PNNL

Abstract

The solvothermal synthesis of a series of Nd dimers decorated with various chalcogenophene carboxylates and 2,2':6',2?-terpyridine of the general formula, [Nd2(µ-XC5H3O2)2(XC5H3O2)4(N3C15H11)2(H2O)2] where X = O, S, Se, and Te, is reported. The solid-state structures were characterized using single-crystal X-ray diffraction (scXRD) and all the complexes are isomorphous, despite substitution of the heterocyclic chalcogen; phase purity was confirmed via powder X-ray diffraction (pXRD). Vibrational spectroscopy was collected and correlations between chalcogen identity and the binding strength of the carboxylate groups of the chalcogenophene ligands with each metal center were shown to be independent of chalcogen identity. All four complexes displayed Nd(III)-based near-IR luminescence and exhibited ligand-sensitized emission. Varying the chalcogenophene chromophore enabled tuning of the sensitizing triplet state energy level, as evidenced by an 8-fold increase in the sensitization of the TeCA-decorated dimer relative to the other chalcogenophene congeners. This behavior was rationalized by comparing the Nd(III) acceptor and ligand donor states across the series. The donor triplet state of each ligand was estimated via low-temperature (77 K) phosphorescence measurements from 1:1 mixtures with Gd(III); these were found to be 24,631 cm–1 for furan-2-carboxylic acid (FCA), 23,764 cm–1 for thiophene-2-carboxylic acid (TCA), 22,548 cm–1 for selenophene-2-carboxylic acid (SeCA), and 21,186 cm–1 for tellurophene-2-carboxylic acid (TeCA). The greater sensitization efficiency of TeCA is the result of well-matched ligand donor and metal acceptor levels and thus suppression of nonradiative back-energy transfer. More broadly, triplet energy level information for these ligands serves as a guide for future application to other target metals based on the electronic properties necessary to effect efficient sensitization.