Bonded Exciplexes. A New Concept in Photochemical Reactions

• Published in: Journal of organic chemistry 2007-08, Vol.72 (18), p.6970-6981
• Main Authors: Wang, Yingsheng, Haze, Olesya, Dinnocenzo, Joseph P, Farid, Samir, Farid, Ramy S, Gould, Ian R
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Free radicals chemistry; General and physical chemistry; Heterocyclic compounds; Heterocyclic compounds with only one n hetero atom and condensed derivatives; Organic chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Preparations and properties; Reactivity and mechanisms
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo071157d

Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5−0.8 eV. Specifically, the reactions with pyridine display rapid quenching when conventional electron transfer is highly endothermic. As an example, the rate constant for quenching of the excited dicyanoanthracene is 3.5 × 109 M-1 s-1, even though formation of a conventional radical ion pair, A•-D•+, is endothermic by ∼0.6 eV. No long-lived radical ions or exciplex intermediates can be detected on the picosecond to microsecond time scale. Instead, the reactions are proposed to proceed via formation of a previously undescribed, short-lived charge-transfer intermediate we call a “bonded exciplex”, A-−D+. The bonded exciplex can be formally thought of as resulting from bond formation between the unpaired electrons of the radical ions A•- and D•+. The covalent bonding interaction significantly lowers the energy of the charge-transfer state. As a result of this interaction, the energy decreases with decreasing separation distance, and near van der Waals contact, the A-−D+ bonded state mixes with the repulsive excited state of the acceptor, allowing efficient reaction to form A-−D+ even when formation of a radical ion pair A•-D•+ is thermodynamically forbidden. Evidence for the bonded exciplex intermediate comes from studies of steric and Coulombic effects on the quenching rate constants and from extensive DFT computations that clearly show a curve crossing between the ground state and the low-energy bonded exciplex state.