The role of terminal and bridge ligands in the molecular upconversion of lanthanide(iii) 1D coordination polymers

Abstract

The upconversion of trivalent lanthanide ions (Ln^III) has gained significant attention in recent decades due to its potential applications in energy conversion and biological imaging. Recently, the concept of Ln^III molecular upconversion in coordination compounds has emerged, offering new possibilities thanks to their ease of synthesis and processing. To address the challenge of enhancing this relatively unexplored phenomenon, we investigated 1D coordination polymers containing the Er^III/Yb^III pair as models to explore how structural parameters tuned by different ligands can enhance upconversion. Our approach utilized two distinct bridging ligands, [(diphenylphosphoryl)R](diphenyl)phosphine oxide, R = ethyl–dppeo or butyl–dppbo, to modulate the polymeric chain conformation and 3D packing. Additionally, three terminal ligands acac (2,4-pentanedione), tfa (1,1,1-trifluoro-2,4-pentadione), and hfa (1,1,1,5,5,5-hexafluoro-2,4-pentanedione) were employed to investigate the effects of steric and electronic factors on structural and upconversion properties. The tfa⁻ and hfa⁻ ligands facilitated the formation of 1D coordination polymers with the formula [Ln(μ-L)(X)₃]_n (Ln = Er^III/Yb^III, L = dppeo or dppbo, X = tfa or hfa), while the combination of acac⁻ and dppeo led to the dinuclear structure [Ln₂(μ-dppeo)(acac)₆(H₂O)₂]. Notably, the F⋯F and F⋯H intra/intermolecular interactions driven by the terminal ligands significantly influenced the polymer conformation, leading to more linear and loosely packed polymeric chains. Upconversion was observed exclusively in structures with the shorter bridging ligand dppeo, suggesting that reduced Ln⋯Ln distances and loosely packed polymeric chains enhance molecular upconversion. Therefore, this study lays the groundwork for understanding the structural factors that govern the molecular upconversion in polynuclear Ln^III complexes.