Triplet-State 1- and 2-fold Cis-Trans Isomerization of Bisstyrylarenes: Balance between Diabatic and Adiabatic Mechanisms

Article abstract of DOI:10.1021/j100196a034

The mechanism for triplet-sensitized cis-trans photoisomerization of three bisstyrylarenes, 1-3, has been studied by quantum yield measurements and laser flash photolysis.The triplet-state lifetimes for all three molecules are long enough to give an isomerization pattern essentially determined by the thermodynamics of the triplet excited-state surface, that is, equilibration processes are much faster than decay processes.Decay from the triplet excited-state surface occurs to various degrees from two equilibrated states, the planar ³E,E^* and another with one double bond twisted, ³E,p^*.Extension of ?-conjugation gives more 2-fold cis-trans isomerization, from the Z,Z isomer to the E,E isomer, due to a more facile adiabatic than diabatic pathway for the isomerization reaction.For one of the molecules, 9,10-bisstyrylanthracene, 3, the Z,Z isomer undergoes exclusively 2-fold isomerization to yield the E,E isomer. 1,4-Bis(3,5-di-tert-butylstyryl)naphthalene, 2, shows mainly 2-fold isomerization from the Z,Z isomer, but there is also a diabatic contribution to this isomerization giving a photostationary state, ZE:EE, 18:82.For 4,4'-bis(3,5-di-tert-butylstyryl)biphenyl, 1, the isomerization is still partly adiabatic.There is no decay from the ³Z,p^* but all decay occurs from ³E,p^*, giving a photostationary state ZE:EE, ca. 50:50.The photostationary states for the bisstyrylnaphthalene and the bisstyrylbiphenyl can be affected by the addition of a triplet quencher, azulene or perylene, to give almost pure E,E.