Abstract
Ruthenium(II) picolinate complex, [Ru(dmb)2(pic)]+ (Ru(pic); dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N∧N)3]2+ (N∧N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λabs < 670 nm) and stronger reduction ability in a one-electron reduced state ( = −1.86 V vs. Ag/AgNO3), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO2 to CO under red-light irradiation (λex > 600 nm). TONCO reached 235 and ΦCO was 8.0%. Under these conditions, [Ru(dmb)3]2+ (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOFCO = 6.7 min−1) and larger TONCO (2347) than Ru(dmb) (TOFCO = 3.6 min−1; TONCO = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.
Highlights
Redox photosensitizers, which absorb visible light and facilitate the electron transfer process, play a key role in various photochemical reactions, such as CO2 reduction (Takeda et al, 2017; Tamaki and Ishitani, 2017), water oxidation (Fukuzumi et al, 2016), hydrogen evolution (Schulz et al, 2012), and organic synthesis (Prier et al, 2013)
We have reported an osmium(II) analog, i.e., [Os(N∧N)3]2+, which could function as a redox photosensitizer utilizing a much wider wavelength range of visible light due to its singlet-to-triplet direct excitation (S-T absorption) and drive photocatalytic CO2 reduction by red-light irradiation in the combination with rhenium(I) catalyst unit (Tamaki et al, 2013b), whereas the high toxicity of OsVIIIO4 inhibits the wider application of osmium complexes
When using Ru(pic) as a photosensitizer and Re as a catalyst, the electron transfer process from OERS of Ru(pic) to Re (Er1e/d2 = −1.76 V) occurs exothermically. These results indicated that Ru(pic) had some advantages with respect to its function as a redox photosensitizer compared with Ru(dmb), including its wider wavelength range of visiblelight absorption and stronger reducing power of OERS, which is effective in the electron transfer to the catalyst
Summary
Redox photosensitizers, which absorb visible light and facilitate the electron transfer process, play a key role in various photochemical reactions, such as CO2 reduction (Takeda et al, 2017; Tamaki and Ishitani, 2017), water oxidation (Fukuzumi et al, 2016), hydrogen evolution (Schulz et al, 2012), and organic synthesis (Prier et al, 2013). Effective photosensitizers should be endowed with three important properties, including (1) visible-light absorption, (2) a long lifetime in the excited state to initiate the electron transfer process, and (3) reducing and/or oxidizing power that is strong enough to donate electrons or holes to the catalyst. Redox Photosensitizer With Wide-Band Absorption redox reactions because these types of complexes exhibit strong absorption in the visible-light region and have a long lifetime in their triplet metal-to-ligand charge-transfer (3MLCT) excited states (Juris et al, 1988; Thompson et al, 2013).
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