TRIPLET-STATE 1-FOLD AND 2-FOLD CIS TRANS ISOMERIZATION OF BISSTYRYLARENES - BALANCE BETWEEN DIABATIC AND ADIABATIC MECHANISMS

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 pi-conjugation gives more 2-fold cis-trans isomerization, from the Z,Z isomer to the E,E isomer, due to aa 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 aa triplet quencher, azulene or perylene, to give almost pure E,E.