Structure-Dependent Electron Transfer Rates for Dihydrophenazine, Phenoxazine, and Phenothiazine Photoredox Catalysts Employed in Atom Transfer Radical Polymerization

Organic photocatalysts (PCs) are gaining popularity in applications of photoredox catalysis, but few studies have explored their modus operandi. We report a detailed mechanistic investigation the electron transfer activation step organocatalyzed atom radical polymerization (O-ATRP) involving electronically excited organic PCs and initiator, methyl 2-bromopropionate (MBP). This study compares nine N-aryl modified possessing dihydrophenazine, phenoxazine, or phenothiazine core chromophores. Transient electronic vibrational absorption spectroscopies over subpicosecond to nanosecond microsecond time intervals, respectively, track spectroscopic signatures both reactants products photoinduced N,N-dimethylformamide, dichloromethane, toluene solutions. The rate coefficients for exhibit range values up ?1010 M–1 s–1 influenced systematically by PC structures. These an order magnitude smaller catalysts with charge character first singlet (S1) triplet (T1) states than locally character. latter species show nearly diffusion-limited MBP. derived kinetic parameters used model contributions from S1 state each different concentrations Comparisons reactivity one phenoxazine reveal that coefficient kET(T1) = (2.7 ± 0.3) × 107 T1 is 2 orders lower state, kET(S1) (2.6 0.4) 109 s–1. trends bimolecular accounted using Marcus theory dissociative transfer.