Construction of multifunctional materials based on Tb3+ and croconic acid, directed by K+ cations: synthesis, structures, fluorescence, magnetic and ferroelectric behaviors

Abstract

In order to obtain multifunctional molecular-based materials, especially multiferroics, a polar structure is essential. In this study, we find that intelligently using the K⁺ cation can effectively achieve this goal in the Tb³⁺-croconate hybrid system. Four rationally designed complexes based on Tb³⁺ and croconic acid have been synthesized and structurally characterized. The complexes have the formula [Tb₂(C₅O₅)₃(H₂O)₁₂]·2H₂O (1), [KTb(C₅O₅)₂(H₂O)₆]·1.5H₂O (2), [Tb(C₅O₅)(Hdpa)(H₂O)₅]·H₂O (3) and [KTb(C₅O₅)₂(H₂O)₅]·2H₂O (4), (H₂C₅O₅ = croconic acid, H₂dpa = 2,5-dipicolinic acid). The structures of the complexes were remarkably different after K⁺ was introduced. Complex 1, constructed by eight-coordinate Tb³⁺ nodes connected with C₅O₅²⁻ and H₂O, is a novel 0D metal–organic architecture with abundant O–H⋯O hydrogen bonds, whereas complex 2 is a 1D chain built by C₅O₅²⁻ spacers bridging five-coordinate K⁺ and eight-coordinate Tb³⁺ ions. Complex 3 has deprotonated C₅O₅²⁻ and Hdpa⁻ anions connected to an eight-coordinate Tb³⁺ center, to give a polar 0D structure. Complex 4, constructed by nine-coordinate Tb³⁺ and ten-coordinate K⁺ nodes connected with C₅O₅²⁻ and H₂O, is a novel, polar 3D metal–organic architecture. The fluorescence spectra of 1–4 show characteristic emission bands of Tb³⁺ ions. Interestingly, the fluorescence intensities of complexes 3 and 4, especially emissions at 370 nm, are significantly enhanced compared to the intensities of 1 and 2, owing to the structural differences. Magnetic susceptibility studies of complexes 1–4 show similar patterns. Furthermore, complex 4 exhibits a ferroelectric hysteresis loop at room temperature, due to its polarity. Thus, in this work, by merely adjusting the polarity of the structure, a series of different, versatile compounds were obtained as potential multifunctional materials.