S₂-S₀ Spectroscopy of Azulene and Its 2-Chloro, 2-Methyl, and 1,3-Dimethyl Derivatives

The laser-induced S₂-S₀ fluorescence excitation and dispersed emission spectra of azulene and three azulene derivatives of C_2v skeletal symmetry, 2-chloro-, 2-methyl-, and 1,3-dimethylazulene, have been measured under ultracold, isolated conditions in a supersonic expansion. The FT-IR spectra of the derivatives have been measured, and the ground state vibrational frequencies have been calculated, with scaling, from the force fields by an ab initio method at the RHF/ CEP-31G level for all four compounds to aid in the assignments of these spectra. To achieve global consistency, the vibrational assignments of the ground and second excited singlet states of azulene have required refinement; the reassignments have implications for the vibronic and vibrational coupling models used in existing interpretations of its S₂-S₀ spectra and the S₂ photophysics. The results suggest that small changes of the energy gap between the S₂ and S₁ states which result from substitutions in the ring produce large changes in both the S₂-S₀ fluorescence excitation spectra and the photophysics of azulene. Substitution at the 2-position accelerates the rate of radiationless relaxation, whereas this rate remains nearly unaffected by substitution at the 1,3-positions when compared with azulene itself. The threshold for intramolecular vibrational redistribution is significantly reduced in the methyl-substituted azulenes, consistent with an increase in the density of coupled states.