Inhomogeneous decay kinetics and apparent solvent relaxation at low temperatures

We have observed time-dependent shifts in the fluorescence spectra of solvatochromic probe molecules in frozen polar solvents. The phenomenon is a general one and is observed in a variety of fluorophores in both hydrogen-bonding and non-hydrogen-bonding solvents. These shifts are not related to solvent relaxation but rather result from inhomogeneous fluorescence decay kinetics. Molecules in different portions of the fluorescence spectrum decay at different rates and thereby cause the spectrum to evolve in time due to nonuniform loss of excited-state population. We have modeled such kinetics based on the assumption that fluorescence decay rates are solvent sensitive only through the relation between the radiative rate and emission frequency, k_rad ∝ v³. Measurements of the effect of excitation wavelength on steady-state fluorescence spectra are used to quantify the inhomogeneous broadening present in a number of fluorophore/solvent systems. These results are then used as input to model their time dependence. In most cases, nearly quantitative agreement between the calculated and observed spectral dynamics is achieved.