Supersonic jet studies on the photophysics of substituted benzenes and naphthalenes

Laser-induced fluorescence (LIF) studies of seven substituted benzene and naphthalene derivatives seeded in a supersonic jet are reported. Dual fluorescence has been observed in aniline, 4-aminobenzonitrile, 1-aminonaphthalene, and 1-cyano-4-aminonaphthalene. The dual fluorescence is comprised of sharp features from a locally excited state and an unstructured, red-shifted background from a state that may be a mixed L-a/CT state. In aniline and 4-aminobenzonitrile the unstructured background is absent when low vibronic levels are excited but appears when higher levels are excited. In I-aminonaphthalene and 1-cyano-4-aminonaphthalene, the background is observable upon excitation of all vibronic levels including the zero-point level. In all of the molecules, the intensity of the background increases with respect to that of the sharp component as the excitation energy is increased. The background becomes the only observable feature in the spectra at high excess energy. The broadening and red shift of the band center is difficult to explain solely by internal vibrational relaxation (IVR). It is attributed in part to emission from another electronic excited state with perhaps some charge-transfer character.