SINGLE AND TWOFOLD CIS TRANS ISOMERIZATION OF Z,Z-1,4-BIS(1-PROPENYL)BENZENE ON TRIPLET-SENSITIZED EXCITATION

The mechanism for triplet sensitized cis-trans photoisomerization of t he isomers of 1,4-bis(1-propenyl) benzene, denoted by ZZ, EZ and EE, h as been studied by quantum yield measurements and laser flash photolys es. The triplet lifetime is long enough to give an isomerization patte rn that is essentially determined by the thermodynamics of the triplet excited state surface, that is, equilibration processes are faster th an decay processes. Decay from the tripler excited state surface occur s predominantly from (3)E,p, but there is also a minor decay from (3) Z,p. Thus, on sensitized excitation of ZZ there are about equal amoun ts of single and twofold isomerization to EZ and EE respectively. The photostationary state composition ZZ:EZ:EE is 1.5:49:49.5. Addition of azulene as a quencher to the reaction mixture increased the amount of twofold isomerization from ZZ to EE as well as the amount of EE at th e photostationary state. The T-1-T-n absorption spectra (lambda(max) = 390 nm) for all three isomers are identical (more than 100 ns after t he excitation pulse) as is the tripler lifetime (tau = 290 ns). Probab ly (3)E,p and (3)EE* are close to isoenergetic (about 1 kcal mol(-1) difference). The tripler energies of the isomers were estimated by mea suring the energy transfer rates from (3)biacetyl to the isomers. Tak en in the order ZZ, EZ and EE, the experimental values of the tripler energies were 56.4, 56.2 and 55.4 kcal mol(-1).