Organic Photocatalyst Utilizing Triplet Excited States for Highly Efficient Visible-Light-Driven Polymerizations.

ConspectusUltraviolet (UV) light has traditionally been used to drive photochemical organic transformations, mainly due to the limited visible-light absorption of most organic molecules. However, the high energy associated with UV light often causes undesirable side reactions. In the late 2000s, MacMillan, Yoon, and Stephenson pioneered the use of visible light in conjunction with photocatalysts (PCs) to initiate organic transformations. This innovative approach overcame the limitations of UV light by utilizing visible-light-absorbing PCs in their photoexcited states for electron or energy transfer, generating reactive radical species and promoting the photoreactions. Furthermore, while the photocatalysis has predominantly relied on transition-metal complexes, concerns over the potential toxicity, cost, and sustainability of these metals have driven the development of organic PCs. These organic PCs eliminate the need for metal removal, offer structural diversity, and enable tuning of their properties, thus paving the way for the creation of a tailored library of PCs.In recent decades, significant advancements have been made in the development of novel organic PCs with diverse scaffolds, with a notable example being the work of Zhang et al. in 2016. They demonstrated that cyanoarene analogues, originally developed by Adachi et al. for thermally activated delayed fluorescence (TADF) in organic light-emitting diodes, could function effectively as PCs. Building on these insights, we developed a PC design platform featuring TADF compounds with twisted donor-acceptor structures, which paved the way for new PC discoveries. We showcased these PCs' ability (i) to generate long-lived lowest triplet excited (T