Tuning Single-Molecule Conductance by Controlled Electric Field-Induced trans-to-cis Isomerisation

External electric fields (EEFs) have proven to be very efficient in catalysing chemical reactions, even those inaccessible via wet-chemical synthesis. At the single-molecule level, oriented EEFs been successfully used promote situ reactions absence of catalysts. Here, we elucidate effect an on structure and conductance a molecular junction. Employing scanning tunnelling microscopy break junction (STM-BJ) experiments, form electrically characterize junctions two tetramethyl carotene isomers. Two discrete signatures show up more prominently at low high applied voltages which are univocally ascribed trans cis isomers carotenoid, respectively. The difference between both cis-/trans- is concordance with previous predictions considering π-quantum interference due presence single gauche defect isomer. Electronic calculations suggest that field polarizes molecule mixes excited states. mixed states (spectroscopically) allowed transition and, therefore, can cis-isomerization participate electron transport. Our work opens new routes for control isomerisation contacts.