIONAL DEPENDENCE OF THE HEAVY-ATOM EFFECT

Larger azulene quenching effects are observed on trans → cis quantum yields than on trans/cis photostationary compositions for the direct photoisomerization of the stilbenes and the p-, m-, and m, m'-bromine substituted derivatives. Analysis of these data together with the temperature dependence of fluorescence quantum yields for each trans isomer in n-pentane indicates that (1) intersystem crossing yields based on photostationary states can be unreliable, (2) the singlet mechanism (1t*→1p*) is confirmed as the major isomerization pathway of Pww-stilbene singlets, (3) significant fractions of bromostilbene 1t* molecules intersystem cross, (4) almost all bromostilbene singlets (ø1p* ≥ 0.60) intersystem cross following twisting to 1p*, and (5) the positional dependence previously inferred for the heavy-atom effect is incorrect. Azulene effects on the cis → trans and the cis → DHP photoisomerizations are reported. The photoisomerization mechanism for the cis-bromostilbenes cannot involve 1p* as an exclusive intermediate. A pathway involving excited dihydrophenanthrenes is tentatively proposed. © 1979, American Chemical Society. All rights reserved.