Fluorescence Dynamics of Three UV-B Sunscreens

Abstract

Polarity of the surrounding medium affects the excited states of UV-B sunscreens. Therefore understanding excited state processes in a mixed polarity model system similar to skin is essential. We report the excited state lifetimes, quantum yields, radiative and non-radiative rates of three sunscreens. Among the three UV-B sunscreens studied, octyl salicylate emits from a single excited state, while padimate O and octyl methoxy cinnamate show multiple states. The radiative rates of salicylate and cinnamate are approximately constant, while that of padimate O depends strongly on solvent. The non-radiative rates of all sunscreens vary with solvent polarity. Compared to salicylate and cinnamate, padimate O is complex to analyze because of its two emission peaks and one peak's strong dependence on the dielectric constant. High absorbance, broad absorption peak with small fluorescence quantum yield, and low radiative rate make octyl methoxy cinnamate a superior UV-B sunscreen ingredient. The complexity in excited-state analysis shows that the lifetimes of the sunscreens are critical parameters, in addition to absorbance and quantum yield. Fluorescence lifetime substantiates the use of polystyrene nanospheres as a model host to study the photo-physical properties of sunscreen in a heterogeneous environment.

Insights into the photoprotection mechanism of the UV filter homosalate

“…As such, we assume that comparisons between the photodynamic behaviours of HMS and EHS are valid. Moreover, EHS also produces an emission peak at 450 nm (as reported by Krishnan and Nordlund) 73 that is very similar to the emission of HMS shown in Fig. S14 (ESI †), in terms of both central wavelength and bandwidth, further validating the comparison between these two molecules.…”

“…This observation is similar to that previously reported for methyl salicylate and ethylhexyl salicylate. 72,73 Moreover, our computational studies reveal that the S 1 -keto tautomer is lower in energy than its S 1 -enol counterpart and it is therefore anticipated that ESIPT would remain barrierless for HMS in solution.…”

“…S15a, ESI †). 73,74 Equivalent TEAS studies on EHS following the same experimental methodology as for HMS have been performed and the results are presented in S5 and S6, residuals Fig. S12, ESI †).…”

Photochemical degradation of the UV filter octyl methoxycinnamate in solution and in aggregates

“…16 Steady-state, time-resolved, Raman and two-photon fluorescence imaging experiments have all been used to investigate different aspects of OMC's excited state dynamics, ground state structures, and reactive oxygen species sensitization. 15,[17][18][19] A wide variance in the amount of loss of OMC absorptivity during irradiation has been reported in the literature. In solution, OMC has been reported to degrade irreversibly, losing up to 90% of its absorbance.…”

On the Validity of Beer–Lambert Law and its Significance for Sunscreens

“…In the cases of EHS and HMS, the lower absorbance limit could be caused by the fluorescence of such molecules, giving rise to additional false light. Indeed, it is reported that EHS shows a certain fluorescence with a quantum yield of 0.01 in 1‐propanol . As HMS possesses the same chromophore, its fluorescence properties are expected to be similar.…”