Fluorescence line-narrowing difference spectra: Dependence of Huang–Rhys factor on excitation wavelength

It is well known that fluorescence line narrowed (FLN) difference spectra (ΔFLN spectra) reduce to single-site fluorescence line shape function in the low-fluence limit. This finding is confirmed by the observation of different line shapes of FLN and ΔFLN spectra obtained for pyrene (Py) in ethanol glass at various excitation wavelengths using time-resolved fluorescence measurements. A peculiar dependence of the Huang–Rhys factor (S) on excitation wavelength was discovered, and discussed in terms of the solute–solvent, intermolecular Lennard–Jones potential. A V-shaped frequency dependence of the S-factor suggests that the repulsive–dispersive potential is not sufficient to account for a complex ‘color effect’.