Perspective: How can ultrafast laser spectroscopy inform the design of new organic photoredox catalysts for chemical and materials synthesis?

Abstract

Photoredox catalysis of chemical reactions, using light-activated molecules which serve as electron donors or acceptors to initiate chemical transformations under mild conditions, is finding widespread use in the synthesis of organic compounds and materials. The transition-metal-centred complexes first developed for these photoredox-catalysed applications are steadily being superseded by more sustainable and lower toxicity organic photocatalysts. While the diversity of possible structures for photoredox-active organic molecules brings benefits of design flexibility, it also presents considerable challenges for optimization of the photocatalyst molecular architecture. Transient absorption spectroscopy over timescales from the femtosecond to microsecond domains can explore the detailed mechanisms of activation and reaction of these organic photocatalysts in solution and, by linking their dynamical properties to their structures, has the potential to establish reliable design principles for future development of improved photocatalysts.

FIG. 1.

Generalized photoredox catalytic cycles. The ground electronic state photocatalyst, PC(S₀), is photo-excited by visible or near-UV light to a higher energy singlet state (S_n) and undergoes internal conversion (IC) or intersystem crossing (ISC) to PC* (S₁ or T₁) before photoinduced electron transfer (PET) with a substrate S. The left-hand cycle shows PC* as a reductant: electron donation to an acceptor S makes the oxidized PC⋅⁺ radical cation and a reduced substrate S·^- which reacts (dashed arrow) to product P⋅^-. The right-hand cycle shows PC* as an oxidant, accepting an electron from the donor S into a vacancy in a valence molecular orbital, and reaction of the oxidized substrate S·⁺. In both cycles, back-electron transfer (BET) between reaction products P⋅^- or P⋅⁺ and the oxidized or reduced photocatalyst radical intermediate completes the cycle.

FIG. 2.

Examples of OPC structures. The first column shows three examples of commonly used OPCs, benzophenone (top), 9,10-dicyanoanthracene (middle), and anthraquinone (bottom). The second column shows OPCs based on aryl-substituted phenoxazine (top), phenothiazine (middle), and 5,10-dihydrophenazine (bottom) cores. Representative substituents at the N-atom (R₁) and core (R₂ and R₃) sites are shown in the third column. Core substituents can extend the conjugation of the chromophore, and choice of both N-atom and core substituents can instill either electron withdrawing or donating character in these distinct regions of the excited state molecules.

FIG. 3.

Characterization of several steps in an organocatalysed atom-transfer radical polymerization reaction using transient absorption spectroscopy. (a) The O-ATRP cycle in which MP· radicals (with the structure shown) are produced by dissociative photoinduced electron transfer (PET) from an excited state of the photocatalyst (PC) to the methyl 2-bromopropionate (MBP) initiator and react with a monomer alkene (M) to commence polymerization. All species in red have been observed by TVAS or TEAS. (b) Structures of selected aryl-substituted dihydrophenazine photocatalysts, denoted by the labels PCH and PCF. (c) Transient IR spectra of a solution of PCF and MBP in dichloromethane obtained at time delays from 1 to 1000 ps after 370-nm photoexcitation. Transient absorption features are assigned to PCF*(S₁), PCF·⁺, and MP· radicals. (d) Time-dependence of absorption intensity on the band centred at 1660 cm^-1, assigned to the MP· radical, for different concentrations of the MBP initiator (indicated by different coloured symbols and fitted curves). A pseudo-first order kinetic analysis is shown in the inset, from which a bimolecular rate coefficient for the PET reaction is determined.