Direct photoisomerization of the 1,6-diphenyl-1,3,5-hexatrienes. Medium effect on triplet and singlet contributions

Quantum yields for the interconversion of the all-trans-, cis,trans,trans- and trans,cis,trans-1,6-diphenyl-1,3,5-hexatrienes (DPH) in methylcyclohexa ne (MCH) or acetonitrile (AN) following 366 nm excitation show these proces ses to be relatively inefficient. Their dependence on the concentration of the DPH reveals significant participation of triplet states in the overall process. Despite very low intersystem crossing quantum yields (0.029 and 0. 010 in MCH and AN, respectively) singlet and triplet contributions in the p hotoisomerization of all-trans-1,6-diphenyl-1,3,5-hexatriene are roughly eq ual in MCH, and, for the trans,cis,trans isomer, in AN. However, in AN the cis,trans,trans isomer forms nearly exclusively by a singlet pathway from t he other two isomers. The cis,cis,trans isomer, a very minor component in p hotostationary states, appears to form primarily from the cis,trans,trans i somer whose excited singlet state also gives another isomer, tentatively id entified as ctc-DPH. The major radiationless channel of the excited singlet state of each DPH isomer is direct decay to the original ground state. Bar riers to torsional relaxation of the planar lowest DPH excited singlet stat es (2(1)A(g) and 1(1)B(u)) must be significantly higher than previously sup posed. Photoisomerization quantum yields of all-trans-DPH in the presence o f fumaronitrile (FN) are also separated into singlet and triplet contributi ons. Fumaronitrile quenches DPH fluorescence and singlet contributions to t he photoisomerization equally, but enhances DPH triplet formation and the t riplet contribution to the photoisomerization. Radical cations of DPH form in AN but do not participate in isomer interconversion.