Philip M. Coulter

Probing the Ultrafast Energy Dissipation Mechanism of the Sunscreen Oxybenzone after UVA Irradiation.

Oxybenzone is a common constituent of many commercially available sunscreens providing photoprotection from ultraviolet light incident on the skin. Femtosecond transient electronic and vibrational absorption spectroscopies have been used to investigate the nonradiative relaxation pathways of oxybenzone in cyclohexane and methanol after excitation in the UVA region. The present data suggest that the photoprotective properties of oxybenzone can be understood in terms of an initial ultrafast excited state enol → keto tautomerization, followed by efficient internal conversion and subsequent vibrational relaxation to the ground state (enol) tautomer.

Ultraviolet Absorption Induces Hydrogen-Atom Transfer in G⋅C Watson–Crick DNA Base Pairs in Solution

Ultrafast deactivation pathways bestow photostability on nucleobases and hence preserve the structural integrity of DNA following absorption of ultraviolet (UV) radiation. One controversial recovery mechanism proposed to account for this photostability involves electron-driven proton transfer (EDPT) in Watson-Crick base pairs. The first direct observation is reported of the EDPT process after UV excitation of individual guanine-cytosine (G⋅C) Watson-Crick base pairs by ultrafast time-resolved UV/visible and mid-infrared spectroscopy. The formation of an intermediate biradical species (G[-H]⋅C[+H]) with a lifetime of 2.9 ps was tracked. The majority of these biradicals return to the original G⋅C Watson-Crick pairs, but up to 10% of the initially excited molecules instead form a stable photoproduct G*⋅C* that has undergone double hydrogen-atom transfer. The observation of these sequential EDPT mechanisms across intermolecular hydrogen bonds confirms an important and long debated pathway for the deactivation of photoexcited base pairs, with possible implications for the UV photochemistry of DNA.

Recombination, Solvation and Reaction of CN Radicals Following Ultraviolet Photolysis of ICN in Organic Solvents

The fates of CN radicals produced by ultraviolet (UV) photolysis of ICN in various organic solvents have been examined by transient electronic and vibrational absorption spectroscopy (TEAS and TVAS). Near-UV and visible bands in the TEAS measurement enable direct observation of the CN radicals and their complexes with the solvent molecules. Complementary TVAS measurements probe the products of CN-radical reactions. Geminate recombination to form ICN and INC is a minor pathway on the 150 fs -1300 ps time scales of our experiments in the chosen organic solvents; nonetheless, large infrared transition dipole moments permit direct observation of INC that is vibrationally excited in the C≡N stretching mode. The time constants for INC vibrational cooling range from 30 ps in tetrahydrofuran (THF) to 1400 ps in more weakly interacting solvents such as chloroform. The major channel for CN removal in the organic solvents is reaction with solvent molecules, as revealed by depletion of solvent absorption bands and growth of product bands in the TVA spectra. HCN is a reaction product of hydrogen atom abstraction in most of the photoexcited solutions, and forms with vibrational excitation in both the C-H and C≡N stretching modes. The vibrational cooling rate of the C≡N stretch in HCN depends on the solvent, and follows the same trend as the cooling rate of the C≡N stretch in INC. However, in acetonitrile solution an additional reaction pathway produces C3H3N2(•) radicals, which release HCN on a much longer time scale.

Reaction Dynamics of CN Radicals in Acetonitrile Solutions

The bimolecular reactions that follow 267 nm ultraviolet photolysis of ICN in acetonitrile solution have been studied using transient absorption spectroscopy on the picosecond time scale. Time-resolved electronic absorption spectroscopy (TEAS) in the ultraviolet and visible spectral regions observes rapid production and loss (with a decay time constant of 0.6 ± 0.1 ps) of the photolytically generated free CN radicals. Some of these radicals convert to a solvated form which decays with a lifetime of 8.5 ± 2.1 ps. Time-resolved vibrational absorption spectroscopy (TVAS) reveals that the free and solvated CN-radicals undergo geminate recombination with I atoms to make ICN and INC, H atom abstraction reactions, and addition reactions to solvent molecules to make C3H3N2 radical species. These radical products have a characteristic absorption band at 2036 cm(-1) that shifts to 2010 cm(-1) when ICN is photolyzed in CD3CN. The HCN yield is low, suggesting the addition pathway competes effectively with H atom abstraction from CH3CN, but the delayed growth of the C3H3N2 radical band is best described by reaction of solvated CN radicals through an unobserved intermediate species. Addition of methanol or tetrahydrofuran as a cosolute promotes H atom abstraction reactions that produce vibrationally hot HCN. The combination of TEAS and TVAS measurements shows that the rate-limiting process for production of ground-state HCN is vibrational cooling, the rate of which is accelerated by the presence of methanol or tetrahydrofuran.

Translational, rotational and vibrational relaxation dynamics of a solute molecule in a non-interacting solvent

Spectroscopically observing the translational and rotational motion of solute molecules in liquid solutions is typically impeded by their interactions with the solvent, which conceal spectral detail through linewidth broadening. Here we show that unique insights into solute dynamics can be made with perfluorinated solvents, which interact weakly with solutes and provide a simplified liquid environment that helps to bridge the gap in our understanding of gas- and liquid-phase dynamics. Specifically, we show that in such solvents, the translational and rotational cooling of an energetic CN radical can be observed directly using ultrafast transient absorption spectroscopy. We observe that translational-energy dissipation within these liquids can be modelled through a series of classic collisions, whereas classically simulated rotational-energy dissipation is shown to be distinctly faster than experimentally measured. We also observe the onset of rotational hindering from nearby solvent molecules, which arises as the average rotational energy of the solute falls below the effective barrier to rotation induced by the solvent.