Are better conducting molecules more rigid?

TL;DR

This paper explores the electronic factors influencing the bending stiffness of conducting molecules, revealing how molecular orbital overlap and doping affect flexibility and how electro-stiffness varies with applied voltage.

Contribution

It introduces the concept of electro-stiffness, linking electronic structure to mechanical flexibility, and analyzes how doping and voltage bias influence molecular stiffness.

Findings

Electro-stiffness depends on orbital overlap and energy gap.

Doped molecules are more flexible.

Voltage bias causes nonlinear changes in molecule length and stiffness.

Abstract

We investigate the electronic origin of the bending stiffness of conducting molecules. It is found that the bending stiffness associated with electronic motion, which we refer to as electro-stiffness, $\kappa_{e}$, is governed by the molecular orbital overlap $t$ and the gap width $u$ between HOMO and LUMO levels, and behaves as $\kappa_{e}\sim t^{2}/\sqrt{u^2+t^{2}}$. To study the effect of doping, we analyze the electron filling-fraction dependence on $\kappa_{e}$ and show that doped molecules are more flexible. In addition, to estimate the contribution of $\kappa_{e}$ to the total stiffness, we consider molecules under a voltage bias, and study the length contraction ratio as a function of the voltage. The molecules are shown to be contracted or dilated, with $\kappa_{e}$ increasing nonlinearly with the applied bias.