Nearly ubiquitous use in sunscreens o Strong UV

 Nearly ubiquitous use in sunscreens o Strong UV

Nearly ubiquitous use in sunscreens o Strong UV absorbers needed for adequate sun protection o Photostability is key to long lived & efficient products Avobenzone (4-tert-butyl-4methoxydibenzoylmethane) o Very high near-UV absorption o Degradation pathways lead to less effective protection M. Yamaji and M. Kida, The Journal of Physical Chemistry A 117 (9), 1946-1951 (2013)

cis-trans isomerization isomerization about single bonds to form rotamers enol-keto tautomerization Key intermediate for photodegradation Found in acetonitrile but not in other solvents A. Cantrell and D. J. McGarvey, Journal of Photochemistry and Photobiology B: Biology 64 (23), 117-122 (2001) M. Yamaji and M. Kida, The Journal of Physical Chemistry A 117 (9), 1946-1951 (2013) Stimulated Emission 350 nm

Transient Absorption Ground State Bleach 350 nm Singular value decomposition (SVD) guides global fitting & analysis Reveals two distinct species: o Fast component is assigned as excited state absorption and stimulated emission o Slow component is vibrationally excited molecules in the ground state tES

tvib Methanol 1.4 0.2 ps 6.3 1.0 ps Cyclohexane 420 40 fs 5.2 0.6 ps Acetonitrile 1.1 0.2 ps 5.9 1.2 ps

Small influence by solvent identity suggests primarily intramolecular IVR Sfeir, M. Visualization and Analysis of Broadband Transient Absorption Data IGOR.6.20.x-4.5. April 29, 2013 350 nm 350 nm Methanol

Cyclohexane Time Constants Acetonitrile t1 1.3 0.3 ps 1.1 0.2 ps 1.4 0.2 ps

t2 30 10 ps 59 8 ps 80 20 ps Amplitude Factors Fast 0.36 0.04

0.54 0.04 0.44 0.04 Slow 0.22 0.04 0.22 0.02 0.13 0.02 Offset

0.42 0.02 0.24 0.04 0.44 0.03 Timescales suggest three distinct non-chelated enol species involved Amplitude factors from fits provide a crude measure of relative yield

Utilized femtosecond resolved transient absorption experiments probing both in the visible and UV to elucidate a more complete picture of the photoisomerization dynamics of Avobenzone Currently working on calculations to back up our observations with respect to solvent dependence of various signals Avobenzone presents a nice target for vibrationally mediated photoisomerization (VIMP) in solution o Perhaps affect the product distribution in the 266 nm probing data? o The 350 nm mx absorption makes pumping at

400 nm an easy wavelength for vib. + elec. Excitation.

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