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

Abstract

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.

Acridinium salts as photoredox organocatalysts for photomediated cationic RAFT and DT polymerizations of vinyl ethers

“…46 This is in contrast to acridine, which has been widely used for photo- or photoredox catalysts for cationic and radical polymerizations. 47–50…”

One‐Dimensional Organic Conjugated Polymers as Recyclable Heterogeneous Photocatalysts

“…In previous work, we found that the D−A type organic photosensitizer can effectively transfer electrons to the substrate and catalyze polymerizations [10] . Similarly, we speculated that one‐dimensional organic D−A conjugated polymers can also efficiently catalyze redox reactions.…”

Developments in the Components of Metal‐Free Photoinitiated Organocatalyzed‐Atom Transfer Radical Polymerization (O‐ATRP)

“…demonstrated 9,9‐dimethyl dihydroacridine as a potential PC in O‐ATRP. By substituting the PC′s active sites with required substituents, a functional structure‐property relationship was established, which helped synthesize the DPDHA‐P2F2 derivative as PC for efficient O‐ATRP [68] . Various other photocatalysts are also found suitable for O‐ATRP, including the derivatives of thienothiophene [69] and carbazoles [70] .…”

“…By substituting the PC's active sites with required substituents, a functional structure-property relationship was established, which helped synthesize the DPDHA-P2F2 derivative as PC for efficient O-ATRP. [68] Various other photocatalysts are also found suitable for O-ATRP, including the derivatives of thienothiophene [69] and carbazoles. [70] All the organic compounds discussed so far have a robust reduction potential in their excited state and an adequate excited-state lifetime, enabling them to mediate the polymerization through oxidative quenching mechanism.…”

“…The dependence of KATRP on other components of the traditional ATRP is shown in Equation 1. [79] The dependence of molecular weight distribution of polymers synthesized via ATRP is shown in Equation 2; it is seen that the molecular weight distribution (Mw/Mn) is dependent on the concentration of Eyosin Y [78] Phenoxazines [63] Erythrosin T [75] Perylene [60] Benzophenone [71] Pyrene [61] Thioxanthones [71] Thienothiophene [69] Camphorquinone [71] Phenazines [65] Fluorescein [73] Carbazoles [70] Benzaldehydes [77] Dihydroacridines [68] --dormant species (PnX), deactivator (Cu II), the rate constants of propagation (kp) and deactivation (kdeact), and monomer conversion (p).…”