Photoisomerization and photodimerization of a widely used UVB filter, 2-ethylhexy-4-methoxycinnamate (EHMC) on a ZnSe surface and baby mouse (<i>Mus musculus</i> Linn.) skin were monitored using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR). Differentiation between the <i>E</i>- and the <i>Z</i>-EHMC could be achieved by examining the infrared (IR) peak at 981 cm<sup>−1</sup> (b peak), which corresponds to the CH rocking deformation vibration of Ph–CH=CH– detected only in the <i>E</i> configuration. By plotting the ratios of the peak area of the b peak and an internal standard peak (1060–998 cm<sup>−1</sup>) against mole percentage of <i>Z</i>-isomer in the <i>E-Z</i> mixtures, a linear calibration plot was obtained. Thus, a simple estimation of the mole percentage of each configuration in a sample was obtained. At the same UVB exposure, photostationary equilibrium of the <i>E</i>/<i>Z</i> isomerization on the surface varied with the applied amounts of EHMC. Photoisomerizations on ZnSe and on baby mouse skin were comparable. Less than 10% of <i>E</i>-EHMC changed configuration when the mouse skins applied with 1.0–4.0 mg/cm<sup>2</sup> <i>E</i>-EHMC were exposed to sunlight for 60 min (UVB radiant exposure of ∼0.30 J/cm<sup>2</sup>). This corresponded to less than 5% loss in UV filtering efficiency. However, at a typical EHMC skin coverage (∼0.2 mg/cm<sup>2</sup>), 0.30 J/cm<sup>2</sup> UVB exposure induced ∼50% photoisomerization resulting in 25% loss of UV filtering efficiency. No photodimerization was detected even at the extreme EHMC coverage of 4.0 mg/cm<sup>2</sup> after a UVB exposure of 0.90 J/cm<sup>2</sup>.

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