RELATING THE POLARITY-DEPENDENT FLUORESCENCE RESPONSE OF PYRENE TO VIBRONIC COUPLING - ACHIEVING A FUNDAMENTAL UNDERSTANDING OF THE PY POLARITY SCALE

We have investigated the fluorescence response of pyrene, a widely used polycyclic aromatic hydrocarbon probe molecule, in a series of solvents that span the empirical ''py'' polarity scale. The relative band intensities in the pyrene fluorescence response are determined by the extent of vibronic coupling between the weakly allowed first excited singlet state and the strongly allowed second excited singlet state. The solvent dependence of vibronic coupling in pyrene can be inferred from steady-state fluorescence measurements in conjunction with fluorescence lifetime and quantum yield measurements. These data, taken collectively, point to the presence of vibronic coupling in pyrene arising from two different mechanisms. The solvent ''polarity'' sensed by the pyrene molecule is related to dipole-induced dipole interactions between the solvent and pyrene vibrational modes of b(3g) symmetry which span the nuclear coordinates of the two excited electronic states.