LAUSR

This paper reports three structurally characterised Ln(III) ternary complexes, [Ln(hfaa)3(Hind)n] {Ln = La (n = 3), Er and Yb (n = 2); hfaa = hexafluoroacetylacetone anion and Hind = indazole}, and their photophysical properties in the pure near infra-red (NIR) region. The hfaa and H-indazole were used as a sensitizer and antenna ligands, respectively, in the complexes. The single crystal XRD and solution NMR studies show that the La complex is nine-coordinate, while the Er and Yb complexes are octa-coordinated. The paramagnetic Er and Yb complexes show very fast longitudinal relaxation rates (ρ = 1/T1), which make them very useful for para-shift relaxation probes used in MRI. Upon UV excitation, erbium and ytterbium exhibit their characteristic luminescence in the pure NIR region (800–1600 nm). These could be very useful in various advanced applications ranging from optical communication to defence. The complexes showed the highest oscillator strength in dichloromethane followed by acetonitrile, and the oscillator strength was lowest in methanol since methanol molecules replaced the Hind ligands and provided a symmetrical ligand field around the Ln ions. The complexes in solid state showed stronger NIR photophysical properties compared to those in solution.