###### Abstract  
External electric fields (EEFs) have proven to be very efficient in catalising chemical reactions, even those inaccessible via wet-chemical synthesis. At the single-molecule level, oriented EEFs have been successfully used to promote in situ single-molecule reactions in the absence of chemical catalysts. Here, we elucidate the effect of an EEFs on the structure and conductance of a molecular junction. Employing scanning tunnelling microscopy break junction (STM-BI) experiments, we form and electrically characterize single-molecule junctions of two tetramethyl caroten isomers. Two discrete conductance signatures show up more prominently at low and high applied voltages which are univocally ascribed to the _trans_ and _cis_ isomers of the carotenoid, respectively. The difference in conductance between both cis-/trans- isomers is in concordance with previous predictions considering \(\pi\)-quantum interference due to the presence of a single _gauche_ defect in the trans isomer. Electronic structure calculations suggest that the electric field polarizes the molecule and mixes the excited states. The mixed states have a (spectroscopically) allowed transition and, therefore, can both promote the cis-isomerization of the molecule and participate in electron transport. Our work opens new routes for the in situ control of isomerisation reactions in single-molecule contacts. molecular electronics; single-molecule junctions; STM break-junction; in-situ isomerisation; carotenoids +
Footnote †: journal: Applied Science  
0021  
Tuning Single-Molecule Conductance by Controlled Electric Field-Induced _trans_-to-_cis_ Isomerisation [PERSON], [PERSON], D.D. molecular configuration constitutes a defining parameter for the intrinsic conducting properties of a molecule. Moreover, measuring single-molecule conductance can be used to follow structural changes in a molecular contact [10; 11; 12; 13], opening the way to real time detection of electrical currents associated with in situ stimuli-induced molecular structural changes [5; 14; 15]. Charge transport across molecular systems as a function of molecule configuration has been widely studied theoretically [16; 17; 18] and experimentally [8; 9; 11; 19; 20; 21; 22], including _cis-/trans_- isomerization. Different isomers exhibit characteristic conductance values that can be attributed to various effects, such as the potential energy barrier variation due to structural change [8; 17; 19], the different contact geometry [8; 9; 16; 21], or the different energies of the frontier molecular orbitals with respect to the electrode Fermi levels for each isomer [17; 18; 22]. Behind unravelling the particular phenomenology for each system, the detection of electrical currents across molecules in metal molecule \(|\) metal junctions [13; 23; 24] have been essential to electrically characterize structural changes of individual molecular systems [25]. Here, we report on single-molecule scanning tunnelling microscopy break junction (STM-BJ) conductance experiments of all-\(E\)-carotene with terminal 1,5-dimethyl-3-methylthiophenyl groups (_trans_-TMC, Figure 1a) and 9-\(Z\)-carotene with terminal 3-methylthiophenyl groups (_cis_-TMC, Figure 1b) carotenoid derivatives. Both, _trans_-TMC and _cis_-TMC-syn molecules are chemically stable at room temperature [22; 26]. Carotenoids contain a delocalized \(\pi\)-conjugated polyene backbone chain with planar configuration maximizing \(\pi\)-orbital overlap resulting in an efficient electron pathway [27; 28; 29]. They serve in several natural photosynthetic systems as energy/electron transfer mediator [30; 31]. Our main findings indicate: (1) applying an STM bias voltage to the high-conductance (HC) _trans_-TMC-based molecular junction promotes the in-situ switching to a low-conductance (LC) _cis_-TMC-based molecular junction whose chemical compound cannot be synthesized wet chemically due to steric constraints. (2) The ratio of LC/HC signatures scales with the Vbias (i.e., EEF) magnitudes.