Cage escape governs photoredox reaction rates and quantum yields

nature chemistry Article https://doi.org/10.1038/s41557-024-01482-4 Cage escape governs photoredox reaction rates and quantum yields Received: 19 March 2023 Cui Wang , Han Li 1,2 , Tobias H. Bürgin1 & Oliver S. Wenger 1 1 Accepted: 20 February 2024 Published online: xx xx xxxx Check for updates Photoredox catalysis relies on light-induced electron transfer leading to a radical pair comprising an oxidized donor and a reduced acceptor in a solvent cage. For productive onward reaction to occur, the oxidized donor and the reduced acceptor must escape from that solvent cage before they undergo spontaneous reverse electron transfer. Here we show the decisive role that cage escape plays in three benchmark photocatalytic reactions, namely, an aerobic hydroxylation, a reductive debromination and an aza-Henry reaction. Using ruthenium(II)- and chromium(III)-based photocatalysts, which provide inherently different cage escape quantum yields, we determined quantitative correlations between the rates of photoredox product formation and the cage escape quantum yields. These findings can be largely rationalized within the framework of Marcus theory for electron transfer. Photocatalysis has become a powerful method in chemistry1–6, but some key mechanistic aspects remain underexplored and poorly understood7–10. Many photoredox studies have monitored the reactivity of electronically excited states by luminescence quenching experiments and equated the disappearance of luminescent excited states to the formation of photochemical products, but this simplification is not generally valid11,12. Diffusion processes can bring together an electronically excited photocatalyst (*PC) and an electron donor in a so-called encounter complex, and then photoinduced electron transfer can quench the photocatalyst’s luminescence. However, the primary quenching products, comprising the reduced photocatalyst (PC·−) and the oxidized donor (D·+), are embedded in a solvent cage (Fig. 1a), from which they must escape for product formation to become possible. Thermal reverse electron transfer from PC·− to D·+ leading to charge recombination can spontaneously occur within the solvent cage13,14, in which case no product is formed even though luminescence quenching is detected. The importance of cage escape has long been overlooked in photoredox catalysis, but is now beginning to be recognized11,12,15–17. Cage escape quantum yields are affected by many different factors, including the driving force, the reorganization energy and the electronic coupling associated with photoinduced electron transfer and thermal in-cage reverse electron transfer16,18,19, spin and heavy-atom effects11,13,14,20–22, solvent polarity and viscosity11,20,23–25, molecular size effects26, ionic strength and ion-pairing effects13,24,25,27, electrostatic interactions28 and temperature24,25,29. Despite numerous fundamental investigations, cage escape quantum yields have remained extremely difficult to predict. In this study, we found that the [Ru(bpz)3]2+ (bpz, 2,2′-bipyrazine) and [Cr(dqp)2]3+ (dqp, 2,6-di(quinolin-8-yl)pyridine) photocatalysts (Fig. 1b) show a similar driving-force dependence for light-induced electron transfer with 12 different donors (Fig. 1c), but the cage escape quantum yields are consistently much higher with the RuII complex than with the CrIII compound. We also determined quantitative correlations between the cage escape quantum yields and the product formation rates in three different photoredox reactions. Each of these three model reactions relies on a multitude of individual elementary steps, and thus the finding that the overall photoredox product formation rates are governed in large part by cage escape seems remarkable. Two key findings emerge from our work: (1) photoredox reaction product formation rates and quantum yields correlate with the cage escape quantum yield (ФCE) and (2) the photocatalyst seems to govern the achievable magnitude of ФCE by dictating the rate of unwanted charge recombination within the solvent cage. Results and discussion Photoinduced electron transfer of Ru and Cr complexes The electron donors shown in Fig. 1c were chosen to explore the influence of the following factors on cage escape: (1) the variation in the driving force for photoinduced electron transfer (ΔGET), Department of Chemistry, University of Basel, Basel, Switzerland. 2Present address: Department of Biology and Chemistry, Osnabrück University, Osnabrück, Germany. e-mail: 1 Nature Chemistry Article https://doi.org/10.1038/s41557-024-01482-4 a b hν N D PC N D *PC PC• Charge recombination D• + Ru N Solvent cage – N N – N N N Cr N N N N N N S• P [Cr(dqp)2] 3+ [Ru(bpz)3] 2+ PC• ΦCE D •+ 3MLCT Cage escape S 2E π* 2T π* t2 t2 c 3 (PF6 ) N N N N 2 (PF6 ) 1 π* t2 OMe O(PEG)3 O(PEG)7 Cl Br I N N N N N N MeO OMe (PEG)3O O(PEG)3 (PEG)7O O(PEG)7 1 TAA-OMe 2 TAA-PEG3 3 TAA-PEG7 N N N H Me OMe 7 DMA 8 DMT Cl Cl 4 TAA-Cl N 9 DMA-OMe 10 THIQ Br Br 5 TAA-Br N 11 TEA I I 6 TAA-I N 12 DIPEA Fig. 1 | Photoinduced electron transfer between metal complexes and tertiary amines. a, Catalytic cycle of a photocatalyst (PC) reacting with an electron donor (D) via a so-called reductive quenching mechanism (oxidative quenching mechanisms are also common but are not considered here)7. Following excitation of the photocatalyst, photoinduced electron transfer leads to the reduced photocatalyst and the oxidized donor embedded in a solvent cage. Escape from this solvent cage competes with unproductive thermal reverse electron transfer (charge recombination). Only successful cage escape can lead to productive photoredox chemistry, here electron transfer to the substrate (S), which reacts onwards to the desired product (P) in subsequent (light-independent) elementary reaction steps. b, Molecular structures of the investigated photocatalysts [Ru(bpz)3]2+ and [Cr(dqp)2]3+ showing the pertinent microstates of the photoactive 3MLCT excited state and the 2E and 2T1 spin-flip excited states, respectively. c, Molecular structures of the investigated electron donors 1–12. PEG, polyethylene glycol. (2) the effect of donor size (TAA-OMe, TAA-PEG3 and TAA-PEG7), (3) the influence of heavy atoms (TAA-Cl, TAA-Br and TAA-I), (4) structural differences in the aromatic amines (triarylamines (TAAs) versus N,N-dimethylanilines) and (5) aromatic amines versus aliphatic amines (2-phenyl-1,2,3,4-tetrahydroisoquinoline (THIQ), triethylamine (TEA) and N,N-diisopropylethylamine (DIPEA)). [Ru(bpz)3]2+ and [Cr(dqp)2]3+ have similar excited-state reduction potentials (1.45 and 1.26 V versus the saturated calomel electrode, respectively)30. All 12 donors quenched the luminescent excited states of the two complexes with rate constants (kq) in the range of 108–1010 M−1 s−1 (Supplementary Figs. 15–40), revealing very similar driving-force dependence in both cases (Fig. 2c), i (...truncated)