Solvent Dependence of Electronic Relaxation in All-trans Retinal Studied by One- and Two-Photon Induced Transient Absorption

Abstract

One- and two-photon excited ultrafast transient absorption measurements are reported for all-trans retinal in polar, nonpolar, and hydrogen-bonding solvents. A comparison of the response following one- and two-photon excitation allows assignment of the observed decays to the 1Bu+-like and 1Ag--like excited states. In the polar, hydrogen-bonding solvent ethanol, the transient absorption changes following one-photon excitation are dominated by a 2-ps response observed both in the decay of transient absorption and in the decay of stimulated emission observed in the region from 600 to 750 nm. Because of the strong stimulated emission gain, this response can be assigned to decay of the strongly optically allowed “1Bu+” state. With two-photon excitation of all-trans retinal in ethanol, a 200−300 fs decay component appears as well and is assigned to decay of the “1Ag-” state, which has a large two-photon cross section. In contrast, for all-trans retinal in hexane, both 200−300 fs and 2-ps decay components are observed following one-photon excitation, and no stimulated emission is detected. A comparison of one- and two-photon induced transient absorption leads to an assignment of the decay events. The ultrafast responses of all-trans retinal in the polar, aprotic solvents acetonitrile and propionitrile are found to be similar to the response in ethanol, indicating that protic solvents act through solvent polarity rather than a specific effect of hydrogen bonding. The effect of hydrogen bonding was studied further for all-trans retinal in mixed solvents with varying concentrations of trifluoroacetic acid, a strong hydrogen-bond donor, in hexane. As the ratio of trifluoroacetic acid to retinal increased from 1:5 to 10:1, the ultrafast response changed smoothly from “hexane-like” to “ethanol-like”, with an increasingly strong stimulated emission component having a 2-ps decay, in agreement with the increased fluorescence quantum yield observed in polar and hydrogen-bonding solvents. A model is proposed incorporating two populations of retinal having different electronic relaxation pathways to explain the solvent-dependent photophysics of all-trans retinal.