N-Heterocyclic Carbene-Triarylamine Hybrids as Redox-Tunable Organophotocatalysts for Radical Cross-Coupling Reactions.

Visible-light-induced photoredox catalysis has emerged as a powerful platform for organic radical transformations under mild conditions. In this field, the development of efficient photocatalysts plays a crucial role. Conventional ruthenium and iridium polypyridyl photocatalysts are limited by the expense of these precious metals. From a sustainable perspective, organic photocatalysts represent cost-effective and mild alternatives to photoredox catalysis; however, tuning their redox properties often requires complicated synthetic approach. Here we introduce N-heterocyclic carbene-triarylamine hybrids as novel donor-acceptor-type organophotocatalysts. These hybrids feature with facile structural modification, broad intramolecular charge-transfer absorption bands, and tunable redox potentials. Furthermore, these new photocatalysts have been successfully utilized for five distinct visible-light-induced redox-neutral radical cross-coupling reactions (94 examples, up to 99% yield). Mechanistic studies indicate that visible-light irradiation of the photocatalyst generates a charge-shifted excited state through photoinduced intramolecular electron transfer. This finding has a guiding significance for the design and development of new organophotocatalysts.