VAN-DER-WAALS COMPLEXES OF 2-CHLOROAZULENE, 2-METHYLAZULENE, AND 1,3-DIMETHYLAZULENE WITH RARE-GASES - MICROSCOPIC SOLVENT SHIFTS, STRUCTURES, AND BINDING-ENERGIES

The S-2-S-0(L-1(a)) fluorescence excitation and emission spectra of th e van der Waals complexes of three azulene (At) derivatives, 2-chloroa zulene (ClAz), 2-methylazulene (MAz), and 1,3-dimethylazulene (DMAz), with the rare gases, Ar, Kr, and Xe, have been measured under jet-cool ed conditions. The microscopic solvent shifts, delta(nu)over bar>, of the origin bands in the S-0-S-2 spectra associated with complexation o f the chromophores with one and two rare gas atoms increase with incre asing polarizability of the adatom(s), consistent with the dominance o f dispersion in the binding. Although there are substantial variations in the relative values of <delta(nu)over bar> among the Az derivative s examined, all of the <delta(nu)over bar> values are relatively small and are similar to those of the L-1(0)(S-0-S-1) transitions in the ra re 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 th at the variations in <delta(nu)over bar> with substitution pattern are primarily electronic in their origin and arise from variations in exc ited state configuration interactions, the magnitude of which depend o n the S-2-S-n energy spacings. These spacings can be varied by placing substituents either along the long axis (2-position) or parallel to t he short axis(1,3-positions) so that they selectively perturb, respect ively, the long axis polarized and the short axis polarized transition s. The structures and binding energies of the complexes of these deriv atives have also been modeled using Lennard-Jones type calculations an d have been compared with those of Az itself. The observed progression s in the low-frequency intermolecular vibrations in each case are assi gned to that excited state bending mode which is parallel to the long axis of the chromophore, in agreement with model calculations using on e-dimensional Morse and Taylor's series potential functions.