Laser-induced flourescence excitation spectroscopy and photophysics of naphthalene bichromophoric molecules in supersonic jets

We present studies of interchromophore interactions under supersonic jet co nditions in a large number of dinaphthyl bichromophoric molecules by measur ing their laser-induced fluorescence (LIF) excitation spectra. The molecule s are composed of two naphthalene chromophores connected by an n-methylene bridge. The length of the bridge was varied as a function of the number of methylene units (n = 0, 1, 2, 4, 6), of the general type NnN(i,j'), were N denotes naphthalene moiety, n the number of methylene units in the bridge, and (i,j') are the alpha or beta positions of the bridge at each of the chr omophores. We obtained high-quality LIF spectra of these bichromophoric mol ecules. In the molecules N1N(2,2'), N1N(1,2'), N2N(2,2'), and N2N(1,2'), th e spectrum is characterized by an intense 0-0 region, with series of low-fr equency progressions. These progressions are assigned as vibrational modes of the bridge. The appearance of several series of progressions is explaine d either by the excitation of different chromophores (in the mixed molecule s) or by the excitation of different populated conformers. The spectrum of N4N(1,1') is different in several aspects from these spectra. The origin is shifted farther to the red, to 31,402 cm(-1). Low-frequency progressions o r other transitions are not observed near the origin, but typical intrachro mophore naphthalene vibrations are intense. The spectra of N6N(1,1') and N6 N(2,2') are also characterized by intense intrachromophore vibrations, howe ver, the spectrum of N6N(2,2') is very complicated dire to many populated c onformations, while that of N6N(1,1') is more simple.