Probing the photodynamics of photoprotective molecules and two-photon activated metal complexes

The work presented in this thesis consists of three distinct areas, each of which is explored using solution-phase femtosecond transient electronic (ultraviolet/visible) absorption spectroscopy. The first area is the effects of the environmental surroundings on the intrinsic photodynamics of a molecule. The second area is the photodynamics responsible for the apparent photostability of a pair of photoprotective molecules. Finally, we explore the effects of two-photon excitation on a ruthenium metal complex. The overarching link between these areas, which at face value appear disconnected, is the idea of the protection of biological systems from the deleterious effects of exposure to solar ultraviolet. The first experiment studies the solvation effects on the photodynamics of 4-tert-butyl-1,2-dihydroxybenzene, a motif of an ultraviolet radiation absorbing chromophore subunit of eumelanin, which serves in the body’s natural photoprotection mechanisms. This work demonstrates that the level of solvent interaction can have a drastic effect on the inherent photodynamics. In the case of a weakly interacting solvent the observed dynamics display a similarity to those observed of the molecule in the isolated environment of the gas-phase, which consist of an ultrafast O–H bond dissociation. However, when placed in a strongly interacting solvent, the dynamics change significantly. Rather than an ultrafast dissociation, a multitude of decay pathways occur instead and take place on the order of nanoseconds. The second experiment studies the ultrafast deactivation route for a pair of sunscreening agents, 3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid and 3-(3,4-dihydroxyphenyl)-2-propenoic acid. In this work, we highlight an ultrafast photoisomerization pathway between the trans- and cis-isomers. This involves a population transfer between two excited states, aided by a conical intersection and subsequently through a second conical intersection between an excited state and ground state along the isomerization coordinate, to regenerate the original trans-isomer or the cis-isomer. The third experiment studies the role of two-photon activation on a ruthenium poly-pyridyl complex and compares this to the one-photon activation process. In this work we demonstrate that both methods of activation result in the same photoproduct and on comparable timescales and yields. In doing so, we demonstrate that the observed excited state dynamics appear to be independent of the excitation method, which may have repercussions in the design of next generation photochemotherapy drugs