Binuclear Ru-Ru, Ir-Ru complexes for deep red emission and photocatalytic water reduction
Four butterfly-like binuclear Ru(II)-Ru(II) or Ir(III)-Ru(II) complexes were designed and synthesized via stepwise method by bridged Ru(II) 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 wavelength ~700 nm. While in Ir(III)-Ru(II) complexes, an additional decay channel is opened for the energy transfer from high energy leveled MLCT state of Ir(III)-coordinated units to the lower-lying 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 H2 in photocatalytic water reduction system. The highest H2 production efficiency can be obtained in heteronuclear IrRu(1) system after an induction period of 12 hours with a TON value of 148, 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 photositizers in lighting/displaying materials and devices, as well as photocatalytic water splitting systems.