Luminescent Bismuth(III)-Organic Compounds: Synthesis and Spectroscopic Properties

Abstract

This work focuses on the synthesis and characterization of bismuth(III)-organic compounds with an eye towards developing structure-photoluminescent behavior relationships in bismuth-based materials. Twenty-eight compounds have been assembled through coordination of Bi3+ with various O- and N-donor ligands; the phases vary in metal ion coordination geometry, nuclearity, and overall dimensionality. The photophysical behavior of the compounds was examined to gain a more thorough understanding of underlying structure-property relationships. Further, computational efforts aimed at identifying the origin of luminescence in several phases were performed. Presented herein are three systems that broadly highlight the structural diversity afforded by bismuth and the rich photophysical properties that can be achieved: (1) bismuth-organic compounds bound to heterocyclic N-donor ligands and/or pyridindicarboxylates, (2) homoleptic Bi- and Ln-doped Bi-thiophenecarboxylates, and (3) heteroleptic Bi- and Eu-doped Bi-organic phases.

Eleven bismuth compounds built from N-donor heterocycles and/or pyridinedicarboxylate were synthesized. The structures consist of discrete molecular units of varying nuclearity linked into supramolecular assemblies via intermolecular π-π stacking and halide-π interactions. These compounds exhibit visible light emission, which based on preliminary computational analysis, may be attributed to ligand-to-ligand and “metal”-to-ligand charge transfer transitions.

Two series of bismuth- and lanthanide-thiophenecarboxylates were synthesized, exhibiting a range of structural motifs, nuclearity, and dimensionality resulting from variations in synthetic parameters. The lanthanide phases differed from the bismuth phases in coordination geometry as the stereochemically active lone pair of Bi3+ imparted significant anisotropy about the metal center. Additionally, disparities in the relative solubility of the Bi- and Ln-units limited doping of the Bi phases. However, delayed introduction of the Ln3+ ion resulted in Ln-doped Bi-TDC and characteristic visible and NIR emission.

To address differences in metal coordination geometries, europium doping into bismuth-organic phases built from terpyridine and 2-thenoyltrifluoroacetone (TTA) were examined. The asymmetric TTA modulated the 6s2 lone pair thereby rendering Bi more Ln-like; isostructural Bi- and Eu-phases were prepared. Still, differences in reaction kinetics limited doping. Characteristic Eu emission was observed, yet with low quantum efficiencies, which may be attributed to less efficient energy migration pathways present in the host material.