Charge-transfer-to-solvent reactions of sodide in tetrahydrofuran investigated by ultrafast pump-probe spectroscopy

Electron transfer reactions in condensed phases are of fundamental importance in chemistry, physics and biology. The goal of this study is to determine how solvent structure and motions define the reaction coordinate for a simple, model liquid-phase charge transfer reaction: the photoinduced charge-transfer-to-solvent (CTTS) reaction of the sodide anion (Na−) dissolved in tetrahydrofuran (THF). Absorption of visible light promotes the detachment of an electron from this species, producing a solvated electron and neutral sodium atom, both of which can be monitored spectroscopically. Published synthesis procedures are used to generate the sodide samples. Ultrafast pump-probe spectroscopy is used to initiate and monitor the products of the charge transfer reaction. The data are fit to kinetic models to obtain reasonable hypotheses concerning the molecular motions responsible for CTTS; these hypotheses are subsequently verified through three-pulse experiments that manipulate the intermediates during the forward and back electron transfer reactions. We find that CTTS of Na− in THF proceeds via solvent-mediated detachment of the electron into different kinds of contact pairs, in which the electron is in relatively close proximity to its parent sodium atom. The electrons in these contact pairs may be manipulated with a sequence of light pulses, providing a new avenue of ultrafast control over a quantum mechanical degree of freedom. Although the mechanisms for CTTS and back electron transfer appear to be similar in solvents other than THF, the stability of each kind of contact pair depends differently on solvent polarity and equilibrium structure. Polarization anisotropy in the bleaching signal of sodide in THF can be explained by modeling the absorption band as a sum of four polarized sub-bands: the relative fraction of each type of contact pair that is produced as a function of excitation energy is determined largely by these same sub-bands.