Study of the photochemical behaviour of sunscreens - benzylidene camphor and derivatives.

Synopsis Benzylidene camphor derivatives have been currently used as ultraviolet filters in sunscreen compositions. When solutions of these compounds are exposed to the light from a solar simulator, a photoinduced cis-trans isomerization is observed. The initial quantum yields of this photochemical isomerization of studied molecules have been determined in several solvents. Their photochemical behaviour depends neither on the concentration nor on the presence or absence of oxygen or other inhibitors such as isopropanol or biacetyl. It means that the lifetime of the precursor states of isomerization is about 10(-12)s, that is to say, too short for them to react with neighbouring molecules. For each compound, the ratio of the respective concentrations of 'E' and 'Z' forms was also determined by photostationary state. A good agreement between theoretical and experimental values of this ratio shows that the studied compounds are only very slightly photodegradable. This was confirmed by the low values of photodegradation quantum yields. In case of the studied compounds, the isomerization process observed is totally reversible. During irradiation a photostationary state is quickly reached, each isomer may change into the other form after absorption of a photon which leads to a mixture of isomers. In practice, it is this mixture of E and Z forms which acts as UV filters. It may be said that a very short lifetime of the excited states, a good light stability and a reaction inertia under light irradiation make these compounds excellent sunscreen agents.