Laser-Induced Flourescence Excitation Spectroscopy and Photophysics of Naphthalene Bichromophoric Molecules in Supersonic Jets

Abstract

We present studies of interchromophore interactions under supersonic jet conditions in a large number of dinaphthyl bichromophoric molecules by measuring their laser-induced fluorescence (LIF) excitation spectra. The molecules 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 α or β positions of the bridge at each of the chromophores. We obtained high-quality LIF spectra of these bichromophoric olecules. In the molecules N1N(2,2′), N1N(1,2′), N2N(2,2′), and N2N(1,2′), the spectrum is characterized by an intense 0–0 region, with series of low-frequency progressions. These progressions are assigned as vibrational modes of the bridge. The appearance of several series of progressions is explained either by the excitation of different chromophores (in the mixed molecules) 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 or other transitions are not observed near the origin, but typical intrachromophore naphthalene vibrations are intense. The spectra of N6N(1,1′) and N6N(2,2′) are also characterized by intense intrachromophore vibrations, however, the spectrum of N6N(2,2′) is very complicated due to many populated conformations, while that of N6N(1,1′) is more simple.