Vacuum-responsive and water-soluble lanthanide(iii)/2-oxonicotinate coordination polymers with high photoluminescence efficiency

Abstract

In this work, we report on the structural and physicochemical characterization and an in-depth photophysical study of a family of isostructural coordination polymers (CPs) with the general formula {[M₂(μ₃-2onic)₄(H₂O)₄](ClO₄)₂·2H₂O}_n (where M(III) = Y (1_Y), Nd (2_Nd), Eu (3_Eu), Gd (4_Gd), Tb (5_Tb), Dy (6_Dy), Er (7_Er) and Yb (8_Yb) and 2onic = 2-oxonicotinate). They consist of a cationic 2D layered structure in which two eight-coordinated rare-earth centres are interconnected by means of 2onic ligands, which demonstrates great flexibility with respect to changes in temperature and pressure (vacuum) derived from partial dehydration implying both lattice and coordination water molecules, which in turn promotes the rearrangement of the hydrogen-bonded network. Similar structural breathing effects are observed under variable gas-pressurization conditions, leading to some metastable phases while vacuum conditions are maintained. Periodic density functional theory (PDFT) calculations performed on 1_Y with variable amounts of water successfully reproduced the structural evolution during dehydration. The fact that the vacuum-/pressure-induced effect is fully reversible in addition to the significantly improved photoluminescence (PL) shown by the vacuum-/pressure-activated compounds shifts the attention towards these compounds as potential pressure and humidity sensors. 2_Nd and 8_Yb, in addition to acting as emitters in the visible range, behave as near-infrared (NIR) emitters, with the former displaying characteristic emission even at room temperature. A thorough analysis of the excitations and energy-transfers by means of semi-empirical methods and multi-configurational calculations on suitable fragment models, and further confirmation by density of states (DOS) theory on the PDFT-optimized structure, allows elucidating the PL mechanism operating in the variable emission in the solid state. The water solubility of the compounds allows the study of the PL properties of the complexes, which surprisingly present quantum efficiencies exceeding those of the solid state especially in the solution of 5_Tb (with the quantum yield (QY) increased from 1.6 to 21.5%).