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).