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van der Waais complexes of perylene with 1-chlorobutane and 1-chloropentane, cooled in a supersonic jet, each exhibit at least three conformational isomers in the electronic ground state, which appear in the fluorescence excitation spectra as distinct band combinations. In emission from the zero-point levels in S,, the spectra of the different conformers are very similar to each other and to that of free perylene but are displaced according to the positions of the excitation resonances. For comparison, the corresponding «-pentane and «-hexane complexes exhibit simple spectra and single isomers. At 355 cm'1 internal energy, the emission spectra of the pentane and hexane complexes, measured with =¿25 cm"1 resolution, each show a symmetric broadening consistent with extensive vibrational coupling, while some conformers of the alkyl halide complexes give rise to shifted spectra. The spectral shifts indicate that activated barrier crossing between different conformers is taking place. A semiquantitative model is presented for differences in the relative well depths for different sites.

The S2(*B1u) -* S0('Alg) fluorescence spectrum of coronene in solution, obtained by using both single-and two-photon excitation, is reported. The spectra, which depend on the excitation conditions, show that even at 300 K vibrational relaxation is substantially incomplete on the time scale of the S2 lifetime. Single site excitation spectroscopy at 12 K in «-heptane has been used for vibrational frequency assignments. With the exception of a weak, matrix-induced resonance at 370 cm"1, the set of vibronically active modes which defines the S2 •*-* S0 spectrum is found to be distinct from that for S, +» S0, although there is some overlap with the phosphorescence spectrum. The minimum line widths observed in fluorescence excitation of S0 -* S2 coronene were ~5 cm"1.

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