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

Abstract

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 subpicosecond 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 k ET(T1) = (2.7 ± 0.3) × 107 M–1 s–1 for electron transfer from the T1 state is 2 orders of magnitude lower than that from the S1 state, k ET(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.

Transient absorption spectroscopy of the electron transfer step in the photochemically activated polymerizations of N-ethylcarbazole and 9-phenylcarbazole

“…The measured values for the lifetimes of the carbazole S 1 states in the absence of an electron acceptor, τ (S 1 ), and the bimolecular rate coefficients for reaction from the S 1 state, k ET (S 1 ), can be combined to estimate quantum yields for S 1 -state quenching by electron transfer, Φ ET (S 1 ), and their dependence on the concentration of the EA, using: 58 …”

Solvent Effects on Ultrafast Photochemical Pathways

“…Recent ultrafast transient absorption spectroscopy studies of the excited state photochemistry of the dihydrophenazine, phenoxazine, and phenothiazine OPCs, as exemplified in Figure , have unraveled the competition between intermolecular electron transfer from the S 1 state to an acceptor, intersystem crossing to T 1 with subsequent electron transfer, and relaxation by radiative or nonradiative pathways back to the S 0 state. ,− The balance of these competing photochemical pathways is highly sensitive to the properties of the solvent, with comparative studies in N , N -dimethylformamide (DMF), dichloromethane (DCM), and toluene showing significant changes to S 1 state lifetimes, ISC yields, and electron transfer rate coefficients. For example, the OPC 4,4′-(phenazine-5,10-diyl)­dibenzonitrile (Figure a) has an S 1 lifetime of 140 ps in DMF solution, which is too short for it to be a strong fluorescence emitter or to undergo significant ISC to populate the T 1 state.…”

Photoredox-HAT Catalysis for Primary Amine α-C–H Alkylation: Mechanistic Insight with Transient Absorption Spectroscopy

“…Organic photocatalysts (OPCs) offer a sustainable and complementary alternative to the archetypal ruthenium- and iridium-based polypyridyl complexes applied in visible-light-mediated synthesis − and controlled polymerization. − One of many exciting advances in this area is the use of OPCs to drive catalytic derivatizations of C­(sp 3 )–H bonds, circumventing the need for prefunctionalized organic substrates. − While some OPCs, such as eosin Y, possess excited states capable of directly abstracting hydrogen atoms from C­(sp 3 )–H bonds, a more typical (and modular) approach is to employ a discrete hydrogen-atom transfer (HAT) cocatalyst that may be photooxidized. , In this vein, one of our research groups (Cresswell and co-workers) recently discovered that azide ion (N 3 – ) is an unusually effective HAT catalyst for the challenging photocatalytic α-C–H alkylation of unprotected, primary alkylamines (Figure a) . The reported alkylation reactions employed acrylates as coupling partners for the synthesis of γ-amino esters (and their derived γ-lactams), with subsequent studies extending the chemistry to vinyl phosphonates and styrenes as radical acceptors.…”