Fluorescence Anisotropy Spectra Disclose the Role of Disorder hi Optical Spectra of Branched Intramolecular-Charge-Transfer Molecules

Fluorescence excitation and emission spectra and corresponding anisotropies of branched intramolecular charge-transfer (ICT) chromophores are reported. To unravel the role of disorder related to polar solvation, we collected spectra of molecules with octupolar (C-3) symmetry in glasses obtained from solvents of different polarity. Thermal disorder related to polar solvation is demonstrated by the dependence of fluorescence excitation/emission spectra and anisotropy spectra on the emission/excitation wavelength. In particular, for excitation in the red edge of the absorption band, the fluorescence anisotropy signal deviates from the 0.1 value expected for octupolar chromophores, approaching the limiting 0.4 value. A qualitatively different result is observed in nonpolar solvents. Based on essential-state models for ICT chromophores, we develop an original approach for the calculation of anisotropy spectra that quantitatively reproduces experimental data. The model, accounting for the coupling between electrons and molecular vibrations and polar solvation, leads to a thorough understanding of the phenomena at the basis of solute-solvent interactions in branched ICT chromophores. A clear distinction is made between symmetry-breaking phenomena in the excited state and disorder-induced lowering of the molecular symmetry. Their relation with red-edge effects is pointed out.