RED-EDGE EXCITATION FLUORESCENCE STUDY OF THE INHOMOGENEOUS BROADENING OF ELECTRONIC-TRANSITIONS IN SOLUTION

Inhomogeneous broadening of the electronic spectra of three dyes in po lar II-bonding solvents at room temperature is investigated. The excit ation-wavelength dependence of the fluorescence decay profiles reveals the existence of an inhomogeneous component. Time-resolved fluorescen ce spectroscopy and the solvent dependence of the fluorescence-decay k inetics reveal the origin of the broadening to be a distribution of so lvent-solute orientations. The inhomogeneous broadening is not static, but has a lifetime related to the dielectric relaxation time of the s olvent; thus the behaviour observed is more properly described as a sp ectral diffusion phenomenon. At low temperature, where solvent relaxat ion is inhibited, a weak dependence-of fluorescence lifetime on static solvent-solute orientation is detected.