Excited-State Properties of Octahedral Hexarhenium(III) Complexes with Redox-active N-heteroaromatic Ligands

New sulfide-capped octahedral hexarhenium(III) complexes containing 4-phenylpyridine (ppy) or 1,2-bis(4-pyridyl)ethane (bpe) ((n-C4H9)4N)[mer-{Re6S8Cl3(ppy)3}] ((Bu4N)[1]), ((n-C4H9)4N)2[trans-{Re6S8Cl4(ppy)2}] ((Bu4N)2[2a]), ((n-C4H9)4N)2[cis-{Re6S8Cl4(ppy)2}] ((Bu4N)2[2b]), ((n-C4H9)4N)2[trans-{Re6S8Cl4(bpe)2}] ((Bu4N)2[3a]), and ((n-C4H9)4N)2[cis-{Re6S8Cl4(bpe)2}] ((Bu4N)2[3b]) were prepared, and X-ray single-crystal structure determination was carried out for (Bu4N)2[2a] and (Bu4N)2[3a]. The photophysical properties of these complexes were studied both in acetonitrile at 298 K and in the solid state at 298 and 80 K, along with those of the known 4,4′-bipyridine (bpy) analogues ((n-C4H9)4N)[mer-{Re6S8Cl3(bpy)3}] ((Bu4N)[4]), ((n-C4H9)4N)2[trans-{Re6S8Cl4(bpy)2}] ((Bu4N)2[5a]), and ((n-C4H9)4N)2[cis-{Re6S8Cl4(bpy)2}] ((Bu4N)2[5b]). The photophysical data of [5a]2− and [5b]2− in solution and in the solid state were significantly different from those of other complexes. On the basis of experimental observations of [2a]2− and [5a]2− and density-functional theory (DFT) calculations, it was concluded that [5a]2− and [5b]2− exhibited metal (Re6S8 core)-to-ligand (bpy) charge transfer (MLCT) type emission. This is the first unambiguous demonstration of MLCT type emissions for the hexarhenium complexes. The MLCT components, where present, are only minor in the case of the emissions of [1]−, [2a]2−, [2b]2−, and [4]−; these can be explained primarily as the contributions of the intracore electronic transitions. The emissions of [3a]2− and [3b]2− can be assigned almost completely to the electronic transitions within the Re6S8 core. The different emission characteristics of the bis(bpy) complexes ([5a]2− and [5b]2−) from the tris(bpy) complex ([4]−) are a result of the increase in the number of nitrogen donors on the Re6S8 core, which stabilizes the Re6S8 core energy to a lower level than the energy of the bpy ligand π* orbital. On the other hand, it has been shown that the emissions of the bis(ppy) ([2a]2− and [2b]2−) and bis(bpe) complexes ([3a]2− and [3b]2−) are best characterized by the higher π* energy level of each N-heteroaromatic ligand, which lead to a stronger metal character in the emissive excited state of the complex.