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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 of the electron transfer activation step of organocatalyzed atom transfer radical polymerization (O-ATRP) involving electronically excited organic PCs and a radical initiator, methyl 2-bromopropionate (MBP). This study compares nine N-aryl modified PCs possessing dihydrophenazine, phenoxazine, or phenothiazine core chromophores. Transient electronic and vibrational absorption spectroscopies over sub-picosecond to nanosecond and microsecond time intervals, respectively, track spectroscopic signatures of both the reactants and products of photoinduced electron transfer in N,N-dimethylformamide, dichloromethane, and toluene solutions. The rate coefficients for electron transfer exhibit a range of values up to ~1010 M-1 s-1 influenced systematically by the PC structures. These rate coefficients are an order of magnitude smaller for catalysts with charge transfer character in their first excited singlet (S1) or triplet (T1) states than for photocatalysts with locally excited character. The latter species show nearly diffusion-limited rate coefficients for the electron transfer to MBP. The derived kinetic parameters are used to model the contributions to electron transfer from the S1 state of each PC for different concentrations of MBP. Comparisons of singlet and triplet reactivity for one of the phenoxazine PCs reveal that the rate coefficient kET (T1 )=(2.7±0.3) × 107 M-1 s-1 for electron transfer from the T1 state is two orders of magnitude lower than that from the S1 state, kET (S1 )=(2.6±0.4) × 109 M-1 s-1. The trends in bimolecular electron transfer rate coefficients are accounted for using a modified Marcus theory for dissociative electron transfer.
Bibliographical noteFunding Information:
We gratefully acknowledge EPSRC (Grant EP/R012695/1) and a previous ERC Advanced Grant CAPRI 290966 for funding this work. M.S. acknowledges award of the Marie Skłodowska-Curie Fellowship (MARCUS 793799). L.L.B. thanks the Bristol Chemical Synthesis Centre for Doctoral Training, funded by EPSRC (EP/L015366/1), and the University of Bristol, for a Ph.D. studentship. The authors are grateful to the Science and Technology Facilities Council for access to the LIFEtime and Ultra Facilities at the STFC Rutherford Appleton Laboratory.
© 2021 American Chemical Society.
- 2 Finished
1/03/18 → 28/02/21