Photochemistry of phenylfulgides XXI: ultrafast photoisomerization and radiationless deactivation processes in α,δ-di-(4-alkoxyphenyl)-fulgides and α,δ-bifluorenylfulgides

Abstract

The photoisomerization processes of two α,δ-di-(4-alkoxyphenyl)-fulgides 1 and 2 and the radiationless deactivation processes of an α,δ-bifluorenylfulgide 3 were investigated by means of a picosecond excite-and-probe beam spectrometer. For 1 and 2 an ultrafast formation of EZ and ZZ isomers in the reaction sequence ▪ was observed for τ ≤ 5 ps. These reactions due to the rotation of the bulky molecular parts may be the fastest which have been observed up to date. Compound 3 exhibits ground state bleaching in the spectral range of the S 1 ← S 0 transition with a recovery time of 12 ps. The observed (S n , n ≥3) ← S 1 excited state absorption disappears with the same decay time. The results are discussed in terms of the photoisomerization model used previously in the case of stilbene. Because of the large degree of steric hindrance in the EE and EZ isomers of 1 and 2 and in compound 3, the torsion about the ethylenic bonds to give perpendicular states which interact strongly and nonadiabatically with the ground state is considered to occur with a strong intramolecular driving force in the excited state, for which no activation barriers could be observed.