Twistable Dipolar Aryl Rings as Electric Field Actuated Conformational Molecular Switches

The ability to control the chemical conformation of a system via external stimuli is a promsing route for developing molecular switches. For eventual deployment as viable sub-nanoscale components that are compatible with current electronic device technology, conformational switching should controllable by a local electric field (i.e. E-field gateable) and accompanied by a rapid change in conduction. In organic chemical systems the degree of π-conjugation is linked to the degree of electronic delocalisation, and thus largely determines the conductivity. Here, by means of accurate first principles calculations, we study the prototypical biphenyl based molecular system in which the dihedral angle between the two rings determines the degree of conjugation. In order to make this a gateable system we create a net dipole by asymmetrically functionalising one ring with electronegative substituents (F, Br and CN) with different polarisabilities. In this way, the application of an E-field interacts with the dipolar system to influence the dihedral angle, thus controlling the conjugation. For all three considered substituents we consider a range of E-fields, and in each case extract conformational energy profiles. Using this data we obtain the minimum E-field required to induce a barrierless switching event for each system. We further extract the estimated switching speeds, the conformational probabliities at finite temperatures, and the effect of applied E-field on electronic structure. Consideration of these data allow us to assess which factors are most important in the design of efficient gateable electrical molecular switches.