Photocatalysis (by semiconductors, molecules and ions) is used in such diverse applications as water hydrolysis for producing hydrogen as fuel, organic synthesis and the recovery of polluted effluents. This tutorial review discusses the common principles of such applications and their role in green chemistry.

In the reactions reviewed, an organic molecule under irradiation catalyzes a chemical reaction. The activation is based either on hydrogen or on electron transfer. Commonly used photoorganocatalysts are aromatic ketones, quinones, heterocycles, dyes; intermediates formed are radicals, radical ions and ions from precursors such as alkanes, alkenes, amines, ethers etc. Oxidation (mainly oxygenation) and reduction processes are obtained along with the α-functionalization of amines and ketones, conjugate additions, cycloadditions etc. The key characteristic of the method is the smooth generation of highly reactive intermediates under mild conditions.

Resveratrol (3,4',5-trihydroxy-trans-stilbene) is a natural phytoalexin found in grapes and wine, which shows antioxidant and antiproliferative activities. In this study we have investigated whether these properties are dependent on similar or different structural determinants of the molecule. To this purpose, resveratrol derivatives, in which all or each single hydroxylic function were selectively substituted with methyl groups, were synthesized. Analogues with the stilbenic double bond reduced or with the stereoisometry modified were also investigated. The antioxidant activity of these compounds was evaluated by measuring the inhibition of citronellal thermo-oxidation, or the reduction of 2,2-diphenyl-1-picrylhydrazyl radical. In addition, the protection against lipid peroxidation was determined in rat liver microsomes, and in human primary cell cultures. The antiproliferative activity was evaluated by a clonogenic assay, and by analysis of cell cycle progression and DNA synthesis. The results showed that the hydroxyl group in 4' position is not the sole determinant for antioxidant activity. In contrast, the presence of 4'-OH together with stereoisometry in the trans-conformation (4'-hydroxystyryl moiety) was absolutely required for inhibition of cell proliferation. Enzymatic assays in vitro demonstrated that inhibition of DNA synthesis was induced by a direct interaction of resveratrol with DNA polymerases alpha and delta.

The photobehavior of fluoroquinolone antibiotics, one of the most successful classes of drugs in therapeutic applications, has recently been the object of increasing interest due to the finding of their phototoxic and photocarcinogenic properties. The main results obtained for a series of structurally related, representative fluoroquinolone drugs is reviewed. Both activation of oxygen and various degradation pathways have been identified and the effects of medium and structure have been rationalized. The results can help in the understanding of the photochemistry occurring in biological environments and in the assessing of the correlation between structural characteristics and biological photodamage.