Mechanism of cis-trans photoisomerization of conjugated trienes

Abstract

Studies of geometric isomerization reactions of alloocimene, a model conjugated triene, have shown significant difference of photochemical behavior of triene triplets and excited singlets. Therefore, intersystem crossing is not an important process in trienes. The results suggest that the triplet intermediates are in rapid equilibrium. Data also show that excited state bond orders cannot be used reliably in predicting the direction of isomerization. Experimentally, the results are more consistent with direction of isomerization controlled by steric factors. Self-quenching is evident because there is a rather drastic dependence of photostationary state composition upon alloocimene concentration. An even more drastic dependence of photostationary state compositions upon quencher (azulene) concentration also exists. Reminiscent to the stilbene system, the triene triplets are quenched selectively to almost 100% central-trans isomers. This is an indication of the quenching processes originated primarily from the central-trans species. Based on a scheme which quantitatively accounts all presently existing data, all the rate constants are determined. The average life of the rapidly equilibrating triplet intermediates of alloocimene is calculated to be 2.9 x 10- 7 sec. Quenching studies for 2, 6-dimethyl-1, 3, 5-heptatriene and 1,3, 5-hexatriene showed similar results. A scheme with a quantum chain is constructed which is similar to but simpler than alloocimene, since only two isomers are involved. Quantum yield expressions showed linear dependence of quantum yield in both directions on triene concentrations, also the sum of quantum yields should always exceed one. Experimentally, the linear relationships is indeed verified, however sum of quantum yield drops below one at lower concentrations for 1,3, 5-hexatriene. Direct decay from the planar trans excited species back to the ground state was postulated to account for the quantum loss. Quantum yield is largest for 2,6-dimethylheptatriene, while those for 1,3, 5-hexatriene is larger than alloocimene. The role of cisoid triene as an internal quencher was invoked to account for the differences.