| Calculation of the excited-state potential energy surface (PES) for a molecule with the size of styrene or 1,3-diphenylpropenes remains a challenge despite recent advances in computation methods. In order to obtain information about the PES of 1,3-diphenylpropenes, which undergo both photoisomerization and the di-pi-methane rearrangement, we developed a kinetic model, combined it with variable temperature measurements, and determined the barriers on the singlet PES. The effects of substituents on the barriers of the singlet photoisomerization and di-pi-methane rearrangement, and the efficiency of di-pi-methane rearrangement were discussed. This powerful methodology was also applied to investigations of the potential energy surfaces for the unimolecular photoisomerization reactions of styrenes and 2-vinylbiphenyls.; The barriers for photoisomerization for styrene and several methylated derivatives were found to show a strong correlation with the ground state dihedral angle between vinyl and benzene ring. Changing the dihedral angle results in an altered electronic structure for the ethylenic double bond in the frontier orbitals and excited states. The large singlet twisting barrier for planar styrenes arises from their shallow S2 potential. On the other hand, the small or zero barriers for twisted styrenes are due to the steep S3 energy surface.; 2-Vinylbiphenyls exist as a mixture of anti and syn rotational isomers. The equilibrium between these rotamers has been investigated by variable temperature NMR and theoretical ground state PES calculations. The rotamers display different photochemical behavior. The more stable anti rotamer is fluorescent, and undergoes singlet E,Z isomerization. The minor syn-rotamer is nonfluorescent and undergoes fast and efficient photocyclization to an intermediate which rearranges thermally to a 9,10-dihyrophenanthrene. 2-Vinyl-1,3-terphenyl exists as a single rotamer which undergoes efficient photoisomerization even at 77 K. The intermediate was first observed in organic glasses at low temperatures under steady-state irradiation conditions. The barrier for the rearrangement of the intermediate was determined to be very low (<2 kcal/mol), which may be ascribed to its highly exergonic nature. |
