The triplet energy levels of UVA and UVB sunscreens

Abstract

The use of WA and UVB sunscreens is certain to become of increasing importance because of environmental factors such as ozone depletion and also the general awareness of the harmful effects of sunlight on human skin. In a recent publication [l], Martincigh et al. drew attention to the possible deleterious effects of sunscreens as photosensitizers, with particular reference to the possibility that they can photopotentiate harmful reactions in the skin, especially the dimerization of thymine. Certainly, it is well known that thymine triplets react with ground state thymine to yield thymine dimers [2], which are potential precursors to skin cancer, and thus it is suggested that there could be drawbacks in using sunscreens whose lowest triplet is higher than or similar to thymine. The lowest triplet energy of thymine, e.g. as thymine monophosphate (IMP), has recently been estimated at 315 kJ mol-’ (75 kcal mol-‘) [l] which is in good agreement with early phosphorescence data [3]. The triplet energy level of para-aminobenzoic acid (PABA) has also been reported as near 75 kcal mol-‘. It has been established that triplet-triplet energy transfer from a donor (D) to an acceptor (A)