Insight into the Photodynamics of Photostabilizer Molecules

Abstract

Solar exposure of avobenzone, one of the most widely used commercial UVA filters on the market, is known to cause significant degradation. This finding has fueled research into developing photostabilizer molecules. In an effort to provide insight into their stand-alone photoprotection properties, the excited state dynamics of the photostabilizer, 3-(3,4,5-trimethoxybenzylidene) pentane-2,4-dione (TMBP), and its phenolic derivative, 3-(4-hydroxy-3,5-dimethoxybenzylidene) pentane-2,4-dione (DMBP), were studied with ultrafast transient absorption spectroscopy. Solutions of TMPB and DMBP in ethanol and in an industry-standard emollient, as well as TMBP and DMBP deposited on synthetic skin mimic, were investigated. These experiments were allied with computational methods to aid interpretation of the experimental data. Upon photoexcitation, these photostabilizers repopulate the electronic ground state via nonradiative decay within a few picoseconds involving a twisted intramolecular charge transfer configuration in the excited state, followed by internal conversion and subsequent vibrational cooling in the ground state. This finding implies that, aside from acting as a photostabilizer to certain UV filters, TMBP and DMBP may offer additional photoprotection in a sunscreen formulation as a stand-alone UV filter. Finally, TMBP and DMBP could also find applications as molecular photon-to-heat converters.

Figure 1

Steady-state UV–visible absorption spectra of TMBP (red) and DMBP (blue) obtained in ethanol. The wavelength of the theoretically predicted strongest transition (corresponding transition orbitals shown inset) for each molecule is presented as a vertical line, with the color matching the corresponding experimental absorption spectrum. Also shown are the structures of TMBP (red) and DMBP (blue), the λ_max, and the extinction coefficient.

Figure 2

TEA spectra presented as a false color heatmap for 1 mM TMBP in (a) ethanol and (b) CCT and for 1 mM DMBP in (c) ethanol and (d) CCT, following photoexcitation at their respective λ_max. In all cases, the pump–probe delay time is presented on a linear scale until 1 ps and then as a logarithmic scale between 1 and 2000 ps. The evolution associated difference spectra (EADS) produced by the fitting procedure are shown in panels e and f for TMBP in ethanol and CCT respectively, and in panels g and h for DMBP in ethanol and CCT, respectively. EADS₄ in panels e and f, together with EADS₃ in panels g and h, are all multiplied by two as a visual aid.

Figure 3

TVA spectra obtained for 30 mM ethanolic solutions of (a) TMBP and (b) DMBP, both photoexcited at their respective λ_max and using a broadband IR probe pulse centered at 1580 cm^–1 for TMBP and 1590 cm^–1 for DMBP. Both spectra are presented as smoothed colored line plots of mΔOD (left-hand y-axis) vs probe wavenumber at selected pump–probe delay times. The steady-state FTIR spectra are shown as black lines in the respective panels, with the transmittance scale shown on the right-hand y-axis. The kinetics of (c) TMBP and (d) DMBP GSB recovery for the prominent vibrational bands (raw data as open circles and fit as solid red line) centered at ∼1580 cm^–1 for TMBP and ∼1590 cm^–1 for DMBP are also reported. In both TVA spectra, the data were fitted with biexponential functions with the delay times plotted linearly until 110 ps; then, there is a break until 600 ps beyond which the 600–1000 ps data are plotted on a logarithmic scale to show incomplete GSB recovery.

Figure 4

UV–visible spectra before and at various times during 2 h of irradiation with solar simulator in a 1 mm cuvette for (a) TMBP and (b) DMBP in ethanol. The downward arrows denote the observed percentage decrease in λ_max absorbance over 2 h of irradiation.