Abstract
The most important characteristics of an effective and safe UV absorber used as a UV-blocking compound in cosmetic sunscreens is photochemical stability. The most widely used organic UV-A absorber is 4-tert-butyl-4'-methoxydibenzoylmethane (BMDBM). However, BMDBM is well-known for its photoinstability under actual conditions of use. The commonly employed method for improving the photostablity of BMDBM is based on the addition of another UV absorber. Octocrylene (OCR) has been found to be most effective at stabilizing BMDBM. In some sunscreen formulations, octyl 4-methoxycinnamate (OMC), the most widely used representative UV-B absorber, is added to the mixture of BMDBM and OCR to provide effective protection throughout the whole UV range of solar radiation because the molar absorption coefficients of OCR are small in the UV-B region. In the present study, we have shown that the observed photostablity of BMDBM in the BMDBM-OCR-OMC combination is lower than that in the BMDBM-OCR combination. To eliminate this problem, we have designed a new UV-B absorber, dioctyl 4-methoxybenzylidenemalonate (DOMBM), which does not undergo cis-trans photoisomerization owing to its molecular structure. We tested the effect of the addition of DOMBM on the photostability of BMDBM in the BMDBM-OCR-DOMBM combination. The photostablity of BMDBM in the BMDBM-OCR-DOMBM combination is higher than those in the BMDBM-OCR and BMDBM-OCR-OMC combinations. It is suggested that intermolecular triplet-triplet energy transfer from BMDBM to DOMBM and the photostable filter effect of DOMBM on BMDBM are possible mechanisms to explain the photostability of BMDBM in the combination of UV-A and UV-B absorbers.
