Bonded exciplexes. A new concept in photochemical reactions

(Graph Presented) 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.