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

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 Tb 3+ -croconate hybrid system. Four rationally designed complexes based on Tb 3+ and croconic acid have been synthesized and structurally characterized. The complexes have the formula [Tb 2 (C 5 O 5 ) 3 (H 2 O) 12 ]·2H 2 O ( 1 ), [KTb(C 5 O 5 ) 2 (H 2 O) 6 ]·1.5H 2 O ( 2 ), [Tb(C 5 O 5 )(Hdpa)(H 2 O) 5 ]·H 2 O ( 3 ) and [KTb(C 5 O 5 ) 2 (H 2 O) 5 ]·2H 2 O ( 4 ), (H 2 C 5 O 5 = croconic acid, H 2 dpa = 2,5-dipicolinic acid). The structures of the complexes were remarkably different after K + was introduced. Complex 1 , constructed by eight-coordinate Tb 3+ nodes connected with C 5 O 5 2− and H 2 O, is a novel 0D metal-organic architecture with abundant O-H O hydrogen bonds, whereas complex 2 is a 1D chain built by C 5 O 5 2− spacers bridging five-coordinate K + and eight-coordinate Tb 3+ ions. Complex 3 has deprotonated C 5 O 5 2− and Hdpa − anions connected to an eight-coordinate Tb 3+ center, to give a polar 0D structure. Complex 4 , constructed by nine-coordinate Tb 3+ and ten-coordinate K + nodes connected with C 5 O 5 2− and H 2 O, is a novel, polar 3D metal-organic architecture. The fluorescence spectra of 1-4 show characteristic emission bands of Tb 3+ 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. By adjusting the polarity of the structure through K + cations, versatile compounds are obtained as potential multifunctional materials.