Free Rotor Styrylcyanine Chromogens

Fluorescence spectra (maximum wavelengthλ F ) and fluorescence quantum yields ( φ F ) were measured for four structurally related styrylcyanine chromogens of the pyridinium and quinolinium type (1–4) in different solvents at ambient temperature and 77 K. The response ofλ Fandφ Fvalues to changes in solvent polarity, solvent hydrogen bonding donor strength, viscosity and temperature was a sensitive function of chromogen structure. The sensitivities of theλ Fandφ Fvalues correlate with the degree of charge transfer character of theS 1 , C Tstate; Stokes shift of fluorescence was progressively decreased whileφ Fvalue was enhanced as the CT character ofS 1 , C Tstate increases. Moreover, a large edge-excitation red shift was observed in ethanol glass at 77 K. The dominant photophysical features for these dyes are discussed in terms of strong emission from an intramolecular CT state characterized by different solvation sites indicated by the observation of the excitation-wavelength dependent phenomenon in ethanol at 77 K and an important non-radiative decay channel involving rotation of the different parts of molecules leading to a more relaxed weakly fluorescentS 1 , C Tcreated in fluid media. The viscosity dependence of fluorescence properties (a marked increase inφ Fwas observed with increasing viscosity) suggests that these dyes can be useful reporters of microviscosity for different sites in various organized assemblies. Moreover, it was suggested that increasing H-bonding donor strength of the solvent activates a rotatory non-radiative decay channel probably by localizing charge densities and decreasing CT nature of theS 1 , C Tstate.