Toward Metal-free Photocatalysis: Photochemical Regeneration of Organic Hydride Donors Using Phenazine-Based Photosensitizers

Abstract

Earth-abundant chromophores and catalysts are important molecular building blocks for artificial photosynthesis applications. Our team previously reported that metal-free hydride donors, such as biomimetic benzoimidazole-based motifs, can reduce CO2 selectively to the formate ion and that they can be electrochemically regenerated using the proton-coupled mechanism. To enable direct utilization of solar energy, we report here the photochemical regeneration of a benzoimidazole-based hydride donor using a phenazine-based metal-free chromophore. The photochemical regeneration was investigated under different experimental conditions involving varying sacrificial donors, proton donors, solvents, and component concentrations. The best hydride regeneration yield of 50% was obtained with phenol as a proton source and thiophenolate as a sacrificial electron donor. The mechanism of photochemical regeneration was studied using steady-state and time-resolved UV/Vis spectroscopies. Based on the results of these studies, we hypothesize that the initial photoinduced electron transfer from photoexcited phenazine chromophores involves the benzoimidazole cation and that this process is likely coupled with proton transfer to generate protonated benzoimidazole-based radical cation. The second photoinduced electron transfer is hypothesized to generate the hydride form. Our findings provide the requisite information for the future development of reductive photocatalysts for solar energy and light-harvesting applications utilizing earth-abundant metal-free materials.

Bis-Imidazolium-Embedded Heterohelicene: A Regenerable NADP+ Cofactor Analogue for Electrocatalytic CO2 Reduction

“…7 Inspired by the fact that biological systems follow a 2e − / 1H + -transfer concept instead of a typical chemical hydride (H − , which is equivalent to 2e − /1H + ) transfer for the regeneration step, recent investigations demonstrated that the regeneration of certain organic hydride compounds (AH) is indeed possible by the electrochemical approach via the proton-coupled electron-transfer process in the presence of a suitable proton source, 3a,8 or by photochemical approach using suitable photosensitizers (Figure 1b). 9 However, because electron-and proton-transfer events with the oxidized organic NADP + analogues are inherently involved, the still-existing key challenges with many organic systems are as follows: (i) facile irreversible dimerization of the one-electron reduced (electrochemically generated) radical species (A • ) prior to the desired reaction and (ii) difficult protonation step of the one-electronreduced radical species (A • ) for its (AH •+ ) further oneelectron reduction to form the hydride (AH). By improving the stability and basicity of the electrochemically generated organic radicals with proper structural modification and also by employing a suitable proton source with matching pK a values, the above limitations were overcome to a certain extent.…”

Sensitizer-Free Photochemical Regeneration of Benzimidazoline Organohydride

“…17 Hasegawa et al reported photochemical hydrodesulfonylation using a catalytic amount of BI + -1Nap-O − in the presence of stoichiometric NaBH 4 (Figure 1c). 21 Recently, Glusac 22 and our group 23,24 independently reported the photochemical regeneration of BIH (Figure 1d,e). Using photochemical regeneration, metal-free CO 2 reduction to formate was achieved (Figure 1e), 24 in which the excited state of the carbazole photosensitizer PS2 with high reducing capability 25−27 performed the first one-electron transfer in the two-electron reduction process of BI + -Me to BIH-Me.…”

Sensitizer-free photochemical regeneration of benzimidazoline organohydride

“…17 Hasegawa et al reported photochemical hydrodesulfonylation using a catalytic amount of BI + -1Nap-Oin the presence of stoichiometric NaBH4 (Figure 1c). 21 Recently, Glusac 22 and our group [23][24] independently reported the photochemical regeneration of BIH (Figure 1d,e). Using photochemical regeneration, metal-free CO2 reduction to formate was achieved (Figure 1e), 24 in which the excited state of the carbazole photosensitizer PS2 with high reducing capability [25][26][27] performed the first one-electron transfer in the two-electron reduction process of BI + -Me to BIH-Me.…”