Chang-Jiang Yao

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

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.

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.

Metallopolymeric films based on a biscyclometalated ruthenium complex bridged by 1,3,6,8-tetra(2-pyridyl)pyrene: applications in near-infrared electrochromic windows.

A biscyclometalated ruthenium complex bridged by the 2,7-deprotonated form of 1,3,6,8-tetra(2-pyridyl)pyrene was deposited onto indium-tin oxide glass electrodes by reductive electropolymerization. The resulting metallopolymeric films exhibited tricolor electrochromic behavior in the near-infrared region upon switching of the two well-separated Ru(II/III) processes at low potentials. A good contrast ratio (35%) at 2050 nm and a long memory time up to 100 min were recorded for this electrochromic behavior. The response time is typically of a few seconds.

Multi-center redox-active system: amine-amine electronic coupling through a cyclometalated bisruthenium segment.

A multicenter redox-active system with a linear N-Ru-Ru-N array, where two distal triarylamine sites are bridged by a cyclometalated bisruthenium segment, has been synthesized and characterized with single-crystal X-ray analysis. This system displays four consecutive and separate anodic redox waves at low potentials, indicating the presence of amine-amine electronic coupling with a distance of 19.16 Å through the cyclometalated bisruthenium segment. In contrast, when a noncyclometalated bisruthenium bridge is used, no amine-amine coupling is present. Upon stepwise oxidation by chemical or electrochemical methods, four-step absorption spectral changes occur in the visible to near-infrared region. In addition, the EPR studies and DFT and TDDFT calculations of the singly oxidized state are presented.

Oligotriarylamines with a pyrene core: a multicenter strategy for enhancing radical cation and dication stability and tuning spin distribution.

Monoamine 1, diamines 2-4, triamine 5, and tetraamine 6 have been synthesized by substituting dianisylamino groups at the 1-, 3-, 6-, and/or 8-positions of pyrene. Diamines 2-4 differ in the positions of the amine substituents. No pyrene-pyrene interactions are evident in the single-crystal packing of 3, 4, and 6. With increasing numbers of amine substituents, the first oxidation potential decreases progressively from the mono- to the tetraamine. These compounds show intense charge-transfer (CT) emission in CH2 Cl2 at around 530 nm with quantum yields of 48-68 %. Upon stepwise oxidation by electrolysis or chemical oxidation, these compounds were transformed into radical cations 1(⋅+) -6(⋅+) and dications 2(2+) -6(2+) , which feature strong visible and near-infrared absorptions. Time-dependent density functional theory studies suggested the presence of localized transitions from the pyrene radical cation and aminium radical cation, intervalence CT, and CT between the pyrene and amine moieties. Spectroscopic studies indicated that these radical cations and dications have good stability. Triamine 5 and tetraamine 6 formed efficient CT complexes with tetracyanoquinodimethane in solution. The results of EPR spectroscopy and density functional theory calculations suggested that the dications 2(2+) -4(2+) have a triplet ground state, whereas 5(2+) and 6(2+) have a singlet ground state. The dication of 1,3-disubstituted diamine 4 exhibits a strong EPR signal.

Ruthenium-Amine Conjugated Organometallic Materials for Multistate Near-IR Electrochromism and Information Storage

Monometallic and dimetallic complexes with the ruthenium-amine conjugated structural unit have been prepared. These complexes display consecutive redox waves with low potentials and rich and intense absorptions in the near-infrared region. The electrochemical and spectroscopic properties can be modulated using substituents or auxiliary ligands with different electronic natures. Through simple functionalization, electropolymerized or monolayer thin films of these complexes have been prepared. These films display multistate near-infrared electrochromism with good contrast ratios and long optical retention times. In addition, flip-flop and flip-flap-flop memories have been demonstrated on the basis of these thin films.

Redox-responsive carbometalated ruthenium and osmium complexes

Organometallic conjugated complexes have become an important type of stimuli-responsive materials because of their appealing electrochemical properties and rich photonic, electronic, and magnetic properties. They are potentially useful in a wide range of applications such as molecular wires, molecular switches, molecular machines, molecular memory, and optoelectronic detections. This review outlines the recent progress on the molecular design of carbometalated ruthenium and osmium complexes and their applications as redox-responsive materials with visible and near-infrared (NIR) absorptions and electron paramagnetic resonance as readout signals. Three molecule systems are introduced, including the symmetric diruthenium complexes, metal-amine conjugated bi-center system, and multi-center redox-active organometallic compounds. Because of the presence of a metal-carbon bond on each metal component and strong electronic coupling between redox sites, these compounds display multiple reversible redox processes at low potentials and each redox state possesses significantly different physical and chemical properties. Using electrochemical potentials as input signals, these materials show reversible NIR absorption spectral changes, making them potentially useful in NIR electrochromism and information storage.

Facile Construction of Structurally Defined Porous Membranes from Supramolecular Hexakistriphenylamine Metallacycles through Electropolymerization.

The construction of well-controlled porous materials is very challenging. Herein, we report the successful preparation of structurally defined porous membranes based on hexakistriphenylamine metallacycles through electropolymerization. The newly designed porous materials were characterized by the typical cyclic voltammograms, XPS, SEM, and TEM investigations. Further investigations revealed that the metallacycle-based polymer films displayed a good size-selective molecular-sieving behavior.