Quantum dot-bridge-fullerene heterodimers with controlled photoinduced electron transfer.

Molecular electronics has been growing rapidly, to a point where the ambition is to miniaturize conventional electronic devices down to the single-molecule scale. One of the essential components of the future molecular circuits is miniaturized power sources. A few reports have addressed this key issue by using a single nanowire as a low-power source for nanoelectronics, such as a piezoelectric nanogenerator based on zinc oxide nanowire and a solar cell made from a single coaxial silicon nanowire. Organic donor–bridge–acceptor (DBA) supramolecules have been at the center of research interest in molecular electronics owing to their rich charge-transport mechanisms and the ability to control the rate of charge transfer through chemical synthesis. DBA systems can be promising power sources for molecular circuits if they efficiently absorb and convert photons into charge carriers through photoinduced charge transfer (CT). Recently, semiconductor quantum dots (QDs) have been combined with dyes, fullerenes, TiO2, or conductive polymers to yield donor–acceptor (DA) charge-transfer systems for dye-sensitized cells or hybrid solar cells. However, the power conversion efficiency of such QD-based devices remains quite low. Tremendous efforts have been devoted to unveiling the fundamentals of photovoltaic processes such as charge separation and recombination in QD-based devices, mainly by ensemble-averaged optical methods such as ultrafast transient photoluminescence and absorption spectroscopy. Recent reports demonstrate that single-molecule spectroscopy (SMS) is a powerful method to unveil the inhomogeneous dynamics of CT obscured by ensemble averaging in a variety of systems, including organic DBA systems, dyes adsorbed on TiO2, or proteins. [7] A limited number of SMS studies addresses charge transfer between QDs and acceptor materials such as TiO2 or an ensemble of dyes adsorbed on a QD. Herein we introduce a method to fabricate electrontransfer DBA heterodimers based on a core/shell CdSe/ZnS QD and a fullerene derivative, the interparticle distance of which is controlled by aminoalkanethiol linkers. The fabricated QD–FMH dimers provide a model system for the single-molecule exploration of photoinduced electron transfer between QDs and electron acceptors, which is an essential process in QD-based solar cells. By varying the linker length and the QD size, we demonstrate control of the rate and of the magnitude of fluctuations of the photoinduced electron transfer at the level of the individual dimers. With excellent, size-dependent light absorption properties conferred by the incorporated QDs, these dimers are promising power-generating units for molecular electronics. The components used for fabricating the DBA heterodimers consist of a water-soluble fullerene derivative (fullerene–malonic acid hexaadduct, FMH); a set of water-soluble carboxy-ended core/shell CdSe/ZnS QDs with varying sizes and colors: QD605, QD565, and QD525, denoting QDs with photoluminescence (PL) emission maxima at 605, 565, and 525 nm, respectively; and a set of aminoalkanethiol linkers (Figure 1a). The electronic spectra of FMH and QDs are shown in Figure 1b. Owing to the lack of overlap between the PL spectra of QDs and the absorption spectrum of FMH, energy transfer from QDs to FMH is ruled out. Therefore, charge transfer from QDs to FMH should be the primary photoinduced interaction under optical excitation at 460 nm. This interaction can be further narrowed down to electron transfer (ET), considering the positioning of the electronic energy levels of CdSe QDs (conduction band 4.3 eV, valence band 6.4 eV for a 4.4 nm CdSe QD) and fullerene (lowest unoccupied molecular orbital (LUMO) 4.7 eV, highest occupied molecular orbital (HOMO) 6.8 eV), which does not favor hole transfer. When QDs and FMH are mixed in aqueous solution, quenching of QDs by FMH is low (15%, see Figure S1a in the Supporting Information), presumably owing to the weak electronic coupling between QDs and FMH, as both have negatively charged carboxy groups at the surface. To enhance the electronic coupling, aminoalkanethiol linkers of varying length (Figure 1a) were used to conjugate QD and FMH components: 6-amino-1-hexanethiol hydrochloride (6AHT), 11-amino-1-undecanethiol hydrochloride (11AUT), and 16amino-1-hexadecanethiol hydrochloride (16AHT). Specifically, the amine end of the linker couples with a carboxy group of the FMH through a standard coupling reaction assisted by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), while the thiolated end of the linker binds to the ZnS surface of the core/shell QD. This procedure enhances quenching of the QD by FMH in solution (up to 42% and for the shortest linker, see Figure S1b in the Supporting Information), thus indicating enhanced ET and therefore successful linking of QDs and FMHs by aminoalkanethiols. Enhanced ET was further confirmed by transient PL and absorption measurements in solution (see Figures S2 and S3 [*] Dr. Z. Xu, Dr. M. Cotlet Center for Functional Nanomaterials Brookhaven National Laboratory, Upton, NY 11973 (USA) E-mail: [email protected]