photoreduction

Photoreduction is a chemical reduction process driven by light, typically via photoinduced electron transfer that generates a reducing species capable of transferring electrons to a substrate. It is a central concept in photoredox catalysis and can occur in homogeneous or heterogeneous systems. In many cases, light excites a photosensitizer or a semiconductor to produce a long-lived reducing intermediate that reduces an organic or inorganic substrate, while the corresponding oxidized partner is regenerated by a sacrificial electron donor or a complementary catalytic cycle.

In homogeneous photoredox systems, a photosensitizer such as a transition-metal complex or an organic dye absorbs light to reach an excited state that donates an electron to the substrate. The photosensitizer becomes oxidized and is regenerated by a sacrificial donor, completing a catalytic cycle. In heterogeneous systems, photoexcited semiconductors (for example TiO2 or CdS) generate conduction-band electrons that can reduce adsorbed substrates, with holes scavenged by donors in solution. Direct excitation of the substrate or a co-catalyst can also contribute to photoreduction in certain designs.

Common catalysts include ruthenium or iridium polypyridyl complexes and various organic dyes, often used with sacrificial donors such as amines or ascorbate. Substrates range from halides and carbonyl compounds to electron-deficient alkenes and metal complexes. Applications span organic synthesis, environmental remediation, and solar-to-chemical energy conversion, enabling reductions under mild conditions with visible light.

Advantages include mild reaction conditions, tunable selectivity, and the use of light as a clean energy source. Limitations involve dependence on light penetration, catalyst cost and availability, potential side reactions, and challenges in scaling up photochemical processes.