Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

Abstract

Efficient excited-state electron transfer between an iron­(III) photosensitizer and organic electron donors was realized with green light irradiation. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe­(phtmeimb)2]+ (phtmeimb = {phenyl­[tris­(3-methyl-imidazolin-2-ylidene)]­borate}, that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru­(bpy)3]2+. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe­(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the ultraviolet to the infrared regions, provided a quantitative description of the catalytic mechanism with associated rate constants and yields.

Anti-dissipative strategies towards more efficient solar energy conversion

“…With this clear excited state description, we noticed that the higher energy excited state 3 MLCT(bpy) of Dm and Tmm was sufficiently long-lived to perform bimolecular electron transfer, 36 and could hence be intercepted before IC/VR for anti-dissipative energy conversion. These higher energy-state could save between 800 cm -1 (100 meV) and 1400 cm -1 (170 meV) for Dm and Tmm, respectively.…”

“…23,37 The formation of oxidized tri-tolylamine (TTA •+ ) was clearly evident in nsTAS (Figures 6a, S14-S17) through the large absorption changes observed in the 15000-20000 cm -1 (500-800 nm) range, with a maximum at 14900 cm -1 (670 nm). 36 To provide insights into the electron transfer mechanism, different amounts of TTA were added to solutions of Dm, and a target analysis of the nsTAS results was applied (Figures 6 and S13-S15). The resulting 3 MLCT(Lm) and 3 MLCT(bpy) decay constants were found to increase upon the addition of TTA (Figure 6e-f and Table S7) which were analyzed according to equation (4).…”

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

“…The more complex the photocatalytic systems become, the more likely this probably gets. 136 , 157 , 158 …”

“…The combination of [Cu(dap) 2 ] + and DCA complements and expands the known photochemical applications of these two individual components when used separately. 68 , 77 , 83 , 89 , 117 , 159 − 167 Red light-driven applications play important roles in other important contexts, for example, hydrogen production, 47 , 48 , 168 , 169 medical applications, 158 , 169 − 173 and polymerizations. 174 − 178 Now, red light as well as multiphoton excitation-based mechanisms seem to become of increasing interest for synthetic organic photoredox chemistry, 50 , 126 , 179 − 181 and we hope the insights gained from our work will be useful in that greater context.…”

Molecular Rubies in Photoredox Catalysis

“…using Mn I , Fe II and Cu I photocatalysts ( Herr, et al, 2021 ; Leis, et al, 2022 ; Hossain, et al, 2019 ). Oxidative processes photoinduced by first row transition metal complexes are comparably scarce and have been reported with iron(III) and cobalt(III) ( Aydogan, et al, 2021 ; Pal, et al, 2018 ) and in particular with chromium(III) photocatalysts ( Stevenson, et al, 2015 ; Higgins, et al, 2016 ). Oxidatively induced photocatalytic processes, e.g.…”