Effects of Connectivity Isomerization on Electron Transport Through Thiophene Heterocyclic Molecular Junction.

Connectivity isomerization of the same aromatic molecular core with different substitution positions profoundly affects electron transport pathways and single-molecule conductance. Herein, we designed and synthesized all connectivity isomers of a thiophene (TP) aromatic ring substituted by two dihydrobenzo[b]thiophene (BT) groups with ethynyl spacers (m,n-TP-BT, (m,n = 2,3; 2,4; 2,5; 3,4)), to systematically probe how connectivity contributes to single-molecule conductance. Single-molecule conductance measurements using a scanning tunneling microscopy break junction (STM-BJ) technique show ∼12-fold change in conductance values, which follow an order of 10-4.83 G0 (2,4-TP-BT) < 10-4.78 G0 (3,4-TP-BT) < 10-4.06 G0 (2,3-TP-BT) < 10-3.75 G0 (2,5-TP-BT). Electronic structure analysis and theoretical simulations show that the connectivity isomerization significantly changes electron delocalization and HOMO-LUMO energy gaps. Moreover, the connectivity-dependent molecular structures lead to different quantum interference (QI) effects in electron transport, e.g., a strong destructive QI near E = EF leads the smallest conductance value for 2,4-TP-BT. This work proves a clear relationship between the connectivity isomerization and single-molecule conductance of thiophene heterocyclic molecular junctions for the future design of molecular devices.

Substituent-mediated quantum interference toward a giant single-molecule conductance variation.

Quantum interference (QI) in single molecular junctions shows a promising perspective for realizing conceptual nanoelectronics. However, controlling and modulating the QI remains a big challenge. Herein, two-type substituents at different positions ofmeta-linked benzene, namely electron-donating methoxy (-OMe) and electron-withdrawing nitryl (-NO2), are designed and synthesized to investigate the substituent effects on QI. The calculated transmission coefficientsT(E) indicates that -OMe and -NO2could remove the antiresonance and destructive quantum interference (DQI)-induced transmission dips at position 2. -OMe could raise the antiresonance energy at position 4 while -NO2groups removes the DQI features. For substituents at position 5, both of them are nonactive for tuning QI. The conductance measurements by scanning tunneling microscopy break junction show a good agreement with the theoretical prediction. More than two order of magnitude single-molecule conductance on/off ratio could be achieved at the different positions of -NO2substituent groups at room temperature. The present work proves chemical substituents can be used for tuning QI features in single molecular junctions, which provides a feasible way toward realization of high-performance molecular devices.

Controlling and Observing Sharp-Valleyed Quantum Interference Effect in Single Molecular Junctions.

The ability to control over the quantum interference (QI) effect in single molecular junctions is attractive in the application of molecular electronics. Herein we report that the QI effect of meta-benzene based molecule with dihydrobenzo[ b]thiophene as the anchoring group ( meta-BT) can be controlled by manipulating the electrode potential of the junctions in electrolyte while the redox state of the molecule does not change. More than 2 orders of magnitude conductance change is observed for meta-BT ranging from <10-6.0 to 10-3.3 G0 with varying the electrode potential, while the upper value is even larger than the conductance of para-BT ( para-benzene based molecule with anchoring group of dihydrobenzo[ b]thiophene). This phenomenon is attributed to the shifting of energy level alignment between the molecule and electrodes under electrode potential control. Calculation is carried out to predict the transmission function of single molecular junction and the work function of Au surface in the presence of the molecule, and good agreement is found between theory and experiments, both showing sharp-valley featured destructive QI effect for the meta-BT. The present work demonstrates that the QI effect can be tuned through electrochemical gating without change of molecular redox states, which provides a feasible way toward realization of effective molecular switches.

Stereoelectronic Switches of Single-Molecule Junctions through Conformation-Modulated Intramolecular Coupling Approaches.

Stereoelectronic effects in single-molecule junctions have been widely utilized to achieve a molecular switch, but high-efficiency and reproducible switching remain challenging. Here, we demonstrate that there are three stable intramolecular conformations in the 9,10-diphenyl-9,10-methanoanthracen-11-one (DPMAO) systems due to steric effect. Interestingly, different electronic coupling approaches including weak coupling (through-space), decoupling, and strong coupling (through-bond) between two terminal benzene rings are accomplished in the three stable conformations, respectively. Theoretical calculations show that the molecular conductance of three stable conformations differs by more than 1 order of magnitude. Furthermore, the populations of the three stable conformations are highly dependent on the solvent effect and the external electric field. Therefore, an excellent molecular switch can be achieved using the DPMAO molecule junctions and external stimuli. Our findings reveal that modulating intramolecular electronic coupling approaches may be a useful manner to enable molecular switches with high switching ratios. This opens up a new route for building high-efficiency molecular switches in single-molecular junctions.

