Intramolecular Donor-Acceptor Systems. 4. Solvent Effects on Radiative and Nonradiative Processes for the Charge-Transfer States of N-Arylaminonaphthalenesulfonates

Fluorescence lifetimes and fluorescence quantum yields for 6,2- and 8,1-N-arylaminonaphthalenesulfonates (ANS) have been measured in dioxane-water solutions having a wide range of solvent polarities, as measured by the empirical solvent polarity parameter, ET(30). The radiative rate constants for change-transfer emission (S1,ct→S0,np) and the nonradiative rate constants for electron-transfer reaction (S1,Ct→S0,np) are both sensitive to solvent. The greater sensitivity of the latter, knr, than the former, kr, to changes in solvent polarity accounts for the strong quenching observed for ANS derivatives in polar solvents. The solvent dependence of (kr)1/2 can be accounted for reasonably well in terms of a relationship based on substituent effects on emission energies. The linear dependence of the transition state free energy ΔG‡ (for knr) on ET(30) is explained by the “migration” of charge (i.e., screening) into the solvent shell. The ΔG‡ correlations for many of the ANS derivatives extrapolate to a common region, which corresponds roughly to the dielectric relaxation rate for water. The intramolecular electron-transfer rate is thus limited by a property of a suitable reaction field, Rs, in the solvent, with intramolecular factors such as donor-acceptor overlap and bond distance changes all being compatible with still higher electron-transfer rates. A mechanism for an intramolecular electron-transfer reaction is thus established. The relationship of the strong solvent effect on emission energies to the strong effect upon ΔG‡ is explained. Analysis of other results from the literature, using the ANS results for kT and knr as references, reveals that some quenching reactions may be electron transfer and some most certainly are not. A new quenching mechanism, formation of an unstable covalent bond, is proposed for several molecules. A classification of decay processes for excited charge-transfer states is also given.