Osmium Complexes with Tridentate 6-Pyrazol-3-yl 2,2′-Bipyridine Ligands : Coarse Tuning of Phosphorescence from the Red to the Near-Infrared Region

We have prepared and characterized a series of osmium complexes [Os₂(CO)₄(fpbpy)₂] (1), [Os(CO)(fpbpy)₂] (2), and [Os(fpbpy)₂] (3) with tridentate 6-pyrazol-3-yl 2,2′-bipyridine chelating ligands. Upon the transformation of complex 2 into 3 through the elimination of the CO ligand, an extremely large change in the phosphorescence wavelength from 655 to 935 nm was observed. The results are rationalized qualitatively by the strong π-accepting character of CO, which lowers the energy of the osmium d_π orbital, in combination with the lower degree of π conjugation in 2 owing to the absence of one possible pyridine-binding site. As a result, the energy gap for both intraligand π-π* charge transfer (ILCT) and metal-to-ligand charge transfer (MLCT) is significantly greater in 2. Firm support for this explanation was also provided by the time-dependent DFT approach, the results of which led to the conclusion that the S₀ → T₁ transition mainly involves MLCT between the osmium center and bipyridine in combination with pyrazolate-to-bipyridine ³π-π* ILCT. The relatively weak near-infrared emission can be rationalized tentatively by the energy-gap law, according to which the radiationless deactivation may be governed by certain low-frequency motions with a high density of states. The information provided should allow the successful design of other emissive tridentate metal complexes, the physical properties of which could be significantly different from those of complexes with only a bidentate chromophore.