Charge Transfer and Reactivity of nπ* and ππ* Organic Triplets, Including Anthraquinonesulfonates, in Interactions with Inorganic Anions: A Comparative Study Based on Classical Marcus Theory

The study of rates and radical yields in charge-transfer (CT) interactions between organic triplets and simple anions has been extended to triplets of 1-sulfonate, 1, 5-disulfonate, and 2, 6-disulfonate derivatives of 9, 10-anthraquinone and of fluorescein dianion. New information is also presented on 1, 4-naphthoquinone. For comparison, H-atom-transfer reactions of the anthraquinone triplets with 2-propanol were also studied. The new triplet-anion results, together with many previously reported data, are analyzed in the framework of a simplified Marcus theory by which the activation energy of formation of the pure charge-transfer exciplex, ΔG^‡, was calculated and correlated with the rate constant k_q. Plots of log k_q vs ΔG^‡(calcd) for the various systems reveal three groups of roughly linear correlations. The energetically favored interactions (mostly for I⁻, N₃⁻, SCN⁻, and NO₂⁻ with ΔG°_CT ≤ 0.2 eV) display the theoretical slope-1 /(2.3RT). For endoergic interactions, two additional straight lines appear with successively smaller slopes that relate both to the respective magnitudes of ΔG°_CT and to specific anion effects. This behavior is interpreted in terms of partial charge transfer in the reaction complex. Our comparative study bears also on the question of nπ* vs ππ* reactivity in charge-transfer interactions. No intrinsic difference in CT reactivity between these two electronic configurations is found either in their quenching kinetics or in the quantum yields of resulting radicals. The reactivity of the organic triplet depends essentially on its thermodynamic properties (reduction potential and triplet energy). That of the anion depends also on specific properties, including its (large) reorganization energy (affecting the quenching kinetics) and spin-orbit coupling within the incipient inorganic radical (affecting the bulk radical yield). For anions that contain H-atom (such as HCO₂⁻), the possibility of H-transfer is suggested in some cases. Also discussed, in light of the new results, is the difference in reactivity between 1- and 2-sulfonated derivatives of anthraquinone, representing respectively “weak” and “strong” sensitizers.