Yu-Wu Zhong

Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles

Injection of photoexcited electrons from colloidal PbS quantum dots into TiO(2) nanoparticles is investigated. The electron affinity and ionization potential of PbS quantum dots, inferred from cyclic voltammetry measurements, show strong size dependence due to quantum confinement. On the basis of the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into TiO(2) only for quantum-dot diameter below approximately 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots. The measured electron transfer time is surprisingly slow ( approximately 100 ns), and implications of this for future photovoltaics will be discussed. Initial results obtained from solar cells sensitized with PbS quantum dots are presented.

Electrogenerated Chemiluminescence from PbS Quantum Dots

We report the first observation of electrogenerated chemiluminescence (ECL) from PbS quantum dots (QDs). Different ECL intensities are observed for different ligands used to passivate the QDs, which indicates that ECL is sensitive to surface chemistry, with the potential to serve as a powerful probe of surface states and charge transfer dynamics in QDs. In particular, passivation of the QD surfaces with trioctylphosphine (TOP) increases ECL intensity by 3 orders of magnitude when compared to passivation with oleic acid alone. The observed overlap of the ECL and photoluminescence spectra suggests a significant reduction of deep surface trap states from the QDs passivated with TOP.

Conductance switching and mechanisms in single-molecule junctions.

From its very start, one of the most intriguing motivations of molecular electronics is to provide unique and low-cost solutions for electronic functions based on molecules, such as diodes, transistors, switches, and memristors, since molecules are probably the smallest units still capable of offering a rich structural variety. However, the ability to control the conductance of molecules at the molecular level by an external mode is still a formidable challenge in this field. Here we report the observation of reproducible conductance switching triggered by external light on a new platform of graphene–molecule junctions, where three photochromic diarylethene derivatives with different substituents are used as key elements. Analyses of both transition voltage spectroscopy and first-principles calculations consistently reveal tunable molecule–electrode coupling, thus demonstrating the photogated inflection (Vtrans) transition when the chargetransport mechanism changes from direct to Fowler–Nordheim (F-N) tunneling. We chose diarylethene derivatives as photosensitive molecular bridges because they, as a typical family of photochromic molecules, can undergo reversible transitions between two distinct isomers with open/closed conformations when exposed to light irradiation (Figure 1a). The closed isomer is nearly planar, but the open isomer adopts a bent conformation with its thiophene rings twisted about 618 out of the plane from the cyclopentene ring. Correspondingly, these two isomers display different absorption spectra, that of the closed form extends towards longer wavelengths up to the visible region, suggesting the delocalization of p electrons over the entire structure (see Figure S1 in the Supporting Information). In the open form, however, delocalization of the p electrons is restricted to each half of the molecule and electronic communication through the unsaturated bond of the middle ring is interrupted. Another remarkable feature of the diarylethene molecules used in this study is that only negligible changes in the molecular length ( 0.2 ) happen when they switch back-and-forth between open/closed states (Figure S2 and Table S1). In conjunction with their superior thermal stability and fatigue resistance, these significant electronic and structural properties place diarylethene molecules as ideal candidates for building light-driven molecular switches as demonstrated theoretically and experimentally. However, a longstanding challenge is to conserve these promising properties in solution when the diarylethene molecules are sandwiched between solid-state molecularscale electrodes. One major reason is due to the quenching effect of the photoexcited states of the diarylethene molecules by the electrodes, which strongly stresses the importance of the molecule–electrode coupling strength to the device performance. To tailor the energy level alignments at the molecule– electrode interface, in this study we intend to modify diarylethene backbones with rationally designed side and anchoring groups (1–3 in Figure 1b). This modification has two specific considerations. The first is to substitute the hydrogenated cyclopentene in 1 by the fluorinated unit (2). In comparison with the hydrogenated cyclopentene, the fluorinated unit is electron-withdrawing and thereby decreases the electron density on the central alkene unit and increases the fatigue resistance of the photochromic properties. The second is to further introduce a methylene group (CH2) between the terminal amine group and the functional center on each side (3). The incorporation of the saturated CH2 groups can cut off p-electron delocalization, thus largely decoupling the electronic interaction between molecules and electrodes. Theoretical calculations were performed to predict the electronic structures of the molecule–electrode contacts as shown in Figure 1c (Table S2). Indeed, the energy levels of 2 are lower than those of 1 because of the electron-withdrawing effect of the fluorinated unit, which is consistent with electrochemical measurements of similar systems. For 3, the energy levels are even lower. More importantly, the calculated molecular orbital diagrams reveal a lower orbital density of states (DOS) at the C sites of the CH2 groups (Figure 1c), which implies that the CH2 groups decrease the strong electronic coupling between diarylethene molecules and electrodes. These results demonstrate the potential of molecular engineering as an efficient tool for tuning the molecule–electrode coupling strength. This tuna[*] C. Jia, J. Wang, Y. Cao, Prof. Z.-R. Liu, Prof. Z.-F. Liu, Prof. X.-F. Guo Center for NanoChemistry Beijing National Laboratory for Molecular Sciences State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University Beijing 100871 (P. R. China) E-mail: zfliu@pku.edu.cn guoxf@pku.edu.cn

Single-Molecule Conductance of Pyridine-Terminated Dithienylethene Switch Molecules

