Photo-induced intramolecular electron transfer (IET) of a series of heterodinuclear Ru(II)–Co(III) complexes has been theoretically investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT). The Ru(II)–Co(III) complexes display Ru → Co metal-to-metal electron transfer (MMET) in the visible region. The photosensitivity involving spectral response range and absorption intensity, and IET rate, are improved by introducing (Z)-N-(1H-isoindol-1-ylidene)-2H-isoindol-1-amine as donor ligands. The Ru → Co IET rate in the newly designed complexes shows at least one order of magnitude larger than that in [(bpy)2-Ru(pytp)Co(tren)]5+. These superior performances indicate that heterodinuclear Ru(II)–Co(III) complexes could be promising as the effective ligand release carriers for the selective cancer treatment.