Helical orbitals in electrical uni-directional molecular motors

Abstract

The generation of unidirectional motion has been a long-standing challenge in engineering of molecular motors. Here, a mechanism driving the rotation is presented based on electron current through helical orbitals on a $\pi$-bonded carbon chain. Such electron current through helical orbitals has been shown to be circulating around the carbon chain. It is natural to expect that the associated electronic angular momentum drives a rotation when the current is turned on. As intuitive as this relation might seem, it is also incomplete because a formal definition of helicality in terms of a physical observable has not yet been given. Such a definition is proposed here. Based on this definition, we show how helicality determines the motor's sense of rotation. We exemplify the relation between helicality and angular momentum in H\"uckel models of linear carbon chains (cumulenes and oligoynes). We attribute the previously reported opposite helicality sense of frontier orbitals (HOMO and LUMO) to the approximate sub-lattice symmetry. For oligoynes, this symmetry is hidden in the sense that it does not reduce to a mere labeling of atoms. Sub-lattice symmetry, combined with time-reversal invariance, allows us to derive Onsager-type reciprocal relations of various linear response coefficients, dictating e.g. an odd energy dependence of angular momentum response to voltage bias. We propose an observable consequence of the approximate sub-lattice symmetry: If the carbon chain is employed as an axle of a molecular rotor, the sense of rotation is independent on the direction of the current.