Binuclear Ru-Ru and Ir-Ru complexes for deep red emission and photocatalytic water reduction

Four butterfly-like binuclear Ru(II)-Ru(II) and Ir(III)-Ru(II) complexes were designed and synthesized via a stepwise method by Ru(II)/Ir(III) metalloligands containing polypyridine (bpy)/phenylpyridine (ppy), phenanthroline (phen) and bibenzimidazole (BiBzIm) moieties. The absorption and photoluminescence of Ru(II)-Ru(II) compounds are dominated by metal-to-ligand charge-transfer (MLCT) transitions from Ru(II) centers to the organic ligand parts, which emit in the deep red region with a wavelength similar to 700 nm. While in Ir(III)-Ru(II) complexes, an additional decay channel is opened for the energy transfer from the higher energy level MLCT state of Ir(III)-coordinated units to the lower-energy level MLCT state of Ru(II)coordinated units, as approved by both experimental and theoretical DFT calculations. Therefore, similar deep red emission profiles originating from Ru(II) units are observed in Ir(III)-Ru(II) systems. These binuclear complexes were further tested as photosensitizers (PSs) to produce H-2 in photocatalytic water reduction systems. The highest H2 production efficiency can be obtained in the heteronuclear IrRu(1) system after 80 hours continuous production with a TON value of 1088 based on the amount of IrRu(1) as PS, much higher than the other binunclear complexes and mononuclear counterparts. The results provide a new insight into the designing guidelines for noble metal complexes as emitting centers and photosensitizers in lighting/display materials and devices, as well as photocatalytic water splitting systems.