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 the isomers of 1,4-bis(1-propenyl) benzene, denoted by ZZ, EZ and EE, has been studied by quantum yield measurements and laser flash photolyses. The triplet lifetime is long enough to give an isomerization pattern that is essentially determined by the thermodynamics of the triplet excited state surface, that is, equilibration processes are faster than decay processes. Decay from the tripler excited state surface occurs 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 amounts 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 the photostationary state. The T-1-T-n absorption spectra (lambda(max) = 390 nm) for all three isomers are identical (more than 100 ns after the excitation pulse) as is the tripler lifetime (tau = 290 ns). Probably (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 measuring the energy transfer rates from (3)biacetyl* to the isomers. Taken 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).