Photophysics and photochemistry of tetrahydrocurcuminoids

The photochemical properties of 10 tetrahydrocurcuminoids (THCs) have been studied for the first time. They display very low fluorescence in ethanol solution at room temperature with quantum yields between 0.9 and 13 × 10−3 whereas their phosphorescence quantum yields in ethanol glass at 77 K are between 0.025 and 0.5. Their disappearance quantum yield measured in ethanol solution, between 5 × 10−4 and 10−2, follows the same trend as their antiradical power efficiency (ARP) previously measured. The reactivity of THCs is enhanced if they include a phenol group in meta or para of the linking chain and a phenol or methoxy group as neighbor. UV absorption difference spectra of the irradiated solutions, HPLC and GC–MS analyses performed on 4-propylguaiacol (4PG), which displays similar behavior as tetrahydrocurcumin (THC2) indicates the formation of quinone methides and ortho-quinone. The formation of a phenoxy radical by photooxidation appears to be the first reaction step. This demonstrates that the general photodegradation mechanism of phenols found for guaiacyl lignins applies also to simplest molecules of natural origin such as THCs. The irradiation of the THCs in carboxymethylcellulose films follows the same trends as the solution studies, except that the quinone methides and ortho-quinones are not detected, probably because they react with the cellulosic matrix. The knowledge obtained in this study probably will help to promote the use of tetrahydrocurcuminoids in various applications where it is necessary to develop antioxidant properties in presence of solar light.