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 Cu I (dmp)(DPEphos) + -type complexes (DPEphos = [2-(diphenylphosphino)phenyl]ether) for photocatalytic CO 2 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 −1 cm −1 ) compared to that of the phenyl (Ph-)-containing analog (ε = 5,700 M −1 cm −1 at λ max = 388 nm). However, introducing 4-R-Ph- groups (R = the electron-withdrawing groups NC-, or NO 2 -) led to a red shift in the band to λ max = 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 −1 cm −1 ) due to the expansion of π-conjugation over the substituted dmp ligand. The photocatalytic reaction for CO 2 reduction was tested using the obtained Cu I complexes as photosensitizers in the presence of a Fe(dmp) 2 (NCS) 2 catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1 H -benzo[d]imidazole as a sacrificial reductant, which showed improved CO generation.

Two dinuclear CuI complexes with two tetradentate ligands consisting of a diimine unit and two diphenylphosphine units connected with −C4H8– chains were synthesized. The CuI dimer with p-CF3 substituents worked as an outstanding redox photosensitizer for photocatalytic CO2 reduction; FeII(dmp)2(NCS)2 (dmp = 2,9-dimethyl-1,10-phenanthroline) was used as a catalyst, and CO was produced with a quantum yield of 14.1%. This is one of the most efficient photocatalytic systems using only abundant elements. This Cu dimer was significantly durable: approximately 80% remained in the reaction solution after irradiation for 48 h, while the turnover number was TONCO = 778 based on the dinuclear CuI complex used.

[Cu(I)(dmp)(P)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline derivatives; P = phosphine ligand) is one of the most promising photosensitizers used in a photo-catalytic system reducing CO2, of which quantum yield is as high as...