SINGLET-STATE DIABATIC AND ADIABATIC CIS-TRANS PHOTOISOMERIZATIONS OF 4,4'-BIS(3,5-DI-TERT-BUTYLSTYRYL)STILBENES

The mechanism for photoisomerization of the six isomers, 1-6, of 4,4'-bis(3,5-di-tert-butylstyryl)stilbene has been investigated. The isomerizations always proceed from cis to trans configurations, and single isomerizations are dominating. By increasing the solvent polarity from methylcyclohexane to toluene and to methylene chloride, an increase of the quantum yields, particularly for isomerization of the central double bond, was observed. Thus, there is a selectivity effect; in methylcyclohexane isomerization of an edge double bond is favored, while in methylene chloride isomerization of a middle double bond is favored. There is also clear evidence of an adiabatic contribution to the isomerization of the central double bond. The diabatic part of the isomerization quantum yield is most strongly affected by the solvent. A solvent with higher polarity lowers the energy of an excited state with 90-degrees twist of the double bond; that is, a polar solvent will stabilize a zwitterionic excited singlet state.