UV Filters and Their Photochemistry

When studying UV filters or sunscreen preparations, it is useful and desirable to have some knowledge about the photochemical behavior of the components under study. It is the absence of this knowledge which, over the last two decades, has led to many misinterpretations and erroneous conclusions. 

All these processes are physical and compete against each other. They allow the excited UV filter molecule to free itself of its excitation energy and return to the ground state, where it can absorb another photon. 

Besides these physical pathways, some molecules may also lose their excitation energy through chemical transformations, some of which are even desirable. When considering the photochemical reactions of UV filters, it is important to distinguish between  

The normal pure synthetic material contains more than 98% the trans-isomer. Upon exposure to UV radiation, it will undergo isomerization to the cis form which itself is a UV filter with a very similar shape of absorption curve. Hence, the ds-isomer will also absorb UV radiation and isomerize back to trans form. Neither of the two isomers are stable and their back and forth isomerization will lead to a photostation-ary equilibrium (60% ds+40% trans)- the two isomers together form a stable system. This equilibration is fast and accompanied by a drop in overall absorbance. This decrease is due to the fact that, although the two isomers have absorption curves very similar in shape, they differ in magnitude of their individual extinction coefficients as shown below. 

The equilibrated mixture will exhibit an absorbance lower by 20% to 25%, depending on the medium, compared to the same sample before exposure (Fig. 3). 

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