Heteroleptic [Os(Cl)(CO)(P^P)(pbi)] complexes bearing bidentate phosphine and 2-(2-pyridyl)benzimidazolate ligands: impact of isomerism on their luminescence properties.

A series of six [Os(Cl)(CO)(P^P)(pbi)] complexes have been synthesized and characterized using FT-IR, 1H NMR, and 31P NMR spectroscopy. Their molecular structures have been confirmed by means of X-ray diffraction studies. For each of the studied bidentate phosphines (P^P = cis-1,2-bis(diphenylphosphino)ethene - dppv, 1,2-bis(diphenylphosphino)ethane - dppe, 1,2-bis(diphenylphosphino)benzene - dppb) the applied synthesis procedure has afforded preparation of two isomers with pseudo-octahedral coordination of the osmium(ii) ion. According to X-ray data, the obtained isomers, green emissive [OC-6-24] and yellow emissive [OC-6-23] species, differ mainly in the arrangement of the 2-(2-pyridyl)benzimidazolate (pbi) anion in their structures. In the [OC-6-24] isomers the benzoimidazole fragment of the pbi ligand is located trans to the coordinated CO molecule, whereas the cis conformation is characteristic of the [OC-6-23] isomers. Each of trans-[Os(Cl)(CO)(P^P)(pbi)] shows intense green emission attributable to the excited triplet state of the pbi ligand, whereas the yellow emission from the excited cis-[Os(Cl)(CO)(P^P)(pbi)] indicates slight metal-to-ligand charge transfer character (from the Os(Cl)(CO)(P^P)+ fragment to the pbi ligand). The investigated complexes are generally well emissive with emission quantum yields up to 0.49 and emission lifetimes in the range of 10-150 μs. Only the yellow emissive cis-[Os(Cl)(CO)(dppv)(pbi)] complex exhibits remarkably different photophysical behaviour despite the fact that all three cis-[Os(Cl)(CO)(P^P)(pbi)] isomers emit in the same spectral region. In the view of DFT/TD-DFT results this has been explained by the presence of an additional excited dark state possessing distinct charge transfer character (from the Os(Cl)(CO)(pbi) fragment to the dppv ligand).