Adiabatic Pathways in the Conformational and Geometrical Photoisomerizations of the 1,2-Distyrylbenzene Isomers

The photophysical behavior of the geometrical isomers of 1,2-distyrylbenzene (1,2-DSB) has been investigated as a function of temperature by stationary and pulsed fluorimetric techniques in a nonpolar solvent. The temperature effect on the spectral properties, fluorescence quantum yields, and decay profiles allowed the role played by the conformational isomers of these molecules in the relaxation processes of the lowest excited states to be clarified. In the case of the EE geometrical isomer, a conformer (or rotamer) re-equilibration, implying an adiabatic interconversion during the lifetime in the S-1 state, was found to be operative. The deactivation channels of the excited cis isomers (ZE and ZZ) markedly depend on the conformational geometry, which is responsible for the adiabatic (rotamer-specific) cis -> trans photoisomerization. The radiative and radiationless relaxation rate parameters of the isomers and conformers of 1,2-DSB, derived from the experimental and calculation results, allowed a complete and reasonably quantitative description of their behavior in the S-1 state.