Enhanced Ultraviolet Photon Capture in Ligand-Sensitized Nanocrystals

Abstract

The small absorption cross-sections (ε < 10 M ) in sensitizing excited states in lanthanide nanocrystals. Here, we detail the photophysics of NaGd 1-x Eu x F 4 nanoparticles featuring surface display of the ligand 3,4,3-LI(1,2-HOPO), an aromatic antenna functioning as the terminal light absorber in this system. The result is a ligand-nanocrystal hybrid which converts UV (250-360 nm) light into red Eu(III) luminescence with an external quantum yield of 3.3%. We analyze this sensitization process, responsible for a 10 4 fold increase in luminescence relative to metal-centered excitation, through a quantitative treatment of energy transfer between ligand and metal states. 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 The problem of spectral mismatch between semiconductor band gaps and Earth's terrestrial solar spectrum remains an issue plaguing the efficiency of modern photovoltaics 1,2 . A number of methods have been proposed and implemented to address this problem, including the incorporation of materials absorbing at different wavelengths in multi-junction photovoltaics, modification of the intrinsic Si band gap through advanced nanofabrication methods, and the respective up-and down-conversion of lowand high-frequency EM radiation into photons suited for bulk Si absorption 3-6 .Currently, research in the field of spectral conversion is dominated by lanthanide photophysics, where exploitation of f-element nanocrystals has resulted in a wide array of potential downconverters 7-12 . However, the maturation of these prototype systems into practical applications has largely been hampered by the low molar absorptivities of f-f transitions (< 10 M -1 cm -1 ) 13,14 . Routes previously explored to address this challenge include the relaxation of Laporte selection rules through the embedding of lanthanide ions in low-symmetry crystal hosts and the employ of parityallowed, d → f charge transfer in divalent lanthanides such as Eu(II) 15 . By contrast, the possibility of photosensitizing nanocrystalline lanthanides with organic ligands remains a relatively novel method of enhancing f-block nanoparticle light absorption, though a recent study has implemented such schemes in light upconversion 16 , and some work exploring the photochemical effects surface ligands have on lanthanide nanoparticle luminescence has been conducted 17-23 .In this report, we describe the construction of Eu-doped, rare-earth nanoparticles featuring a hydroxypyridinone (HOPO) ligand derivative, 3,4,3-LI(1,2-HOPO) (abbreviated '343'), as an ultraviolet photosensitizer. These constructs depart significantly from known systems such as dyesensitized TiO 2 nanoparticles, where the stated purpose of photon absorption is generation of delocalized charge carriers, rather than the efficient production and radiative decay of lower-frequency excitons, the central focus of this work.It has been previously shown that population of Eu(III) ex...

Amplified luminescence in organo-curium nanocrystal hybrids

“…25 Despite the ~70-fold dopant excess used in those studies relative to the curium investigations here, the sensitized-europium system was found to display a quantum efficiency only an order of magnitude (Φ = 3.3%), greater than what we observe for the curium system. 25 It is worth noting that the 5% doping level used for the europium study did not place the system in a concentration-quenching regime, suggesting a significantly higher efficiency for ligand-curium energy transfer relative to the europium nanoparticle analog, a finding consistent with an earlier study of luminescence in the 3,4,3-LI(1,2-HOPO)-Cm molecular complex. 21 Time-dependent luminescence was investigated through pulsed excitation of the ligand band at 350 nm, with concurrent monitoring of sample emission at 598 nm (SI).…”

Bond Covalency and Oxidation State of Actinide Ions Complexed with Therapeutic Chelating Agent 3,4,3-LI(1,2-HOPO)

“…A HOPO ligand has been proposed as a Gd­(III) chelator for use as a contrast agent in magnetic resonance imaging, − and HOPO ligands have been suggested as complexants in radiopharmaceutical treatments using 89 Zr­(IV), 225 Ac­(III), and 227 Th­(IV). , In addition to their manifest medical utility, HOPO ligands have analytical uses. 3,4,3-LI­(1,2-HOPO) is known to sensitize the luminescence of several Ln­(III), An­(III), and An­(IV) cations through the antenna effect, − allowing these systems to be characterized spectroscopically and opening up a range of innovative applications including fluorescence-based bioassays. ,,− A recent report has shown 3,4,3-LI­(1,2-HOPO) capable of stabilizing Bk in the +4 oxidation statethe first stable Bk­(IV) species observed in solutiona development potentially leading to novel Bk separation and purification techniques …”

Ligand-Sensitized Lanthanide Nanocrystals: Merging Solid-State Photophysics and Molecular Solution Chemistry

“…(b) Transmission electron micrograph of hexagonal NaGd 1– x Eu x F 4 nanoparticles capped with the ligand 3,4,3-LI­(1,2-HOPO). Adapted from ref . Copyright 2016 American Chemical Society.…”