The four schools in China, the United States and Germany have cooperated to achieve a new breakthrough in molecular topology insulators

Recently, Professors Latha Venkataraman and Luis M. Campos from Columbia University, Yin Xiaodong, Special Researcher of the School of Chemistry and Chemical Engineering of Beijing Institute of Technology, and Ferdinand of the University of Regensburg in Germany Professor Evers et al. systematically studied the electrical properties of a series of bis(triaryl amine) molecular conductors containing oligomeric benzene bridges in electrically neutral, monovalent and divalent cationic states. On July 7, 2022, the study was published in the journal Nature Chemistry under the title “Highly conducting single-molecule topological insulators based on mono- and di-radical cations.”

Moore’s Law in the field of semiconductors heralds the trend of miniaturization or even miniaturization of devices, making people’s eyes gradually focus on the emerging field of molecular electronics. At present, most of the research on conductive elements of single-molecule circuits is based on the construction of molecular wires by conjugated units, and is usually conducted through coherent and non-resonance mechanisms. But in this case, the conductivity decreases exponentially as the length of the molecular wire increases. Therefore, since the 1980s, the idea of constructing molecular wires with reverse conductivity attenuation has been proposed, that is, the conductivity of molecular lines increases with the increase of length. In such studies, one way to achieve an anomalous conductivity-length relationship is to design molecular lines with dual radical characteristics so that they follow the Su-Schrieffer-Heeger (SSH) model (e.g., polyacetylene) of organic one-dimensional topological insulators. In the past series of work, people have tried to use polymethyltylene, cumulenes and porphyrin oligomers as single-molecule wires. However, the electron transport of these molecules is far from the conductivity quanta (G0 = 2e2/h) expected in the SHH model. In order to develop a wire based on the SHH model, it is an urgent problem to design an extended π conjugate system to achieve highly conductive dual radical characteristics, and stable structural characteristics to insulate the terminal radicals from the environment.

Based on the above background, Professor Latha Venkataraman and Professor Luis M. Campos of Columbia University, Yin Xiaodong, Special Researcher of the School of Chemistry and Chemical Engineering of Beijing Institute of Technology, and Professor Ferdinand Evers of the University of Regensburg, Germany, have systematically studied the single-molecule electrical properties of a series of bis(triaryl amine) molecular conductors containing oligomer benzene bridges in electrically neutral, monovalent and divalent cationic states. Studies have shown that divalent cation wires exhibit quinone-type characteristics with terminal radical cations on nitrogen atoms, similar to polyacetylene lines described in the SSH model. Among them, B32+ molecular wires show an ultra-high conductivity significantly higher than 0.1 G0 (2e2/h, conductivity quantum), 5400 times higher than neutral molecules, and the molecular wire length reaches 2.6 nm. The researchers also observed that the cationic molecular wires after oxidation showed negative correlation characteristics of conductance and wire length, which were β (B1-3+) = -0.9/benzene, respectively; β (B1-32+) = -0.3/benzene indicates that this class of molecular wires has the characteristics of one-dimensional topological insulators described by SSH models. Professor Ferdinand Evers et al. also elucidated the orbital delocation of leading radicals through DFT calculations leading to the observed non-classical quasimetallic behavior, theoretically validating the experimental results.

Figure 1: (A) Chemical structure of nitrogen derivatives of Thiele and Chichibabin hydrocarbons, and their mono- and dioxidic forms; (B) Cyclic voltammetry of B1-B4; (C) UV-VIS spectrum of B1-B4 in neutral (top) and monovalent cation (bottom).

Figure 2: Conductivity measurements from the Bn, Bn+ and Bn2+ series. (A-C) conductivity histogram of bis(triarylamine) lines in (A) neutral (B) monovalent cation and (C) divalent cationic states; (D) For all oxidation states, the conductance of the molecule in relation to length. For the neutral (square), monovalent cation (round), and divalent cation (diamond)series, the β values for each phenyl are 1.7, -0.9, and -0.3, respectively. (E-H) Two-dimensional histogram of B12+−B42+.

Figure 3: Transmission calculations for the neutral Bn series. (A) Relaxation structure of neutral B3 bound to au electrode; (B) Transfer function of neutral B1-B4 molecular wires. The EF (Fermi Energy Level) is represented by a dotted line, and the transmission resonance of the HOMO and HOMO-1 orbits is marked with a solid and dotted arrow, respectively. (C) Exponential attenuation of the transmission at EF, showing that the attenuation constant is approximately β = 1.4/benzene.

Figure 4: Transfer calculations for the oxidized Bn+ and Bn2+ series. (A) Schematic diagram of the DFT calculation model for the single and double cation series. (B) Compare an isolated B4+ molecule with a theoretical SOMO orbital with a B4+ junction with a gold electrode. (C) The eigenvalues of the molecular orbitals of the B4+ molecular front were calculated by PBE and Hartree-Fock methods, respectively. (D) Spin polarization transport function of monovalent cationic molecular leads, with most spin channels and a few spin channels labeled as dashed and solid lines, respectively. (E) Calculation of closed-shell transport of divalent cationic molecular wires. EF is represented by black dotted lines in (D) and (E). (F-G) Compare the theoretical conductivity of calculated and measured monovalent and divalent cation molecular wires, respectively. Both series show reverse conductance attenuation (calculated β value: -0.6) from B1+/2+ to B3+/2+[一价阳离子]and -0.4[二价阳离子]and conductivity degradation for B4+ and B42+.

Through simple molecular system design, sophisticated molecular electronic testing, and rigorous theoretical calculation support, the work achieves long organic molecular wires with ultra-high conductivity and negative conductivity attenuation. A series of results show that it has the characteristics of a one-dimensional topological insulator based on the SSH model, which provides an important theoretical and experimental basis for the design of molecular electronic components, and is expected to promote the further development of molecular electronics from theory to practical application. (Source: Science Network)

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