Theoretical Study on Electrical Properties of Molecular Junctions of Viologen Derivatives

本文基于密度泛函(DFT)结合非平衡格林函数(NEGF)的方法,以具有氧化还原中心的紫罗碱衍生物(N,N′-bis(4-thioalkyl)-4,4′-bipyridinium, HS-4V4-SH)功能分子构造Au(111)/S-4V4-S/Au(111)分子结,详细分析了分子在三种价态V、V+和V2+下的电学性质与分子的几何结构和电子结构的关系。基于对三种价态透射系数分析结果表明,在零偏压下,V与V+的电导值比V2+高了两个数量级,4V4分子结的电导随两个吡啶环之间夹角的增大呈线性减小。同时,理论计算结果也表明,增加烷基链(HS-nVn-SH, n = 2 ~ 7)的数目,发现分子结电导值呈指数形式衰减,其每个亚甲基的衰减因子约为1,与烷基二硫醇分子的接近。In this paper, the electrical properties of molecular junctions formed N,N′-bis(4-thioalkyl)-4,4′-bipyridinium (viologen) moiety between two gold (Au) electrodes have been investigated by combining density functional theory and non-equilibrium Green’s functional approach. To modulate the viologen molecule to be a cation with one and two positive charges (V+ and V2+), we introduce one and two trifluoroacetic acid ions (TFA-) around the molecule, respectively. The valence states of V+ and V2+ are confirmed by checking Mulliken and NBO charges. Then the relationship between molecular conductance and electronic structures of the neutral state V, the radical state V+ and dication V2+ are analyzed in detail. The results in analyzing transmission spectra of the three states reveal that the conductance values of V and V+ are two orders of magnitude larger than that of V2+. This suggests that the redox states of viologen molecules can be used to realize the function of molecular switches. Our calculated results also show that increasing the torsion angle between two pyridine rings of the S-4V4-S molecule will decrease the conductance. By comparing different ions of TFA、PF6 and BF4, the calculated results show that the molecular junction conductance decreases about 3 times when the torsion angle increases by about 6°. It indicates that increasing the torsion angle of the dication V2+ can improve significantly switching ratio of viologen derivatives molecules. At the same time, the calculated results show that increasing the number of methylene groups in alky chains (HS-nVn-SH, n = 2 ~ 7), the conductance values of molecular junctions decrease exponentially, and the attenuation factor of each methylene is about 1 close to alkanedithiol molecules in literatures experimentally and theoretically. This also shows that as the alkyl chain length increases, the DFT-NEGF theoretical method can better predict the zero-bias conductance of the viologen derivative molecule.国家自然科学基金项目(21533006);国家自然科学基金项目(21621091);国家自然科学基金项目(21773197);福建省创新人才通讯作者:吴德印E-mail:[email protected]:De-YinWuE-mail:[email protected]厦门大学化学化工学院化学系,固体表面物理化学国家重点实验室,福建 厦门 361005Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, Chin

A DFT and SERS study of synergistic roles of thermodynamics and kinetics during the electrocatalytic reduction of benzyl chloride at silver cathodes

International audienceThe reductive cleavage of carbon-halogen bonds is one of the classical model systems of concerted versus nonconcertedbond breaking electron transfer in molecular electrochemistry, but most studies did not consider theinfluence of adsorption of reaction intermediates on this phenomenon. We performed density functional theory(DFT) calculations to understand the electrochemical reduction of benzyl chloride at silver electrodes throughpredicting the surface adsorption effect of the reactant and of follow-up intermediates and analyzing the energeticsof the overall dissociative electron transfer. The ensuing DFT calculations of surface-enhanced Ramanspectroscopic (SERS) signals provided some key information for characterizing different surface species at differentpotentials at the molecular level. The most intense and broad spectral peak observed near 800 cm−1 inthe electrochemical SERS was thus assigned to the CH2 wagging vibration closely associated with the coadsorptionof benzyl radical and chlorine. After the dissociation of the C-Cl benzyl chloride bond, the chlorideanion adsorbs at the three-fold hollow site on silver electrodes, as indicated by a low vibrational frequency anda weak Raman signal. Conversely, the formed benzyl radical intermediate is found to adsorb at the top site withthe characteristic vibrational frequency and strong Raman intensity. DFT calculations further confirmed thatthe kinetic effect underlying the electrocatalytic activity of silver electrodes is closely associated with theintrinsic energy barrier and the dissociative adsorption state of benzyl radical and chlorine on silver electrodes.Finally, we discuss the fundamental meaning of the kinetic rate constant of the first-step dissociative electrontransfer for benzyl chloride on silver electrodes based on the different electron transfer models, indicating thesynergistic roles of thermodynamics and kinetics

The first data release (DR1) of the LAMOST regular survey

The Large sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) general survey is a spectroscopic survey that will eventually cover approximately half of the celestial sphere and collect 10 million spectra of stars, galaxies and QSOs. Objects in both the pilot survey and the first year regular survey are included in the LAMOST DR1. The pilot survey started in October 2011 and ended in June 2012, and the data have been released to the public as the LAMOST Pilot Data Release in August 2012. The regular survey started in September 2012, and completed its first year of operation in June 2013. The LAMOST DR1 includes a total of 1202 plates containing 2 955 336 spectra, of which 1 790 879 spectra have observed signal-to-noise ratio (SNR)>= 1 0. All data with SNR >= 2 are formally released as LAMOST DR1 under the LAMOST data policy. This data release contains a total of 2 204 696 spectra, of which 1 944 329 are stellar spectra, 12 082 are galaxy spectra and 5017 are quasars. The DR1 not only includes spectra, but also three stellar catalogs with measured parameters: late A, FGK-type stars with high quality spectra (1 061 918 entries), A-type stars (100 073 entries), and M-type stars (121 522 entries). This paper introduces the survey design, the observational and instrumental limitations, data reduction and analysis, and some caveats. A description of the FITS structure of spectral files and parameter catalogs is also provided