Triazole-based lanthanide(III) adducts: Photo- and thermochromic luminescence

Highlights: • Molecular design of Ln(III) hybrid complexes based on triazole ligands for best luminescence characteristics. • The role of LMCT states on the luminescence. • DTF calculations for better understanding of the emission data. • Terbium and Europium complexes show thermomodulated luminescent behavior on unique energy transfer between triazole molecules and Ln(III) ions. New series of Ln (III) adducts with triazole-based ligands has been synthesized and characterized as potential fluorescent emitters. The general formulas are Ln (C₂H₆O₂) (H₂O)₂(NO₃)₃∙2bpt∙2H₂O where Ln – Sm (1), Eu (2), Gd (3), Tb (4) and Dy (5); bpt - 3,5-bis(pyridine-4-yl)-1,2,4-triazole. The crystal structures of 2 and 4 were determined by single-crystal X-ray analysis. Luminescence studies have been carried out for the complexes in the solid-state phase. All luminescence spectra showed characteristic emission bands of lanthanide ions and broad band of ligand-centred emission that indicates about incomplete suppression of the ligand-centred fluorescence is observed. As a result, it leads to an inefficient ligand-to-metal energy transfer. Complexes of Tb(III) and Eu(III) ions demonstrate thermosensing properties based on dual emission as result of energy transfer between 3,5-bis(pyridine-4-yl)-1,2,4-triazole molecules and Ln (III) ions.