Synthesis and structure of a series of new luminescent Ag–Ln coordination polymers and the influence of the introduction of an Ag(i) ion on NIR luminescence from the Ln(iii) centre

Abstract

A series of new Ag–Ln coordination polymers [LnAg₅(bdc)₄]_n (Ln = Eu (1); Yb (2); Er (3); Ho (4); H₂bdc = 1,2-benzenedicarbolylic acid) has been synthesized by a hydrothermal method and characterized by single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, IR spectra, UV-Vis-NIR absorption spectra and fluorescence spectra. X-ray structural investigations reveal that these polymers are isomorphous and exhibit a 2D layer structure formed by a coordination bond and weak metal⋯metal interaction. Polymers 1–4 exhibit a characteristic emission corresponding to the Ln(III) ions in the near-infrared (NIR) region or in the visible region. Especially, the NIR emission bands of polymers 2–4 present evidently shift or splitting compared with the isolated Ln(III) ions due to formation of the Ag–Ln coordination polymer. In the Ag–Ln coordination polymer, the Ag⋯Ln separation is much shorter (3.71–3.75 Å), thus the d-orbital of the Ag(I) ion and f-orbital of the Ln(III) ion may interact and influence each other, which probably caused the inner levels of the Ag–Ln system to be tuned. This can have an important impact on the NIR emission band presenting shift or splitting, which can be confirmed by the UV-Vis-NIR absorption spectra. Solid 1 shows intense characteristic emissions of the Eu(III) ion in the visible region, which is attributed to sensitization from the ligands and Ag-block (Ag-ligand section).