Entropy-Powered Endothermic Energy Transfer from CsPbBr₃ Nanocrystals for Photon Upconversion

Abstract

Colloidal semiconductor nanocrystals as triplet photosensitizers are characterized by a negligible intersystem crossing energy loss as compared to that of traditional molecular sensitizers. This property in principle allows for a large apparent anti-Stokes shift in sensitized triplet–triplet annihilation photon upconversion (TTA-UC) for a variety of applications. In previous systems, however, this advantage is largely erased by the energy loss associated with energy transfer from nanocrystals to surface-anchored triplet transmitter molecules. Here we report visible-to-ultraviolet TTA-UC from 473 to 355 nm, corresponding to an apparent anti-Stokes shift of 0.87 eV, with a quantum efficiency that reaches 4.5% (normalized at 100%). The system consists of CsPbBr3 nanocrystal sensitizers, phenanthrene transmitters, and diphenyloxazole annihilators. Time-resolved spectroscopy reveals that triplet energy transfer from CsPbBr3 nanocrystals to phenanthrene can be endothermic yet efficient thanks to a sizable entropic gain. This study exemplifies how entropic effects can be harnessed to enhance or control a plethora of applications with nanocrystals as photosensitizers.

Ligand Chemistry of Inorganic Lead Halide Perovskite Nanocrystals

“…It should be noted that the energy transfer is also mediated by charge transfer. The molecules are adapted from various references: (1) ref , (2) ref , (3) ref , (4) ref , (5) refs and , (6) ref , (7) ref , (8) ref , (9) ref , (10) ref , (11) ref , (12) ref , (13) ref , and (14) ref .…”

Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals

“…The latter scheme was proposed to explain the prominent "delayed" emission in comparable NCs. 29 Last, we note the rising appreciation of the role of entropy in influencing TET in functionalized systems, 57 which could add ambiguity to the relationship between the observed thermodynamics and the underlying state energies. However, the modest ligand loading (⟨n⟩ large = 6.7 ± 0.1) gives an upper bound on the entropic contribution to the overall free energy, which we expect is small compared to the overall exothermicity of triplet energy transfer.…”

“…Using this analysis, we find that the per-ligand quenching rate constant is accelerated by a factor of 6.7 in the small NCs compared to the larger NCs, from k large = 0.0096 ns –1 ligand –1 to k small = 0.064 ns –1 ligand –1 (before accounting for enhanced wave function leakage ( vide infra )). We also expect little differential effect from entropic considerations between the two sizes of NCs, due to the comparable average ligand loadings. Building up the total picture from our size-dependent experiments and the relative energetics for carrier and exciton transfer (Figure a), our results show that the transfer of photoexcitations from the NC is significantly accelerated when the energetic barrier for the transfer of a single hole from the NC to the ligand is reduced from 100 ± 25 meV to 50 ± 25 meV.…”

Aufwärtskonversion von blauem Licht zu UVB‐Strahlung, Lösungsmittelsensibilisierung und anspruchsvolle Bindungsaktivierungen durch einen Benzol‐Annihilator

“…[79,80] In guter Übereinstimmung mit den über DFT berechneten Energien wurde für bTIPS-Bz eine Triplett-Energie von 2,64 eV durch Phosphoreszenzmessungen bei 77 K bestimmt (Abbildung S17). Da eine isoenergetische und sogar leicht endotherme Sensibilisierung möglich ist, [31,[81][82][83] liegt TTA-UC mit bTIPS-Bz als Annihilator unter Verwendung blauer Photonen eindeutig im Rahmen des Möglichen.…”