9,9-Dimethyl Dihydroacridine-Based Organic Photocatalyst for Atom Transfer Radical Polymerization from Modifying Unstable Electron Donor

Organic photocatalytic atom transfer radical polymerization (ATRP) has recently become a research highlight. Organic photocatalysts based on phenothiazine, phenoxazine, and phenazine have been reported and exhibited remarkable performance. All of those structures contain two heteroatoms, which makes the oxidative state (i.e., the radical cation) of the photocatalysts stable enough to complete the catalytic cycle. However, despite the similar structure, 9,9-dimethyl dihydroacridine (DHA) was rarely used for constructing photocatalyst due to its unstable oxidative state. DHA is a weak electron donor that was widely applied in blue-emitting thermally activated delayed fluorescence (TADF) molecules. Its weaker electron-donating ability will contribute to a higher energy level of the excited state. Also, the higher oxidation potential of its radical cation will contribute to better controllability due to fast reversible dormancy. In this study, we found that substitution on the active sites of DHA could make it stable enough to be the donor part of a donor-acceptor (D-A)-type photocatalyst for ATRP. Moreover, chemical modification is necessary for both stabilizing the radical cation and improving the controllability in the polymerization process. Further modification was made to construct a rapid equilibrium between initiation and reversible dormancy, and polymerization with quantitative initiator efficiencies was achieved with a polydispersity of 1.14. It is notable that such modification can probably apply to different kinds of electron donors, and various organic chromophores could thus be applied to construct organic photocatalyst with superior performance.