Competition between vibrational relaxation and photochemistry: Relevance of vibronic quantum effects

Two molecules showing photochemistry but no fluorescence have been investig ated at 80 K in a rigid matrix regarding the behavior of the quantum yield for bond fragmentation as a function of the vibrational/vibronic level and electronic excited state. A new equation was developed to determine the pho tochemical quantum yield under ambient conditions (80 K). The levels/bands involved were those within a given vibrational progression, in different pr ogressions as well as in combination. The yield was low (Phi = 0.1) with ex citation into the n = 0 level of S-1 but very rapidly increased with excita tion into higher levels whether they were harmonies or combination levels. A parallel result was observed upon excitation into S-2. Vibrational relaxa tion/deactivation occurs only between levels of the same vibrational progre ssion. Deactivation from the 0 level Of S-2 does not occur via levels of S- 1. The photochemically active modes correspond to the vibrational modes pre sent in the region of the molecule where bond breakage occurs. These result s add further proof of the complex nature and number of processes that can occur within excited states of photochemically active molecules.