The influence of structural isomerism on luminescence properties of [Re(CO) 2 (dppv)(pbi)] complexes containing cis -1,2-bis(diphenylphosphino)-ethene and 2-(2-pyridyl)benzimidazole ligands

Simple reaction procedure, boiling of an equimolar mixture of [Re(Cl)(CO) 3 (dppv)] (dppv— cis -1,2-bis(diphenylphosphino)-ethene), 2-(2-pyridyl)benzimidazole (pbi), and TlClO 4 in o -dichlorobenzene solution, leads to two, chromatographically separable, isomers of [Re(CO) 2 (dppv)(pbi)] complex. The obtained species, the yellow-emissive [ OC -6–54] and the orange-emissive [ OC -6–53] isomers, have been identified using FT-IR, 1 H NMR, and 31 P NMR spectroscopy and their structures confirmed by means of X-ray investigations. According to obtained crystallography results, the investigated isomers differ mainly in the arrangement of the 2-(2-pyridyl)benzimidazole anion in their structures. Diverse arrangement of the dppv and pbi ligand around the central cis -[Re(CO) 2 ] + core results in well-pronounced changes in luminescence properties of the studied luminophores. According to results from performed DFT and TDA-DFT computations, the [ OC -6–54] isomer emission occurs from the intra-ligand excitation localized mainly within the pbi ligand. In contrary to that, the [ OC -6–53] isomer emission indicates some metal-to-ligand charge-transfer character associated with the charge transfer from the Re(CO) 2 (dppv) + fragment to the pbi ligand. Moreover, both isomers exhibit different character of their emission decays, mono-exponential and bi-exponential for the [ OC -6–54] and [ OC -6–53] ones, respectively. In the view of obtained DFT/TDA-DFT results, this has been explained by the presence of an additional excited dark state, operative only in the case of the [ OC -6–53] isomer and associated with the charge transfer from the Re(CO) 2 (pbi) fragment to the dppv ligand.