Lanthanides are routinely incorporated into quantum dots to act as down-shifting and up-converting phosphors in display and lighting applications due to their high photoluminescence quantum yields (PLQY). Recent efforts in the field have demonstrated that trivalent lanthanide, Ln­(III), incorporated into ZnAl2O4 spinel nanocrystals can achieve PLQYs of 50% for down-shifting nanophosphors using earth abundant materials. The high PLQY is surprising as the Al­(III) site in a spinel is centrosymmetric, which should lead to poor performance for these nanophosphors. However, spinels are prone to formation of an admixture of inverse and normal spinel lattices when the cation size ratio is not optimal. Such behavior can produce local cation disorder that can influence the phosphor performance. Herein, we describe the use of Tb­(III) as an optical probe to evaluate the fractional population of the inverse and normal spinel structures within Tb x ZnAl2‑x O4. The experimental data exhibits a Tb­(III) concentration dependent change in the fractional population that results in a maximum PLQY of 37% with 3.56% Tb­(III) incorporation. A decrease in the degree of inversion (cation disorder) leads to larger amounts of the cubic Fd m phase resulting in the observed photoluminescence behavior. The correlation of NMR, pXRD, and optical methods provides direct insight into the high PLQY behavior for this class of nanophosphor.

Latva’s empirical rule states that the energy separation between a molecular sensitizer and a lanthanide ion excited state must lie within 2000 to 4000 cm–1 for optimal energy transfer. At energies below 2000 cm–1, back energy transfer will impact the process resulting in the reduction of the photoluminescence quantum yield (PLQY). The role of excited triplet state (3π*) energy and intralanthanide ion energy hopping is assessed for a series of β-diketonate molecular sensitizers coordinated to the surface of a 2 nm 3.56% Tb­(III):ZnAl2O4 nanospinel. It is observed that energy transfer from the β-diketonate to a 2 nm nanospinel lies within the critical radii for energy transfer and the presence of efficient energy hopping minimizes back energy transfer contributions. In contradiction to Latva’s rule, the highest PLQY of 39% is achieved following sensitization by hexafluoroacetylacetonate, with an energy difference (3π*-5D4) of only 1534 cm–1. The measured PLQY is consistent with other reports of Tb­(III) doped nanocrystal hosts lattices, suggesting that energy hopping within the lattice enhances the Tb­(III) phosphor performance. Although not measured, the energy gap plot suggests that a PLQY approaching 58% may be achievable by ligand design.

A new three dimensional lanthanide-containing coordination polymer assembled from 2,2′-bithiophene-5,5′-dicarboxylic acid ([Ln2(C10H4O4S2)3(H2O)6]n, Ln = Sm–Yb) was solvothermally synthesized and used as a sensor to detect nitroaromatic compounds via luminescence quenching.