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The visible-light responsive Cu(I)-complex photosensitizers were developed by introducing various aromatic substituents at the 4,7-positions of a 2,9-dimethyl-1,10-phenanthroline (dmp) ligand in a heteroleptic CuI(dmp)(DPEphos)+-type complexes (DPEphos = [2-(diphenylphosphino)phenyl]ether) for photocatalytic CO2 reduction. Introducing biphenyl groups (Bp-) on the dmp ligand enhanced the molar extinction coefficient (ε) of the metal-to-ligand charge transfer (MLCT) band in the visible region (ε = 7,500 M−1cm−1) compared to that of the phenyl (Ph-)-containing analog (ε = 5,700 M−1cm−1 at λmax = 388 nm). However, introducing 4-R-Ph- groups (R = the electron-withdrawing groups NC-, or NO2-) led to a red shift in the band to λmax = 390, 400, and 401 nm, respectively. Single-crystal X-ray analysis showed the Ph- groups were twisted because of the steric repulsion between the 2,6-protons of the Ph- groups and 5,6-protons of the dmp ligand. The result strongly indicated that the π-conjugation effect of the 4-R-Ph- groups is so weak that the lowering of the energy of the dmp π* orbitals is small. However, when 4-R-ph- was substituted by a 5-membered heterorings, there was a larger red shift, leading to an increase in the ε value of the MLCT absorption band. Thus, the substitution to 2-benzofuranyl- groups resulted in visible-light absorption up to 500 nm and a shoulder peak at around 420 nm (ε = 12,300 M−1cm−1) due to the expansion of π-conjugation over the substituted dmp ligand. The photocatalytic reaction for CO2 reduction was tested using the obtained CuI complexes as photosensitizers in the presence of a Fe(dmp)2(NCS)2 catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as a sacrificial reductant, which showed improved CO generation. more...

A cyclometalated iridium(III) complex having 2-(pyren-1-yl)-4-methylquinoline ligands [Ir(pyr)] has a strong absorption band in the visible region (ε444nm = 67,000 M−1 cm−1) but does not act as a photosensitizer for photochemical reduction reactions in the presence of triethylamine as an electron donor. Here, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H) was used instead of the amine, demonstrating that BI(OH)H efficiently quenched the excited state of Ir(pyr) and can undergo the photochemical carbon dioxide (CO2) reduction catalyzed by trans(Cl)-Ru(dmb)(CO)2Cl2 (dmb = 4,4′-dimethyl-2,2′-bipyridine, Ru) to produce formate as the main product. We also synthesized a binuclear complex combining Ir(pyr) and Ruvia an ethylene bridge and investigated its photochemical CO2 reduction activity in the presence of BI(OH)H. more...

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 (E1/2red = −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. more...