Femtosecond transient spectroscopy of the photoionization of indole in water

The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. Indole is a chromophore of many nitrogen heterocyclic biomolecules, such as, the essential amino acid tryptophan or the highly effective anticancer agent indole-3-carbinol. It is a model compound to study the response of these molecules to photoexcitation. Photoionization of organic compounds in solution is of large interest since it contributes to the damage of biological substances by UV radiation. The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. In cyclohexane population, transfer from the optically excited La to the Lb state happens within 7 ps. In ethanol, ultrafast state reversal is observed, followed by population transfer from the Lb to the La state within 6 ps. In water, ultrafast branching occurs between the fluorescing state and the charge-transfer-to-solvent state. Presolvated electrons, formed together with indole radicals within our time resolution, solvate on a timescale of 350 fs.