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Van der Waals Complexes of 2-Chloro-, 2-Methyl-, and 1,3-Dimethylazulene with Rare Gases: Microscopic Solvent Shifts, Structures, and Binding Energies

The S2-S0( 1 La) fluorescence excitation and emission spectra of the van der Waals complexes of three azulene (Az) derivatives, 2-chloroazulene (ClAz), 2-methylazulene (MAz), and 1,3-dimethylazulene (DMAz), with the rare gases, Ar, Kr, and Xe, have been measured under jet-cooled conditions. The microscopic solvent shifts, ‰O j, of the origin bands in the S0-S2 spectra associated with complexation of the chromophores with one and two rare gas atoms increase with increasing polarizability of the adatom(s), consistent with the dominance of dispersion in the binding. Although there are substantial variations in the relative values of ‰O j among the Az derivatives examined, all of the ‰O j values are relatively small and are similar to those of the 1 L b(S0-S1) transitions in the rare gas complexes of naphthalene and its methyl-substituted derivatives. The theory of microscopic solvent shifts of Jortner et al. has been used to analyze the solvent shift data. Comparisons of the sources of the oscillator strengths and van der Waals binding interactions in the azulene and naphthalene-rare gas systems are revealing and suggest that the variations in ‰O j with substitution pattern are primarily electronic in their origin and arise from variations in excited state configuration interactions, the magnitude of which depend on the S2-Sn energy spacings. These spacings can be varied by placing substituents either along the long axis (2-position) or parallel to the short axis( 1,3-positions) so that they selectively perturb, respectively, the long axis polarized and the short axis polarized transitions. The structures and binding energies of the complexes of these derivatives have also been modeled using Lennard-Jones type calculations and have been compared with those of Az itself. The observed progressions in the low-frequency intermolecular vibrations in each case are assigned to that excited state bending mode which is parallel to the long axis of the chromophore, in agreement with model calculations using one-dimensional Morse and Taylor’s series potential functions.