(Keynote) Radical Ion Excited States As Super-Redox Agents for Artificial Photosynthesis

We will discuss several examples in which photoexcitation of a stable radical anion or cation with visible or NIR light results in production of a high-potential radical ion excited state that can carry out difficult redox reactions relevant to artificial photosynthesis.

First, we will show that selective excitation of a naphthalenediimide radical anion (NDI^•−) covalently linked to the 4-, 5, or 6-positions of the bipyridine (bpy) in the Re(bpy)(CO)₃X (X = Cl or pyridine) carbon dioxide reduction catalyst results in electron transfer from ^2*NDI^•− to Re(bpy)(CO)₃X to form Re(bpy^•−)(CO)₃X, the first intermediate in the photocatalytic reduction of CO₂. Femtosecond UV/Vis, near-IR and mid-IR spectroscopy on these constitutional isomers show that systematically varying the electronic coupling as well as the reaction free energy increases the lifetime of Re(bpy^•−)(CO)₃X by an order of magnitude when the NDI chromophore is attached to the 6-position of bpy.

Electrochemical reduction of the corresponding Mn(bpy)(CO)₃X CO₂ reduction catalyst is thought to proceed by the initial reduction of Mn^I to Mn⁰. We have covalently attached a naphthalenediimide radical anion (NDI^•-) chromophore to the 4-, 5-, or 6-positions of the bpy via a phenyl bridge to produce Mn(NDI^•--bpy)(CO)₃X, where X = Br, CH₃CN, or DMF, and have used femtosecond and nanosecond transient IR spectroscopy to directly observe the intermediates produced by two electron transfer reactions following selective photo-excitation of NDI^•−. In complexes where NDI^•− is attached at the 4- or 5-positions of bipyridine, only the reaction Mn(^2*NDI^•−-bpy)(CO)₃X → Mn(NDI-bpy^•−)(CO)₃X is observed, while in the complex where NDI^•− is attached to the 6-position of bipyridine, the reaction sequence: Mn(^2*NDI^•−-bpy)(CO)₃X → Mn(NDI-bpy^•−)(CO)₃X → Mn⁰(NDI-bpy)(CO)₃ is observed. Moreover, in the complexes with an NDI^•- bound to the 6-position of bipyridine, Mn⁰(NDI-bpy)(CO)₃ exhibits a lifetime that is ~10⁵ times longer than those in complexes with an NDI^•- bound at the 4- or 5- positions of the bipyridine.

On the oxidative side, we will discuss the 10-phenyl-10H-phenothiazine radical cation (PTZ^+•), which has a manifold of excited doublet states accessible using visible and NIR light that can serve as super-photo-oxidants with excited state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photo-excitation of PTZ^+• in CH₃CN with a 517 nm laser pulse populates a D_n electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D₁' (τ < 0.3 ps), followed by relaxation to D₁ (τ = 10.9 ± 0.4 ps), which finally decays to D₀ (τ = 32.3 ± 0.8 ps). D₁' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D₁' → D₁ relaxation time (τ = 19 ± 2 ps). To probe the oxidative power of PTZ^+• photoexcited doublet states, PTZ^+• was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenyl-anthracene (DPA), and 10-phenyl-9-anthracene-carbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs. SCE, respectively. In all cases, photoexcitation of PTZ^+• result in ultrafast oxidation of Per, DPA, and ACN.

The photoexcited peri-xanthenoxanthene radical cation (PXX^+•) is another super-oxidant that has a 124 ps electronic excited state (D₁) lifetime and can deliver +2.1 V vs. SCE of oxidizing potential. Photoexcitation of PXX^+• covalently attached to electron deficient 9,10-bis(trifluoromethyl)anthracene (TMFA) using an 885 nm laser pulse drives oxidation of TFMA with unity quantum yield. Extending the PXX^+•-TFMA dyad to a molecular triad having a 9,10-diphenylanthracene terminal hole acceptor, PXX^+•-TFMA-DPA, and selectively exciting PXX^+• results in formation of PXX-TFMA-DPA^+• with a 46% quantum yield and a τ = 11.5 ± 0.6 ns lifetime. This work demonstrates that the PXX^+• D₁ electronic excited state can serve as a promising super-oxidant for challenging oxidation reactions relevant to solar-energy applications.