Visible-light sensitisation of near-infrared luminescence from Yb(iii), Nd(iii) and Er(iii) complexes of 3,6-bis(2-pyridyl)tetrazine

Reaction of the potentially bis-bidentate bridging ligand 3, 6-bis (2-pyridyl) tetrazine (BPTZ) with various lanthanide complexes Ln (tta) 3 (H2O) 2 Htta = thenoyl (trifluoro) acetone; Ln = La, Nd, Gd, Er, Yb in aqueous ethanol afforded the mononuclear complexes Ln (tta) 3 (BPTZ) (Ln = La, Nd) or the dinuclear complexes Ln (tta) 32 (μ-BPTZ) (Ln = Gd, Er, Yb) in which one or two, respectively, lanthanide tris-diketonate Ln (tta) 3 units are bound to the N, N-bidentate compartments of BPTZ. Crystal structures of the dinuclear complexes Yb (tta) 32 (μ-BPTZ) ·CH2Cl2 and Gd (tta) 32 (μ-BPTZ) ·2CH2Cl2 show that the metal centres have an approximately square-antiprismatic eight-coordinate geometry; there are close contacts above and below the plane of the BPTZ bridging ligand between peripheral trifluoromethyl groups from a tta ligand associated with each metal centre. It is not apparent why the larger lanthanides La and Nd only give a mononuclear complex whereas the smaller lanthanides Gd, Er and Yb give the dinuclear complex in each case. UV/Vis spectroscopic titrations of a solution of BPTZ in CH2Cl2 with increasing amounts of Ln (tta) 3 (H2O) 2 (Ln = La and Yb) show very similar behaviour, with stepwise binding constants K1 and K2 for association of the two Ln (tta) 3 units of ca. 106 M−1 and 105 M−1; allowing for the expected statistical factor of 4 there is an additional reduction in the value of K2 compared to K1 which may be associated with a steric interaction between the two Ln (tta) 3 units when the dinuclear complex forms. Steady-state and time-resolved luminescence studies on the complexes with Yb, Nd and Er, both in the solid state and CH2Cl2 solution, show that near-IR luminescence on the microsecond timescale can be sensitised by irradiation either at 337 nm into the tta-based transition, or at 520 nm into the low-energy BPTZ-centred transition.