We have investigated the conductance of individual optically switchable dithienylethene molecules in both their conducting closed configuration and nonconducting open configuration, using the technique of repeatedly formed break-junctions. We employed pyridine groups to link the molecules to gold electrodes in order to achieve relatively well-defined molecular contacts and stable conductance. For the closed form of each molecule, we observed a peak in the conductance histogram constructed without any data selection, allowing us to determine the conductance of the fully stretched molecules. For two different dithienylethene derivatives, these closed-configuration conductances were (3.3 ± 0.5) × 10(-5)G(0) and (1.5 ± 0.5) × 10(-6)G(0), where G(0) is the conductance quantum. For the open configuration of the molecules, the existence of electrical conduction via the molecule was evident in traces of conductance versus junction displacement, but the conductance of the fully stretched molecules was less than the noise floor of our measurement. We can set a lower limit of 30 for the on/off ratio for the simplest dithienylethene derivative we have investigated. Density functional theory calculations predict an on/off ratio consistent with this result.

Electronic Coupling between Two Cyclometalated Ruthenium Centers Bridged by 1,3,6,8-Tetra(2-pyridyl)pyrene (tppyr)

A new cyclometalating bridging ligand 1,3,6,8-tetra(2-pyridyl)pyrene was designed and synthesized through 4-fold Suzuki couplings between 1,3,6,8-tetrabromopyrene and 2-pyridylboronate. A bis-cyclometalated bisruthenium complex bridged by this ligand showed the presence of an electronic coupling between individual metal centers, as indicated by electrochemical and spectroscopic studies.

Photocurrent-Generating Properties of Organometallic Fullerene Molecules on an Electrode

Compact, rigid, five-legged fullerene derivatives C60R5Me and M(C60R5)Cp (M = Fe and Ru; R = C6H4COOH, C6H4C6H4COOH, and CH2COOH) were synthesized and arrayed on an indium-tin oxide (ITO) surface. These devices exhibit a respectable quantum yield with photocurrent generation up to 18%, and, more importantly, the direction of the photocurrent can be changed not only by the molecular structure itself but also by changing the geometric configuration of the photoactive acceptor (fullerene) and donor (metal atom) on the ITO surface.

Dithienylcyclopentenes-containing transition metal bisterpyridine complexes directed toward molecular electronic applications.

There is continuing interest in the design and synthesis of functional materials for applications in molecular electronics and information storage. Of particular interest are systems that can provide multiple means for controlling transport through well-defined and stable electronic and/or redox states. We report herein the synthesis and characterization of a system containing transition-metal complexes along with dithienylethene (DTE) units so as to achieve photo and redox control of transport. A facile synthetic methodology was developed to assemble and couple metal terpyridine (M-tpy) complexes with the photochromic DTE unit in a linear structure with M-DTE-M or DTE-M-DTE arrangements, with emphasis on the latter series. The photochromic properties of these assemblies were examined by monitoring the changes in their UV/vis spectra upon irradiation at specific wavelengths capable of triggering the open/closed isomerization in the DTE units. Their electrochromic properties were studied via cyclic voltammetry and controlled potential electrolysis experiments. Complexes 10 (PhDTE-Fe-DTEPh) and 11 (PhDTE-Co-DTEPh) with phenyl-ending groups were found to be both photochromic and electrochromic, so they represent excellent candidates for further elaborations. The Fe(II)-containing complex 8 (ClDTE-Fe-DTECl) with chloride-ending groups was photochromically inactive but could undergo the electrochemically induced open-to-closed isomerization. On the contrary, the electrochromically inactive complex 9 (ClDTE-Co-DTECl) underwent cyclization under ultraviolet irradiation.

Electronic communication between two amine redox centers bridged by a bis(terpyridine)ruthenium(II) complex.

Two bis(terpyridine)ruthenium(II) complexes 2 and 3 appended with one or two di-p-anisylamino groups, respectively, were synthesized and fully characterized. Their electronic properties were studied by electrochemical and spectroscopic analyses. Electronic communication between individual amine sites of 3 was estimated by intervalence charge-transfer band analyses.

Three-state near-infrared electrochromism at the molecular scale.

Self-assembled monolayer films of a cyclometalated ruthenium complex with a redox-active amine substituent and three carboxylic acid groups have been prepared on ITO electrode surfaces. The obtained thin films show three-state electrochromic switching with low electrochemical potential inputs and high near-infrared absorbance outputs. Thanks to the long retention time of each oxidation states, these films have been used to demonstrate surface-confined flip-flop memory functions with high ON/OFF ratios at the molecular scale.

A Molecular Platform for Multistate Near‐Infrared Electrochromism and Flip‐Flop, Flip‐Flap‐Flop, and Ternary Memory

A diruthenium complex with a redox-active amine bridge has been designed, synthesized, and studied by single-crystal X-ray analysis and DFT and TDDFT calculations. It shows three well-separated redox processes with exclusive near-infrared (NIR) absorbance at each redox state. The electropolymerized film of a related vinyl-functionalized complex displays multistate NIR electrochromism with low operational potential, good contrast ratio, and long retention time. Flip-flop, flip-flap-flop, and ternary memories have been realized by using the obtained film (ca. 15-20 nm thick) with three electrochemical inputs and three NIR optical outputs that each displays three levels of signal intensity.