Photochemistry of phenylfulgides XXII: Competing ultrafast radiationless deactivation, E—Z isomerization and electrocyclic ring-closure reactions

The photoisomerization about the αβ and γδ double bonds and the electrocyclic ring-closure reaction are competing processes within the excited singlet state of the phenylfulgides. Both processes are induced by the torsion of bulky molecular parts. Because of the extreme steric hindrance the torsion processes occur on potential curves without any activation barrier and consequently the photoisomerization, the photocyclization and the radiationless deactivation of the phenylfulgides are ultrafast processes proceeding within a few picoseconds, or even less, as the absorption measurements performed on a picosecond excite-and-probe beam spectrometer reveal. The competition between the E—Z isomerization and the cyclization is expressed by means of the partial quantum yields of these processes. The experimental results are interpreted in terms of the πbond orders and the sums of free-valence indices for the bondforming atoms in the S₁ state calculated by the Pariser—Parr—People method.