Intermolecular Weak Interactions and Their Effects on Quantum Electronic Devices

分子間弱相互作用及其在量子電子器件中的影響

Student thesis: Doctoral Thesis

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Award date10 Aug 2017

Abstract

Noncovalent interactions between aromatic rings, usually involving π-π and CH-π interactions, are abundant in chemical and biological processes, spanning from self-assembly, molecular recognition, to electronic transport and catalysis. Different from intramolecular chemical bonds, the interactions between aromatic rings have dual natures that both dispersion and electrostatic interactions contribute to the total energy. Most of the previous calculations were committed to the study of geometries and energies with high-accuracy and to give analyses of interaction components based on calculations using complicated quantum chemical methods and basis sets. The intuitionistic quantum-mechanical-level nature of the π-π interaction has not been revealed. In this thesis, employing first-principles calculations, we systematically investigate the electronic delocalization and molecular orbital interaction in a benzene sandwich dimer and dimers with other configurations, including parallel-displaced, T-shaped and edge-to-face configurations, and then study the effects of π-π and CH-π interaction on electronic transport properties using non-equilibrium Green’s function (NEGF) theory.

In chapter 1, we introduce the fundamentals of intermolecular weak interactions involving π-π and CH-π interactions, and then review the investigations on the effect of intermolecular interaction on transport properties.

In chapter 2, we focus on the most popular computational methods, including molecular orbital theory, density functional theory and NEGF employed in this work.

In chapter 3, a prototype of aromatic π-π stacking system – benzene sandwich dimer is investigated by ab initio calculations based on second-order Møller–Plesset perturbation theory (MP2) and Minnesota hybrid functional M06-2X. In succession to our recent exploitation of CH-π complex, definite orbital interaction is also found in the benzene sandwich dimer. Similar to interatomic orbital interaction, the intermolecular orbital interaction also forms “bonding” and “anti-bonding” orbitals. Both occupied and unoccupied molecular orbitals are involved in strong orbital overlap, and conspicuous deformation can be found in HOMO-2, HOMO-3, HOMO-9, LUMO+3, and LUMO+4. Our results shed light on the nature of π-π interaction from the view of intermolecular orbital interactions.

In Chapter 4, following our finding of strong orbital interaction in benzene sandwich dimer, we further studied the intermolecular interaction in benzene dimers with eight different configurations, including sandwich, parallel-displaced, T-shaped, and edge-to-face configurations, based on ab initio calculation at MP2/cc-pVTZ and M06-2X/6-311++G** levels. From the view point of interaction energy, the parallel-displaced configurations are the most stable, followed by T-shaped and edge-to-face ones, and the sandwich one is the least stable. All of those dimers with different configurations exhibit orbital interactions, involving occupied and unoccupied molecular orbitals. The orbital interaction in parallel-displaced configurations is remarkable, similar to the sandwich one; while in T-shaped and edge-to-face configurations, the orbital overlap is largely reduced. The electron density difference presents an overall consequence of π-π interaction. Together with the electronic energy levels, the role that electrostatic repulsion plays in intermolecular interaction is qualitatively understood. The absorption spectra of the system reveal the ultraviolet and far-ultraviolet absorption. In chapter 3, we verified that, even with same energy levels, the strong orbital interaction occurs between the π and π* orbitals with substantial delocalization of benzene molecules. Herein, we further conclude that the orbital interaction is determined by not only the closeness of the energy levels of original two π or π* orbitals from benzene molecules, but also the matching of spatial distributions of them. Thus, we conclude three criteria determining the orbital interaction in benzene dimer systems: the closeness of energy level, the π or π* orbitals, and the matching of spatial distributions.

In chapter 5, using the fully self-consistent non-equilibrium Green’s function method combined with density functional theory, we studied the effect of intermolecular interactions on electronic transport properties of molecular junctions containing benzenethiol dimers and biphenylthiol dimers with sandwich, parallel-displaced dimer, and edge-to-face configurations, together with parallel dithiolbiphenyl dimers with different separation distances. The results of benzenethiol and biphenylthiol bimolecular junctions show that although the intermolecular interaction in sandwich configuration is the most intense, its contribution to electronic transport is not the largest. For benzenethiol bimolecular junctions, edge-to-face configuration has the highest conductance, in which intermolecular interaction is the weakest and the conductance mainly comes from the direct electronic tunneling through molecules to the opposite electrodes; the conductance of sandwich configuration follows, where the intermolecular interaction is the strongest and does not favor the direct tunneling and conductance; the conductance of parallel-displaced configuration is the lowest, where the conductance mainly originates from the intermolecular interaction instead of direct tunneling due to longer distance between two electrodes. For the biphenylthiol dimers, the conductance is totally contributed by the intermolecular interaction: the conductance of parallel displaced configuration is the highest, follows by that of sandwich configuration, and that of edge-to-face is the lowest. For the parallel dithiolbiphenyl bimolecular junction, separation distance influences the intermolecular interaction and thus has a major effect on the conductance; anchoring symmetry has minor effect on conductance. The DFT results of conductance and transmission are consistent with those of the tight-binding models.

In chapter 6, a conclusion of the whole thesis is presented about our systematic study on the intermolecular orbital interaction in benzene dimers with different configurations and the effects of intermolecular interaction on electronic transport properties.

    Research areas

  • Intermolecular weak interaction, orbital interaction, electronic transport