Mapping the multi-step mechanism of a photoredox catalyzed atom-transfer radical polymerization reaction by direct observation of the reactive intermediates

Abstract

Short-lived intermediates are tracked in real-time by transient absorption spectroscopy during a multi-step photoredox catalysed polymerization reaction.

Solvent-dependent photochemical dynamics of a phenoxazine-based photoredox catalyst

“…[14][15][16][17] Transient absorption spectroscopy has also been applied to other examples of photoredox reactions and over timescales spanning sub-picosecond to millisecond to explore multi-step reaction mechanisms. 15,[18][19][20][21] The complex photochemistry of these OPCs depends sensitively on the molecular architecture and the choice of solvent. For example, the ordering of excited states, de-activation pathways to the ground state, rates of intersystem crossing (ISC), triplet-state quantum yields, and rates of bimolecular SET reactions can all be modified by changes to the excited-state electronic character 22,23 and by different solute-solvent interactions.…”

“…The outcomes of our recent experimental studies of the photochemistry of dihydrophenazine-based OPCs in solution contrast with the current observations for the phenoxazine-based NPP. We previously reported S1 lifetimes for selected dihydrophenazine OPCs that ranged from 0.6 -16 ns, with considerable dependence on solvent (in experiments that also compared the photochemistry in DMF, DCM and toluene), and triplet quantum yields below 20% 14,19. For NPP, the S1 state lifetimes are consistently ~2 ns irrespective of solvent choice, and in toluene (81  4) % of the vibrationally equilibrated S1-state molecules cross to the triplet manifold of states before decaying to S0.…”

Emerging Organic Photoredox Catalysts for Organic Transformations

“…The latter mostly arise from an unclear understanding of the reaction mechanism. In fact, there still remain unresolved questions regarding the overall picture of organic photoredox catalysis, although the detailed mechanisms have been revealed for several specific systems:, , Which excited state, singlet or triplet, is in charge during photophysical processes? What is transferred from the excited‐state catalyst to the substrate, i.e.…”

“…In addition to strong absorption and appropriate redox potentials, the population of the lowest triplet excited state (T 1 ) plays a critical role since the PC in the excited state has to diffuse to the substrate of interest before returning to its ground state. Here, the involvement of the lowest singlet excited state (S 1 ) has been recently claimed by Orr‐Ewing and co‐workers from the investigation of photochemical dynamics of organocatalyzed atom transfer radical polymerization (O‐ATRP) , . However, because of the longer lifetime of T 1 , the efficient generation of T 1 has been mainly considered for the development of highly efficient organic PCs.…”

Metal‐Free Organocatalyzed Atom Transfer Radical Polymerization: Synthesis, Applications, and Future Perspectives

“…[ 92,93 ] PCs that exhibit intramolecular charge transfer (CT) are capable of promoting controlled photopolymerization in a wide range of solvents with different polarities, from nonpolar like hexane to highly polar as DMA, however excited intermediates of the charge transfer are stabilized preferentially by polar solvents. [ 94,95 ] When the photocatalyst does not have a CT, its capacity to mediate controlled polymerization is lower. Therefore, the influence of solvent polarity in the control of the reaction system is higher when the catalyst does not present CT. [ 93 ]…”