  Helicenes are inherently chiral polyaromatic molecules composed of all-ortho
fused benzene rings possessing a spring-like structure. Here, using a
combination of density functional theory and tight-binding calculations, it is
demonstrated that controlling the length of the helicene molecule by
mechanically stretching or compressing the molecular junction can dramatically
change the electronic properties of the helicene, leading to a tunable
switching behavior of the conductance and thermopower of the junction with
on/off ratios of several orders of magnitude. Furthermore, control over the
helicene length and number of rings is shown to lead to more than an order of
magnitude increase in the thermopower and thermoelectric figure-of-merit over
typical molecular junctions, presenting new possibilities of making efficient
thermoelectric molecular devices. The physical origin of the strong dependence
of the transport properties of the junction is investigated, and found to be
related to a shift in the position of the molecular orbitals.
