(4,4')-Bipyridine in Vacuo and in Solvents: A Quantum Chemical Study of a Prototypical Floppy Molecule From a Molecular Transport Perspective

TL;DR

This study uses quantum chemical calculations to analyze the structure, vibrational properties, and transport characteristics of (4,4')-bipyridine in various environments, highlighting solvent effects and conformational influences on conductance.

Contribution

It provides a detailed quantum chemical analysis of (4,4')-bipyridine's structural and electronic properties, incorporating solvent effects and proposing a model for molecular transport behavior.

Findings

Solvent acts as an effective gate electrode.

Conductance correlates with the cosine squared of the twist angle.

Significant transmission occurs in the perpendicular conformation.

Abstract

We report results of quantum chemical calculations for the neutral and anionic species of (4,4{'})-bipyridine (44BPY), a prototypical molecule with a floppy degree of freedom, placed in vacuo and in solvents. In addition to equilibrium geometries and vibrational frequencies and spectra, we present adiabatic energy curves for the vibrational modes with significant intramolecular reorganization upon charge transfer. Special attention is paid to the floppy strongly anharmonic degree of freedom of 44BPY, which is related to the most salient structural feature, namely the twist angle $\theta$ between the two pyridine rings. The relevance of the present results for molecular transport will be emphasized. We show that the solvent acts as an effective gate electrode and propose a scissor operator to account for solvent effects on molecular transport. Our result on the conductance $G$ vs. $\cos^2\theta$ is consistent with a significant transmission in perpendicular conformation indicated by previous microscopic analysis.