Solvent Effects on Charge-transfer Fluorescence Bands

Abstract

CHARGE-TRANSFER (CT) transitions in complexes with an ionic ground state, leading as they do to annihilation of the charge, are particularly sensitive to the polarity of the solvent. Such transitions have been used to derive the empirical solvent polarity parameters Z (ref. 1) and E_T (ref. 2) which correlate well with the transition energies of other complexes with charged ground states^3–5. Complexes in which the ground state is non-ionic show, on the other hand, little correlation between the energy of the charge-transfer transition and E_T (or Z), because of the Franck–Condon restriction on reorganization of the solvent during the transition. The CT fluorescence transitions in such complexes should, however, correlate well with Z and E_T provided that the lifetime of the excited state is sufficient to allow reorganization of the solvent. The energy of the CT fluorescence transition should thus decrease with an increase in solvent polarity, the reverse of the trend observed for the absorption process. Few complexes have been found to fluoresce in fluid solution^6,7, and these only in a few solvents, mainly of very low polarity. The fluorescence band maxima measured in a range of solvents using a Baird-Atomic SF100 spectrofluorimeter for the complex hexamethylbenzene–tetrachlorophthalic anhydride are shown in Table 1, and Fig. 1 is a plot of the fluorescence band energy, E_CT, against the solvent polarity parameter, E_T. A good straight line with a slope close to unity (0.91) is obtained, showing that solvent stabilization of the excited state is very similar to that of the ground state in ionic complexes.