posted on 2018-03-07, 00:00authored byBlaine
G. McCarthy, Ryan M. Pearson, Chern-Hooi Lim, Steven M. Sartor, Niels H. Damrauer, Garret M. Miyake
Through the study of structure–property
relationships using
a combination of experimental and computational analyses, a number
of phenoxazine derivatives have been developed as visible light absorbing,
organic photoredox catalysts (PCs) with excited state reduction potentials
rivaling those of highly reducing transition metal PCs. Time-dependent
density functional theory (TD-DFT) computational modeling of the photoexcitation
of N-aryl and core modified phenoxazines guided the
design of PCs with absorption profiles in the visible regime. In accordance
with our previous work with N,N-diaryl
dihydrophenazines, characterization of noncore modified N-aryl phenoxazines in the excited state demonstrated that the nature
of the N-aryl substituent dictates the ability of
the PC to access a charge transfer excited state. However, our current
analysis of core modified phenoxazines revealed that these molecules
can access a different type of CT excited state which we posit involves
a core substituent as the electron acceptor. Modification of the core
of phenoxazine derivatives with electron-donating and electron-withdrawing
substituents was used to alter triplet energies, excited state reduction
potentials, and oxidation potentials of the phenoxazine derivatives.
The catalytic activity of these molecules was explored using organocatalyzed
atom transfer radical polymerization (O-ATRP) for the synthesis of
poly(methyl methacrylate) (PMMA) using white light irradiation. All
of the derivatives were determined to be suitable PCs for O-ATRP as
indicated by a linear growth of polymer molecular weight as a function
of monomer conversion and the ability to synthesize PMMA with moderate
to low dispersity (dispersity less than or equal to 1.5) and initiator
efficiencies typically greater than 70% at high conversions. However,
only PCs that exhibit strong absorption of visible light and strong
triplet excited state reduction potentials maintain control over the
polymerization during the entire course of the reaction. The structure–property
relationships established here will enable the application of these
organic PCs for O-ATRP and other photoredox-catalyzed small molecule
and polymer syntheses.