Concentration-Dependent Photoinduced Electron-Transfer Reactions. 1:1 and 1:2 Radical-Ion Complexes

For some exciplexes of 9,10-dicyanoanthracene (DCA) with naphthalenes and phenanthrenes as donors, the fluorescence quantum yields and lifetimes decrease with increasing donor concentration. The rate constants for these self-quenching reactions increase with decreasing redox energy of the pair and with increasing solvent polarity. Both parameters increase the charge-transfer (CT) character of the exciplex. When the CT character is low, the quenching rate constants are too small to measure (<10⁷ M⁻¹ s⁻¹), but with increasing CT character, to the limit that the exciplex becomes essentially a contact radical-ion pair, the interception rate constant approaches the diffusion-controlled limit. The product of the self-quenching reactions is a 1:2 radical-ion complex. In addition to exciplex self-quenching, the quantum yields for formation of separated radical ions decrease with increasing donor concentration when experiments are performed in the polar solvent acetonitrile. From these data and from an analysis of the spectral distribution of the exciplex fluorescence, information is obtained concerning the driving force dependence of the rates of return electron transfer in the 1:1 and 1:2 contact and the 1:1 solvent-separated radical ion complexes. The 1:1 solvent-separated pair has the highest solvent reorganization energy, λ_s (ca. 1.6 eV), and the 1:1 contact pair the lowest (ca. 0.35 eV). The 1:2 contact complex has an intermediate λ_s of ca. 0.9 eV. The electronic coupling matrix element for return electron transfer in the contact pairs is larger than that for the solvent-separated pairs by ca. 2 orders of magnitude. The corresponding matrix element for the 1:2 contact complex is closer to that for the contact pair than that for the solvent-separated pair. Due to the relatively high matrix element and λ_s, the rates of return electron transfer in the 1:2 contact pairs are high compared to the rates of solvation, which results in the smaller yields of separated radical ions at high donor concentrations.