Chirdon-et-al-2016-tuning-iridium-photocatalysts-and-light-irradiation-for-enhanced-co2-reduction.pdf

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Abstract Summary

The study examines a family of “all-in-one’’ Ir(III) complexes that act simultaneously as photosensitiser and catalyst for the visible-light reduction of CO₂ to CO in acetonitrile/triethanolamine (TEOA). By appending progressively bulkier aromatic groups to the 4′-position of the terpyridine (tpy) ligand and by lowering the blue-light intensity, the authors stretch the catalyst lifetime from a few hours to more than ten days and lift the turnover number (TON) from ≈90 to 265 with a quantum yield of 0.10.

Advances and Special Progress

• Ligand engineering: introduction of a 9-anthryl group on tpy (complex 3) provides steric shielding and an additional triplet energy-dissipation pathway that curbs deactivation.
• Photoregulation: operating with low-power 450 nm LEDs diminishes the steady-state concentration of reactive reduced intermediates, thereby suppressing formation of an off-cycle, “green’’ degradation product.
• Record performance for this Ir(III) catalyst class: TON(CO) = 265 and continuous activity for >250 h without external photosensitisers or precious-metal co-catalysts.

Additional Remarks

• Relevance: Selective CO evolution from CO₂ is a key step toward carbon-neutral synthetic fuels.
• Sustainability: The system functions in wet acetonitrile and tolerates O₂ traces, simplifying practical deployment.
• Challenge: The high cost and rarity of iridium remain obstacles; lessons on photoprotection gleaned here are transferable to earth-abundant platforms.

Content of the Published Article in Detail

The complexes have the general formula [Ir(ppy)(tpy-R)Cl]⁺ where ppy = 2-phenylpyridine and R = H (1), 4′-phenyl (2), 4′-9-anthryl (3), 4′-p-fluorophenyl (4).

1. **Photophysical & electrochemical characterisation**

  * First reversible reduction at −0.84 V vs NHE is tpy-centred for all derivatives.  
  * Visible MLCT absorption tails to 520 nm; emission lifetimes ≈4 µs except for 3 (3.4 µs, lower Φ due to anthryl triplet quenching).  
  * Stern–Volmer analysis shows similar reductive-quenching rates with TEOA (k_q ~6–7 × 10⁸ M⁻¹ s⁻¹).

2. **Proposed mechanism (words)**

  a. 450 nm excitation → *Ir(III)*¹* (³MLCT).  
  b. Reductive quenching by TEOA gives Ir(II) one-electron-reduced species (OERS).  
  c. Chloride or solvent dissociation opens a vacant site; CO₂ binds and is reduced to CO, regenerating Ir(III).  
  d. Competing dark reaction of OERS affords a long-lived “green’’ species that cannot enter the catalytic cycle; accumulation of this species deactivates the system.  
  e. Bulky R groups and low photon flux slow steps (c) and/or (d) that lead to deactivation.

3. **Operando UV–vis / spectroelectrochemistry**

  * Growth of a 700 nm band parallels loss of activity; spectra match those formed during prolonged electrolysis of OERS, confirming an off-cycle product.  
  * Complex 3 accumulates this band more slowly, correlating with its longer operational life.

Catalyst

• Class: Cyclometalated Ir(III) complexes bearing a bidentate ppy and tridentate tpy-R ligand.
• Function: Acts as photosensitiser, electron relay, and CO₂-reduction site in a single molecule.
• Unique properties:

 - Visible absorption to 520 nm, enabling blue-LED excitation.  
 - Water tolerance.  
 - Tunable steric/electronic environment via tpy 4′-substitution; the anthryl derivative (3) displays built-in photoprotective energy-dissipation.

Photosensitizer

No external photosensitiser is employed; the Ir complex itself harvests light. The MLCT excited state (³[Ir(ppy)(tpy-R)Cl]⁺*) is reductively quenched by TEOA to start the catalytic electron chain.

Investigation

cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	solvent A	λ_exc [nm]	TON_CO
1	500	— (intrinsic)	0.5	TEOA	≈2.7	CH₃CN (83 %)	450	91
2	500	—	0.5	TEOA	≈2.7	CH₃CN (83 %)	450	144
3	500	—	0.5	TEOA	≈2.7	CH₃CN (83 %)	450	172
3 (low-light)	500	—	0.5	TEOA	≈2.7	CH₃CN (83 %)	450 (0.43 mW cm⁻²)	265

• All experiments used a 1 : 5 v/v TEOA/CH₃CN mixture (4 mL, CO₂-saturated, ambient T) and continuous irradiation.