Red-edge excitation fluorescence study of the inhomogeneous broadening of electronic transitions in solution

Inhomogeneous broadening of the electronic spectra of three dyes in polar H-bonding solvents at room temperature is investigated. The excitation-wavelength dependence of the fluorescence decay profiles reveals the existence of an inhomogeneous component. Time-resolved fluorescence spectroscopy and the solvent dependence of the fluorescence-decay kinetics reveal the origin of the broadening to be a distribution of solvent-solute orientations. The inhomogeneous broadening is not static, but has a lifetime related to the dielectric relaxation time of the solvent; thus the behaviour observed is more properly described as a spectral diffusion phenomenon. At low temperature, where solvent relaxation is inhibited, a weak dependence of fluorescence lifetime on static solvent-solute orientation is detected.