Monomer and Excimer Fluorescence of 2-Phenylindole in p-Xylene at 20 °C Studied by Frequency-Domain Fluorometry

The effect of 2-phenylindole (2PI) concentration in p-xylene upon the fluorescence decay time was investigated by frequency-domain fluorometry. The fluorescence decay of a dilute (10~5 M) 2PI solution in p-xylene at 20 °C is single exponential with the mean decay time r = 1.87 ns and practically independent of the observation wavelength (λobs) in the fluorescence band. The fluorescence decay in a highly concentrated solution (6 x 10−3 M) is heterogenic and depends on λobs. For a given modulation frequency (~ 200 Hz), the fluorescence phase shifts at the longwave observation (440–480 nm) distinctly exceed 90°, thus clearly indicating that the observed (excimer) fluorescence results from a process taking place in the excited state. The fluorescence decays were measured at seven values of λobs (360–480 nm), and all the results were analyzed comprehensively (τ1 and τ2 are equal for all λohs; the amplitudes are different). The lifetimes of the excimer state is relatively short (~ 3.5 ns). Radiative (τe) and non-radiative (τ1) rate constants for monomer fluorescence of 2PI in p-xylene were determined using appropriate data from photo-and radioluminescence measurements.