S-2-S-0 SPECTROSCOPY OF AZULENE AND ITS 2-CHLORO, 2-METHYL, AND 1,3-DIMETHYL DERIVATIVES

The laser-induced S-2-S-0 fluorescence excitation and dispersed emissi on spectra of azulene and three azulene derivatives of C-2 upsilon Ske letal symmetry, 2-chloro-, 2-methyl-, and 1,3-dimethylazulene, have be en measured under ultracold isolated conditions in a supersonic expans ion. The FT-IR spectra of the derivatives have been measured, and the ground state vibrational frequencies have been calculated, with scalin g, from the force fields by an nh initio method at the RHF/CEP-31G lev el for all four compounds to aid in the assignments of these spectra. To achieve global consistency, the vibrational assignments of the grou nd and second excited singlet states of azulene have required refineme nt; the reassignments have implications for the vibronic and vibration al coupling models used in existing interpretations of its S-2-S-0 spe ctra and the S-2 photophysics. The results suggest that small changes of the energy gap between the S-2 and S-1 states which result from sub stitutions in the ring produce large changes in both the S-2-S-0 fluor escence excitation spectra and the photophysics of azulene. Substituti on 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 intra molecular vibrational redistribution is significantly reduced in the m ethyl-substituted azulenes, consistent with an increase in the density of coupled states. (C) 1997 Academic Press.