Fast redox perturbation of aqueous solution by photoexcitation of pyranine

Intense illumination (60-120 MW/cm²) of an oxygen-free aqueous solution of pyranine (8-hydroxypyrene-1,3,6-trisulfonate) by the third harmonic frequency of an Nd-Yag laser (355 nm) drives a two successive-photon oxidative process of the dye. The first photon excites the dye to its first electronic singlet state. The second photon interacts with the excited molecule, ejects an electron to the solution and deactivates the molecule to a ground state of the oxidized dye (Φ^+•). The oxidized product, Φ^+•, is an intensely colored compound (λ_max = 445 nm, ∈ = 43 000 ± 1000 M^-1 cm^-1) that reacts with a variety of electron donors like quinols, ascorbate and ferrous compounds. In the absence of added reductant, Φ^+• is stable, having a lifetime of ≃10 min. In acidic solutions the solvated electrons generated by the photochemical reaction react preferentially with H⁺. In alkaline solution the favored electron acceptor is the ground-state pyranine anion and a radical, Φ^-•, of the reduced dye is formed. The reduced product is well distinguished from the oxidized one, having its maximal absorption at 510 nm with ∈ = 25 000 ± 2000 M^-1 cm^-1. The oxidized radical can be reduced either by Φ^-• or by other electron donors. The apparent second-order rate constants of these reactions, which vary from 10⁶ up to 1O⁹ M^-1 s^-1, are slower than the rates of diffusion-controlled reactions. Thus the redox reactions are limited by an energy barrier for electron transfer within the encounter complex between the reactants.