Photoredox Chemistry with Organic Catalysts: Role of Computational Methods

Abstract

Organic catalysts have the potential to carry out a wide range of otherwise thermally inaccessible reactions via photoredox routes. Early demonstrated successes of organic photoredox catalysts include one-electron CO₂ reduction and H₂ generation via water splitting. Photoredox systems are challenging to study and design owing to the sheer number and diversity of phenomena involved, including light absorption, emission, intersystem crossing, partial or complete charge transfer, and bond breaking or formation. Designing a viable photoredox route therefore requires consideration of a host of factors such as absorption wavelength, solvent, choice of electron donor or acceptor, and so on. Quantum chemistry methods can play a critical role in demystifying photoredox phenomena. Using one-electron CO₂ reduction with phenylene-based chromophores as an illustrative example, this perspective highlights recent developments in quantum chemistry that can advance our understanding of photoredox processes and proposes a way forward for driving the design and discovery of organic catalysts.

Figure 1

Oxidative and reductive quenching cycles of a photoredox catalyst. Reprinted (adapted) with permission from ref (1) Copyright 2016 American Chemical Society.

Figure 2

Chemical structures of common organic photocatalysts. Reprinted (adapted) with permission from ref (13) Copyright 2018 Wiley.

Figure 3

Catalytic cycle of one-electron reduction of CO₂ with OPP, where n represents the number of interior phenylenes (i.e., n = 1 corresponds to OPP-3) and R represents terminal, para substitutions (e.g., −OH, −CN, −F). Reprinted (adapted) with permission from ref (15) Copyright 2020 American Chemical Society.

Figure 4

Proposed photophysical (blue), charge transfer (green), and degradation via bond-breaking/formation (red) processes in photoredox CO₂ reduction with OPP-3 as the chromophore and TEA (Et₃N) as the sacrificial electron donor. Methods for characterizing interfragment interactions at various stages of the photoredox cycle are also described (gray box). Reprinted (adapted) with permission from ref (15) Copyright 2020 American Chemical Society.

Figure 5

Qualitative energy vs reaction coordinate representation for an ET step, shown here for ET between OPP^•– and CO₂. The pink curve represents the adiabatic limit arising from the strong coupling. The black curve reflects the charge distribution of the initial state (IS, reactant complex) and blue represents that of the final state (FS, product complex). ΔG is the free energy difference between the two states and λ corresponds to the energy of reorganization of the solute and solvent. Reprinted (adapted) with permission from ref (15) Copyright 2020 American Chemical Society.

Figure 6

Highest occupied molecular orbital (HOMO, red) and lowest unoccupied molecular orbital (LUMO, blue) energies (eV) vs σ_p for substituted OPPs. Inset: canonical molecular HOMO and LUMO for unsubstituted OPP. Trendlines, developed using the modified Akiake information criterion, reflect quadratic dependence on σ_p (R_HOMO² = 0.94, R_LUMO² = 0.90). A qualitative depiction, pending future analysis with variational EDA, of regions of varying extents of orbital mixing (OOM: occupied–occupied mixing; VVM: virtual–virtual mixing) is also shown. Reprinted (adapted) with permission from ref (15) Copyright 2020 American Chemical Society.