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)(3)}(2) (mu-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)(3)}(2) (mu-BPTZ)] .CH2Cl2 and [{Gd(tta)(3)}(2)(mu-BPTZ)].2CH(2)Cl(2) 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 K-1 and K-2 for association of the two {Ln(tta)(3)} units of ca. 10(6) M-1 and 10(5) M-1; allowing for the expected statistical factor of 4 there is an additional reduction in the value of K-2 compared to K-1 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.