Mechanically Induced Switching between Two Discrete Conductance States: A Potential Single-Molecule Variable Resistor.

The fabrication of solid-state single-molecule switches with high on-off conductance ratios has been proposed to advance conventional technology in areas such as molecular electronics. Herein, we employed the scanning tunneling microscope break junction (STM-BJ) technique to modulate conductance in single-molecule junctions using mechanically induced stretching. Compound 1a possesses two dihydrobenzothiophene (DHBT) anchoring groups at the opposite ends linked with rigid alkyne side arms to form a gold-molecule-gold junction, while 1b contains 4-pyridine-anchoring groups. The incorporation of ferrocene into the backbone of each compound allows rotational freedom to the cyclopentadienyl (Cp) rings to give two distinct conductance states (high and low) for each. Various control experiments and suspended junction compression/retraction measurements indicate that these high- and low-conductance plateaus are the results of conformational changes within the junctions (extended and folded states) brought about by mechanically induced stretching. A high-low switching factor of 42 was achieved for 1a, whereas an exceptional conductance ratio in excess of 2 orders of magnitude (205) was observed for 1b. To the best of our knowledge, this is the highest experimental on-off conductance switching ratio for a single-molecule junction exploiting the mechanically induced STM-BJ method. Computational studies indicated that the two disparate conductance states observed for 1a and 1b result from mechanically induced conformational changes due to an interplay between conductance and the dihedral angles associated with the electrode-molecule interfaces. Our study reveals the structure-function relationship that determines conductance in such flexible and dynamic systems and promotes the development of a single-molecule variable resistor with high on-off switching factors.

Constructing Dual-Molecule Junctions to Probe Intermolecular Crosstalk.

Understanding and controlling charge transport across multiple parallel molecules are fundamental to the creation of innovative functional electronic components, as future molecular devices will likely be multimolecular. The smallest possible molecular ensemble to address this challenge is a dual-molecule junction device, which has potential to unravel the effects of intermolecular crosstalk on electronic transport at the molecular level that cannot be elucidated using either conventional single-molecule or self-assembled monolayer (SAM) techniques. Herein, we demonstrate the fabrication of a scanning tunneling microscopy (STM) dual-molecule junction device, which utilizes noncovalent interactions and allows for direct comparison to the conventional STM single-molecule device. STM-break junction (BJ) measurements reveal a decrease in conductance of 10% per molecule from the dual-molecule to the single-molecule junction device. Quantum transport simulations indicate that this decrease is attributable to intermolecular crosstalk (i.e., intermolecular π-π interactions), with possible contributions from substrate-mediated coupling (i.e., molecule-electrode). This study provides the first experimental evidence to interpret intermolecular crosstalk in electronic transport at the STM-BJ level and translates the experimental observations into meaningful molecular information to enhance our fundamental knowledge of this subject matter. This approach is pertinent to the design and development of future multimolecular electronic components and also to other dual-molecular systems where such crosstalk is mediated by various noncovalent intermolecular interactions (e.g., electrostatic and hydrogen bonding).

New concept and clinical application of colorectal intraepithelial neoplasia and carcinoma / 中华外科杂志

<p><b>OBJECTIVE</b>To introduce the WHO 2000 diagnostic criteria of biopsy of colorectal intraepithelial neoplasia and carcinoma and to enhance diagnostic accuracy and avoid overdiagnosis and underdiagnosis.</p><p><b>METHOD</b>The postoperative pathological examination and preoperative biopsy in 56 patients diagnosed as colorectal intraepithelial neoplasia and carcinoma before operation from January 2001 to October 2005 were compared retrospectively.</p><p><b>RESULTS</b>Among the 56 cases, 16 patients were diagnosed by preoperative biopsy as carcinoma in situ, intramucosal carcinoma and adenocarcinoma, but according to the new standard, of them 14 cases should be revised to be higher grade colorectal intraepithelial neoplasia.</p><p><b>CONCLUSIONS</b>Strictly adhere to the new WHO criteria, colorectal intraepithelial neoplasia and carcinoma can be diagnosed properly, but for the cases that submucosal muscular layer would not presented in biopsy, the diagnosis should be made by combining clinical findings and various examination results so as to avoid underdiagnosis and delay of treatment.</p>