Ln(iii) complexes with a chiral 1H-pyrazolo[3,4-b]pyridine derivative fused with a (−)-α-pinene moiety: synthesis, crystal structure, and photophysical studies in solution and in the solid state

Chiral lanthanide(III) complexes, [LnL2(NO3)3]·MeCN (Ln = Sm (1), Eu (2), Tb (3)), containing an optically active 1H-pyrazolo[3,4-b]pyridine derivative fused with a natural monoterpene (−)-α-pinene moiety (L) have been synthesized and structurally characterized. The mononuclear complexes 1–3 are isostructural and crystallize in the non-centrosymmetric space group P21212. The lanthanide ions are ten-coordinated with three bidentate chelating nitrate ions and two bidentate N,N-donor ligands L. Complexes 1–3 show metal-centered luminescence at room temperature in solution and in the solid state. The solid-state and solution-phase luminescence quantum yields increase in the order 1 < 2 < 3. Complex 3 possesses a very high luminescence quantum yield both in acetonitrile solution (φf = 0.736) and in the solid state (φf = 0.890), indicating efficient energy transfer from two ligands L to the Ln center. The luminescence quantum yields of complexes 1 and 2 in the acetonitrile solutions are almost two and four times higher, respectively, than those in the solid state. DFT and TD-DFT studies were carried out to explore geometry and electronic structure of L and to gain insight into its absorption properties.