Evidence for bi-exponential decay of pyrene in condensed phase: Possibility of complex formation with solvent molecules

Lifetime of pyrene fluorescence has been measured in methanol and benzene–acetonitrile binary solvent of permittivity 15 along with the quenching effect by dimethylaniline (DMA) in these two solvents. In all cases pyrene fluorescence exhibits a bi-exponential decay. The data strongly points to the possibility of reversible complex formation of excited pyrene molecule with the solvent molecules (a few percent only) in addition to the existence of free excited pyrene molecules (majority). It has been observed that the fluorescence spectra of these two species differ significantly.