Complementary Lock-and-Key Ligand Binding of a Triplet Transmitter to a Nanocrystal Photosensitizer

Angewandte Communications Chemie International Edition: DOI: 10.1002/anie.201701929 German Edition: DOI: 10.1002/ange.201701929 Nanocrystals Complementary Lock-and-Key Ligand Binding of a Triplet Transmitter to a Nanocrystal Photosensitizer Xin Li, Alexander Fast, Zhiyuan Huang, Dmitry A. Fishman, and Ming Lee Tang* Abstract: Owing to the difficulty in comprehensively charac- terizing nanocrystal (NC) surfaces, clear guidance for ligand design is lacking. In this work, a series of bidentate bis- (pyridine) anthracene isomers (2,3-PyAn, 3,3-PyAn, 2,2- PyAn) that differ in their binding geometries were designed to find the best complementary fit to the NC surface. The efficiency of triplet energy transfer (TET) from the CdSe NC donor to a diphenylanthracene (DPA) acceptor mediated by these isomers was used as a proxy for the efficacy of orbital overlap and therefore ligand binding. 2,3-PyAn, with an intramolecular N–N distance of 8.2 , provided the best match to the surface of CdSe NCs. When serving as a trans- mitter for photon upconversion, 2,3-PyAn yielded the highest upconversion quantum yield (QY) of 12.1 1.3 %, followed by 3,3-PyAn and 2,2-PyAn. The TET quantum efficiencies determined by ultrafast transient absorption measurements showed the same trend. T he design of ligands for nanocrystals is challenging because the high surface area to volume ratio modifies the bulk lattice in ways that are difficult to predict. [1] Ligand development requires an understanding of the binding mechanism(s), as well as the nature of the available binding sites. The latter is difficult to characterize, [2] owing to the disordered nature of the organic–inorganic interface and the inherent distribution in the size and shape of colloidally synthesized nanocrystals. Ligands that are structurally complementary to the nano- crystal surface are critical in enhancing exciton delocalization within the hybrid structure. Herein, we examine the conse- quence of subtle perturbations in the structure of ligands that serve as transmitters for triplet excitons from nanocrystal donors. Much like Fishers lock-and-key mechanism for the binding of substrates to an enzyme active site, [3] only one of the transmitter ligands complements the nanocrystal surface, resulting in a high photon upconversion quantum yield (QY) that arises from efficient triplet energy transfer (TET). Conjugated organic ligands can drastically enhance energy transfer from a nanocrystal (NC) donor to an acceptor. This is especially evident in a hybrid complex consisting of NC sensitizers and acene annihilators. For example, a combination [*] X. Li, Z. Huang, Prof. M. L. Tang Chemistry, University of California, Riverside 501 Big Springs Road, Riverside, CA 92521 (USA) E-mail: minglee.tang@ucr.edu A. Fast, Dr. D. A. Fishman Chemistry, University of California, Irvine (USA) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.doi.org/10.1002/anie.201701929. Figure 1. a) The distances between the pyridine N atoms in the three isomers are shown together with the {0001} facet of wurtzite CdSe NCs for which the distances between neighboring cations are given. b) Absorption and emission spectra of the three bis(pyridine) anthra- cene isomers and 2.4 nm diameter CdSe NCs. All spectra were measured in n-hexane at room temperature. c) Possible binding geo- metries and the energy transfer in this hybrid photon upconversion platform. The energy diagram depicts the triplet excitonic states of the CdSe NC, the anthracene transmitter, and the 9,10-diphenylanthracene annihilator. 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2017, 56, 1 – 6 of PbS nanocrystals and tetracene derivatives was shown to efficiently upconvert near-infrared into visible light under one sun conditions. [4] In this process, low-energy photons har- vested by the NCs are funneled through surface-anchored transmitters to free annihilators in solution. Two annihilators in their triplet excited state collide, converting two low-energy photons into one high-energy photon through triplet–triplet annihilation (TTA). In this hybrid system, triplet energy transfer (TET) from the NC to transmitter molecules is the bottleneck limiting the photon upconversion QY. Therefore, designing a transmitter ligand with a high binding affinity for the NC donor in a geometry that facilitates orbital overlap is crucial in increasing the rate and the overall quantum efficiency of TET. A series of isomeric bidentate bis(pyridine) anthracene transmitter ligands were designed to simultaneously fulfill these two requirements (Figure 1 a). The bidentate nature of these transmitters implies that the anthracene core is parallel to the NC surface if the ligand binds via the two pyridine N atoms. This increases the orbital overlap between the NC donor and the acene acceptor, which is necessary for efficient Dexter energy transfer. In contrast, previous work solely involved monodentate transmitters with conjugated cores that could be either perpendicular to or parallel with the NC surface. [4b, 5] Pyridine (Py) was selected to lock the anthracene core in a fixed geometry because it is widely used as a coordination ligand in organometallic complexes. [6] In These are not the final page numbers!