photoredox

Photoredox refers to chemical reactions initiated by light-activated catalysts that mediate oxidation and reduction via single-electron transfer or energy transfer. The approach uses visible light and photocatalysts to generate reactive radical or ionic intermediates under mild conditions.

In a typical cycle, an excited photocatalyst (often Ru or Ir complexes, or organic dyes) has altered redox properties. It can act as a strong oxidant or reductant. Through reductive or oxidative quenching, it accepts or donates electrons to substrates, generating radical species.

Energy transfer pathways also operate, in which the excited catalyst transfers energy to substrates to promote reactions without changing oxidation state. Some systems employ proton-coupled electron transfer to couple electron transfer with proton movements, enabling transformations under milder conditions.

Common catalysts include Ru(bpy)3^2+, Ir(ppy)3, and organic photosensitizers like eosin Y or phenothiazine derivatives. Reactions are typically driven by blue or visible LEDs, and may require sacrificial electron donors or co-catalysts such as nickel, palladium, or copper.

Applications span C–C and C–heteroatom bond formation, dehalogenations, hydrofunctionalization of alkenes, and late-stage functionalization. Photoredox is often paired with other catalytic modes, notably nickel catalysis, to access cross-electrophile couplings and broader radical chemistry.

History and development: The field emerged in the late 2000s with foundational work by MacMillan and Yoon, followed by rapid growth of catalysts, mechanisms, and reactor designs, including flow photochemistry that improves scalability and light penetration.

Advantages and limitations: Photoredox enables mild conditions, broad functional-group tolerance, and access to otherwise difficult radical pathways. Limitations include cost of some catalysts, air sensitivity, and scale-up challenges, though flow techniques mitigate several issues.