Visible-light driven photocatalytic CO2 reduction promoted by organic photosensitizers and a Mn(I) catalyst

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Abstract Summary

A visible‐light three-component system for the photocatalytic reduction of CO₂ was constructed from 1) a sterically protected Mn(I) bipyridine tricarbonyl complex (catalyst), 2) purely organic thermally activated delayed fluorescence (TADF) dyes (photosensitisers, PS), and 3) 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole (BIH) as sacrificial electron donor (e-D). Among five TADF dyes evaluated, the diphenyl-amine derivative 4DPAIPN proved optimal, delivering a total turnover number (TON) for CO + HCOOH > 650 and a quantum yield Φ_CO+HCOOH = 22.8 % under 470 nm irradiation with equimolar PS : catalyst (50 µM each). Mechanistic studies show that the long-lived TADF state of 4DPAIPN is reductively quenched by BIH; the resulting PS^•- then reduces the Mn catalyst which subsequently converts CO₂ into CO and formic acid.

Advances and Special Progress

• Demonstrates the highest quantum yield yet reported for CO₂ reduction driven by **metal-free TADF photosensitisers** (22.8 %).
• Operates with a **1 : 1 PS : catalyst ratio**, unlike earlier TADF systems that required a large PS excess, thus improving atom economy and durability.
• Identifies electronic and photophysical design rules: strong visible absorption (>450 nm), deep one-electron reduction potential (E_red ≤ -2.0 V vs Fc^+/0) and a μs–ms TADF lifetime are all critical for efficient electron transfer to Mn(I).
• Uses an **earth-abundant Mn(I) complex** featuring an ortho-phenyl “umbrella” that suppresses the usual Mn–Mn dimerisation pathway, prolonging catalyst life.

Additional Remarks

The study advances sustainable solar-fuel research by replacing scarce Ru/Ir chromophores with inexpensive organic dyes while retaining high catalytic activity. Remaining challenges include (i) mitigating PS photobleaching during extended operation and (ii) understanding the speciation of Mn intermediates that selectively yield CO versus HCOOH.

Content of the Published Article in Detail

1. **Photophysical screening** – Five donor-acceptor TADF dyes (two diphenylamine-based and three carbazole-based) were compared. Only the DPA dyes (4DPAIPN and 3DPAFIPN) combine red-shifted absorption (onset ≈ 500 nm) with suitably negative E_red to reduce the Mn catalyst. 2. **Electrochemistry** – Cyclic voltammetry revealed E_red(PS/PS^•-) of -2.08 V (4DPAIPN) and -1.94 V (3DPAFIPN) versus Fc^+/0, both more negative than Mn(E^p) = -1.83 V, enabling thermodynamically downhill electron transfer. 3. **Mechanism (word description)**

  * hv excites 4DPAIPN → S1 (ns) ⇄ T1 (≈ 80 µs) via RISC (TADF).  
  * T1 is **reductively quenched** by BIH (kq ≈ 1.5 × 108 M-1s-1) giving 4DPAIPN•- + BIH•+.  
  * 4DPAIPN•- transfers an electron to Mn(I)→Mn(0)•-; BIH•+ is deprotonated by TEOA, preventing charge recombination.  
  * Mn(0)•- binds CO2; subsequent proton-coupled electron transfer steps (second e- from BI• or extra PS•-) furnish CO or HCOOH and regenerate Mn(I).  

4. **Product analysis** – Isotopic labelling with ¹³CO₂ confirmed that both CO and HCOOH originate from CO₂ (m/z = 29 for ¹³CO, +1 mass shift for H¹³COOH). 5. **Control experiments** – No conversion is observed without light, CO₂, PS, catalyst, or BIH, validating the cooperative triad design.

Catalyst

• **Class & Structure**: A facial tricarbonyl Mn(I) complex with 4,4′-dimethyl-2,2′-bipyridine bearing an ortho-phenyl substituent at the 6-position.
• **Function**: Accepts electrons from the reduced PS, forms a reduced Mn(0) species that binds and activates CO₂, and ultimately mediates its two-electron/proton reduction to CO and/or formate.
• **Unique Property**: The pendant phenyl ring sterically blocks Mn–Mn dimerisation—a degradation pathway common in Mn(bpy)(CO)₃Br catalysts—thereby enhancing longevity under photoreduction conditions.

Photosensitizer

• **Lead Dye**: 4DPAIPN (2,4,5,6-tetracyano-1,3-dipyrene-substituted diphenylamine).
• **Role**: Harvests visible photons (ε ≈ 6.5 × 10³ M^-1 cm^-1 at 470 nm), undergoes TADF providing a long-lived excited state that can be reductively quenched, generating a strongly reducing radical anion capable of electron transfer to Mn(I).
• **Advantage over Carbazole Analogues**: More red-shifted absorption and deeper ground-state reduction potential, allowing operation with lower PS loading and longer‐wavelength light.

Investigation

	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	solvent A	λexc [nm]	.
1.	50	Molecule:100992	0.05	BIH	0.1	DMA/TEOA (1.5 M)	470
2.	50	Molecule:100992	0.05	BIH	0.1	DMA/TEOA (1.5 M)	440
3.	50	3DPAFIPN	0.05	BIH	0.1	DMA/TEOA (1.5 M)	470
4.	50	3DPAFIPN	0.05	BIH	0.1	DMA/TEOA (1.5 M)	440
5.	50	Molecule:100992	0.25	BIH	0.1	DMA/TEOA (1.5 M)	440

Investigation-Name: inv0ExportRefresh

(The table lists representative experiments; “solvent A” refers to N,N-dimethylacetamide containing triethanolamine as base/proton source.)