Near infra-red emitting Ru(ii) complexes of tridentate ligands: electrochemical and photophysical consequences of a strong donor ligand with large bite angles

Abstract

A novel N^N^N tridentate ligand dgpy (dgpy = 2,6-diguanidylpyridine) was synthesized by a Pd-catalyzed C–N bond-forming reaction. A novel family of [Ru^II(tpy')(dgpy)](PF₆)₂ (1 and 2) or [Ru^II(dpt')(dgpy)](PF₆)₂ (3 and 4) (tpy' = substituted-2,2′:6′,2′-terpyridine, dpt' = substituted-2,4-dipyrid-2′-yl-1,3,5-triazine) complexes are reported. The dgpy ligand (80%) and the heteroleptic complexes 1–4 (37–60%) were obtained in modest to good yields. The dgpy ligand and its complexes were fully characterized by a variety of techniques including X-ray crystallography and density functional theory (DFT). In cyclic voltammetric studies, the complexes exhibit a Ru^III/II couple, which is 600–800 mV less positive than the Ru^III/II couple in [Ru(tpy)₂]²⁺. The ¹MLCT absorption maxima of all the complexes (620–740 nm) are considerably red-shifted as compared to that of [Ru(tpy)₂]²⁺ (474 nm). The ³MLCT emission maxima of complexes 1 and 2 are also red-shifted by about 270 nm compared to that of [Ru(tpy)₂]²⁺ (629 nm) at room temperature (298 K), whereas the corresponding maxima for complexes 3 and 4 are shifted by about 330 nm at 77 K. The relative trends in redox potentials and ¹MLCT maxima are in good agreement with DFT and TD-DFT calculations. Complexes 1 and 2 emit from a Ru^II-to-tpy ³MLCT state, which is rarely the emitting state at λ > 850 nm in [Ru(tpy)(N^N^N)]²⁺ complexes when the ancillary ligand is neutral. Complexes 1 and 2 also exhibit long excited-state lifetimes (τ ∼ 100 ns) at room temperature with associated quantum yield (Φ) of 0.001. The reported τ and Φ values are approximately 400–500 times and 1000 times higher compared to those of [Ru(tpy)₂]²⁺ (τ = 0.25 ns, Φ ≤ 5 × 10⁻⁶), respectively. Complexes 3 and 4 emit from a Ru^II-to-dpt ³MLCT state, albeit only at 77 K (τ = 0.25 ns) due to rapid deactivation of their ³MLCT state according to the energy-gap law. The improved photophysical properties of the complexes are consequences of enlarged separation of the ³MLCT–³MC states, due to the strong donation and larger bite angles of the dgpy ligand.