New Insight into the Photoprotection Mechanism of Plant Sunscreens: Adiabatic Relaxation Competing with Nonadiabatic Relaxation in the → Photoisomerization of Methyl Sinapate.

A great deal of thermally instable form photoisomerization products will be formed from the thermally stable form of the plant sunscreens sinapate esters upon ultraviolet radiation. To reveal the photoisomerization mechanism of the -isomer, we explore the photodynamics of a model plant sunscreen methyl sinapate (MS) in the form in organic solution. The high photoisomerization quantum yield of the -isomer results in the relatively higher photostability of -MS. By utilizing femtosecond transient absorption spectroscopy and quantum chemical calculation, we propose that an adiabatic relaxation competes with nonadiabatic relaxation for the excited-state form of methyl sinapate. These results suggest that the photoprotection mechanism of the form of sinapate esters is significantly different from that of the form of sinapate esters and plays an important role in the overall photoprotection effect.