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

A novel N boolean AND N boolean AND 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 (MLCT)-M-1 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 (MLCT)-M-3 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 (MLCT)-M-1 maxima are in good agreement with DFT and TD-DFT calculations. Complexes 1 and 2 emit from a Ru-II-to-tpy (MLCT)-M-3 state, which is rarely the emitting state at lambda > 850 nm in [Ru(tpy)(N boolean AND N boolean AND N)](2+) complexes when the ancillary ligand is neutral. Complexes 1 and 2 also exhibit long excited-state lifetimes (tau similar to 100 ns) at room temperature with associated quantum yield (Phi) of 0.001. The reported tau and Phi values are approximately 400-500 times and 1000 times higher compared to those of [Ru(tpy)(2)](2+) (tau = 0.25 ns, Phi <= 5 x 10(-6)), respectively. Complexes 3 and 4 emit from a Ru-II-to-dpt (MLCT)-M-3 state, albeit only at 77 K (tau = 0.25 ns) due to rapid deactivation of their (MLCT)-M-3 state according to the energy-gap law. The improved photophysical properties of the complexes are consequences of enlarged separation of the (MLCT)-M-3- (MC)-M-3 states, due to the strong donation and larger bite angles of the dgpy ligand.