Anthracene Diphosphate Ligands for CdSe Quantum Dots; Molecular Design for Efficient Upconversion

Abstract

Quantum dot (QD) sensitized photon upconversion follows a multi-step energy transfer process from QD to transmitter ligand to a soluble annihilator. Using a novel 10-R-anthracene-1,8diphosphoric acid (R = octyl, 2-hexyldecyl, phenyl) ligand with high binding affinity for CdSe quantum dot (QD) surfaces, we demonstrate a photon upconversion process that is limited by the transmitter to annihilator transfer efficiency. Using 1 H NMR spectroscopy we demonstrate that these bidentate diphosphate ligands rapidly and irreversibly displace two carboxylate ligands. These ligands mediate energy transfer from the photoexcited QDs to a triplet annihilator (1,10-diphenylanthracene), producing overall photon upconversion quantum efficiencies as high as 17%, the highest for QDs with no shells. Transient absorption spectroscopy shows that the ADP ligand supports a 3.4 fold longer triplet state lifetime compared to 9-ACA (299.9 ± 9.5 vs 88.2 ± 2.1 μs), increasing the probability of the energy transfer.

Green-to-Blue Triplet Fusion Upconversion Sensitized by Anisotropic CdSe Nanoplatelets

“…25 Therefore, QDs can act as efficient triplet sensitizers with minimal energy losses with reports of UC efficiencies exceeding 16%. [25][26][27][28][29] However, QDs often suffer from low quantum yields (QYs), necessitating inorganic core-shell structures 30,31 and additionally require long passivating ligands for surface passivation and colloidal stability. These long ligands hinder efficient TET due to its exponential distance dependence.…”

“…49 In comparison, we obtain a 16% efficiency for the ACA-coupled QDs, which is comparable with values reported for spherical CdSe by Tang and co-workers. 29 Table 2 shows the calculated UC efficiencies for both the NPL and QD-based UC systems for all ACA concentrations. For the efficiency calculations we refer to the Supporting Information and Figure S6 for more details.…”

High Photon Upconversion Efficiency with Hybrid Triplet Sensitizers by Ultrafast Hole‐Routing in Electronic‐Doped Nanocrystals

“…Furthermore, NCs feature a versatile surface chemistry that enables their postsynthesis functionalization with a variety of molecular species, including suitable derivatives of conjugated organic dyes. [ 23 ] As a result, as sketched in Figure A for the case of CdSe NCs functionalized with 9‐anthracenecarboxylic acid (9‐ACA), NCs and conjugated dyes can be rationally combined so as to create hybrid sensitizers [ 16–18,24 ] in which the NC absorbs and transfers the luminous energy via Dexter ET (ET′ in Figure 1A) to the triplet state of a surface‐attached organic moiety (hereafter referred to as triplet acceptor). The resulting excited triplet state acts as an energy bridge to populate, via a second Dexter‐type ET process (ET″), the long‐lived triplet state of a free emitter, whose fusion with the excited triplet state of a second emitter (generated through an identical multistep process) finally generates a highly emissive singlet responsible for the upconverted luminescence.…”

“…Over the years, tremendous progress in the design of such NC‐organic hybrid sensitizers has enabled a substantial growth of the sTTA‐UC performance, as highlighted in Figure 1B, where we report the chronological evolution of the QY uc since their original appearance in 2015. [ 18,21,22,24–29 ] However, despite such advancements, hybrid sTTA‐UC systems are still not as performing as the best fully organic counterparts. [ 21,25,26,30 ] This is mainly due to parasitic processes that can limit the efficiency of the ET′ step that populates the triplets of ligands.…”

Engineering Sensitized Photon Upconversion Efficiency via Nanocrystal Wavefunction and Molecular Geometry

“…Besides NC parameters,molecular design also has an important impact on TET.T he efficiencies of TET from CdSe NCs to anthracene acceptors functionalized with carboxylate,dithiocarbamate or diphosphate groups were found to be very different. [8] Interestingly,the substitution position of the functional group on the molecule is important as well. Forexample,the photon upconversion efficiencies of CdSe NCs with different carboxylated anthracene isomers differed by more than 10fold.…”