Breaking Latva’s Rule by Energy Hopping in a Tb(III):ZnAl₂O₄ Nanospinel

Abstract

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.

Theoretical study of 8‐hydroxyquinoline derivatives as potential antennas in lanthanide complexes: Photophysical properties and elucidation of energy transfer pathways

“…Considering that the energy transfer mechanism in complexes with similar ligands has been reported in the range, 10 7 -10 10 s À1 [67,68]. We conclude that if the Latva rule [69] is accomplished, these ligands can act as efficient antennas.…”

“…Ligands were treated exactly at the same level of theory regarding an active space of eight electrons in eight orbitals CAS(8,8) carefully selected from the analysis of the absorption spectra obtained at the TDDFT level. The resulting states were used to establish the most probable sensitization pathway mechanism by applying the Reinhoudt and Latva rules for inter‐system crossing (ISC) and energy transfer (ET), respectively [66, 69]. At this point, it is important to clarify that the Reinhoudt rule for the ISC rate represents a guideline in terms of the energy difference for higher ISC efficiency and does not necessarily establish that this mechanism occurs between S 1 and T 1 (were the S1‐T1 energy difference is the adiabatic energy difference between the first excited singlet and triplet states at their own geometries without any ZPE correction).…”

Structure–Function Correlation: Engineering High Quantum Yields in Down-Shifting Nanophosphors

“…It is believed the Tb(III) center occupies an Al(III) site to ensure charge compensation and coordination number, consistent with our earlier findings for Eu(III) incorporation into ZnAl 2 O 4 . 1,17 In the presence of lattice inversion the Al(III) site occupies a formally Zn(II) location, allowing the Tb(III) to potentially occupy the Zn(II) site.…”

“…1,8 Research into the broader class of Ln(III) doped quantum dots with molecular sensitizers have shown variable photoluminescence quantum yield (PLQY) performance depending on the host lattice structure, 9,10 guest ion concentration, 11−15 and choice of sensitizer. 16,17 The quantum yield for the down-shifting phosphor will be impacted by the sensitization efficiency from the passivant to the emitting Ln(III) center and by the site symmetry of the Ln(III) ion reflecting the optical selection rules. In noncentrosymmetric lattices, the electric dipole allowed transitions in f orbitals become favorable and lead to higher intrinsic quantum yield, 9,10,18 as reported for Ln(III) incorporation into molecular sensitizer passivated Y 2 O 3 8 and NaYF 4 .…”

“…In the phosphor field, the incorporation of a trivalent lanthanide, Ln­(III), guest ion into a metal oxide quantum dot produces materials capable of exhibiting down-shifting or up-converting photoluminescence that can form the color center for a phosphor-converted light emitting diode (pcLED). − Quantum dots have passivating ligands that can act as a molecular antenna to enhance photon capture and energy conversion of down-shifting phosphors. , Research into the broader class of Ln­(III) doped quantum dots with molecular sensitizers have shown variable photoluminescence quantum yield (PLQY) performance depending on the host lattice structure, , guest ion concentration, − and choice of sensitizer. , The quantum yield for the down-shifting phosphor will be impacted by the sensitization efficiency from the passivant to the emitting Ln­(III) center and by the site symmetry of the Ln­(III) ion reflecting the optical selection rules. In noncentrosymmetric lattices, the electric dipole allowed transitions in f orbitals become favorable and lead to higher intrinsic quantum yield, ,, as reported for Ln­(III) incorporation into molecular sensitizer passivated Y 2 O 3 and NaYF 4 . , The downshifting phosphor performance can be engineered to produce high quantum yields with improved color purity by introducing a lowered symmetry for the Ln­(III) site through cation size mismatch or choice of a noncentrosymmetric lattice.…”