1(b)) can be used as an experimental control parameter that can be tuned to alter and probe the properties of the HMJ. We suggest that the distance r between the two metallic electrodes of a helicene molecular junction ( Fig. The four points in (c) mark the contact points of the rings with the electrode used for the tight-binding model. The tight-binding parameters for nearest neighbor interaction ( t), inter-stack coupling ( t 1) and molecule–electrode level broadening Γ are noted. (c) Graphic representation of the tight-binding model of the helicene-based molecular junction. (b) 2,31-Diazahelicene (9) molecule connected to two gold(1,1,1) contacts, r denoting the distance between the electrodes. Chirality issues were not a subject of this study, however, all the investigated systems were of ( M) helicity. 1 Schematic illustration of the system studied: (a) chemical structure of diazahelicenes (1)–(9) used in the DFT and tight-binding calculations. 40–42 Furthermore, as a first step towards realizing single-molecule junctions, helicenes have been placed on surfaces and probed with atomic force and scanning tunneling microscopes. Although the number of applications of helicenes is so far rather limited, they have already found astonishing applications to enantioselective organo- or transition metal catalysis, 29,30 molecular recognition, 31,32 self-assembly, 33 nonlinear optical materials, 34–36 chiral materials, 37–39 and chiroptical electronic devices. 25–28), and a renewed and growing interest in helical aromatics is clearly visible within the last decade. The chemistry of helicenes has been systematically explored for more than half a century (for recent reviews see, e.g. Helicenes are aromatic compounds in which n benzene rings or other (hetero)aromatics are angularly annulated to give helically-shaped molecules with a spring-like structure ( Fig. In this letter, we propose to focus on helicene-based molecular junctions (HMJs), which can be used as a tool to help answer the above question. The natural question that arises in this context is then: is there a way to in situ increase the thermopower, S, and the thermoelectric FOM, ZT, of MJs? Increasing the thermopower of molecular junctions is an essential step if MJs are ever to become relevant energy conversion technologies. 4,15,16 While there have already been several demonstrations of thermoelectric energy conversion in single-molecule junctions, 17–24 unfortunately the typical thermopower is rather small ( S ∼ 5–50 μV K −1), much smaller than that of commercially available semi-conductor-based thermoelectrics. The thermopower and the thermoelectric figure of merit (FOM) are measures of a junction's ability to convert a temperature difference into electric power, and MJs were suggested as candidates for efficient and high-power thermoelectric devices 5–14 due to their low dimensionality, large versatility and low thermal conductance. 1 In recent years it has been realized that molecular junctions (MJs) can potentially fulfill additional functionalities ranging from opto-electronics and spintronics to thermoelectric energy conversion, 2–4 which is the focus of this work. Introduction Interest in single-molecule junctions arose from their originally proposed role as functional elements in electronic 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. 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. 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. Helicenes are inherently chiral polyaromatic molecules composed of all- ortho fused benzene rings possessing a spring-like structure.