## 4 Conclusions  
In the present study, an in situ electric field-dependent isomerisation in a single-molecule junction is demonstrated. Our experimental platform allows modulating the EEF by modifying the bias voltage magnitude to well-defined nano-gaps between two metal leads, while detecting single-molecule current events. By characterizing both a _trans_-TMC and a synthetically homologous _cis_-TMC version, we have univocally assigned molecular conductance features to each isomer. _Trans_-TMC is then ascribed to the molecular junction displaying average conductance values of ca. 3 \(\times\) 10\({}^{-4}\) Go, and the in situ generated _cis_-TMC is ascribed to molecular junctions average conductance values of ca. 1.6 \(\times\) 10\({}^{-4}\) Go. We show that controlled voltage pulses can be used to in situ isomerize the trapped polyene molecular backbone in the nanoscale molecular junction. Our combined experimental results and DFT calculations ascertain that a controlled isomerisation irreversibly occurs in  
Figure 5: Orbitals for the S\({}_{0}\)\(\rightarrow\) S\({}_{1}\) natural transition of the trans-TMC in the absence (top) or presence (bottom) of an EEF (0\({}^{\circ}\) dihedral angle). Left numerical column indicates the weight for each orbital.  
the _trans_-TMC-based single-molecule junctions when converted into a more structurally stable _cis_-TMC-based junction. The isomerisation rate dependence on the EEF strength is supported by the increasing yield of junction formation of the _cis_-TMC form as a function of the EEF magnitude. Under high bias regimes (i.e., strong EEF), the _cis_-TMC LC feature dominates the conductance histogram. We conclude that the isomerisation from the _trans_-to the _cis_-TMC is promoted in a controlled way under a specific EEF strength in the single-molecule junctions. These findings have a special relevance in the field of electrocatalysis, since obtaining the _cis_-TMC isomer is unfeasible using traditional bulk synthetic routes due to steric constrains.  
Conceptualisation, A.C.A., I.D.-P., S.K.; Synthesis: S.K., methodology, A.C.A.; software, A.C.A., D.A. ([PERSON]); validation, C.S.Q., A.C.A., D.A. ([PERSON])., I.D.-P.; formal analysis, C.S.Q., A.C.A., D.A. ([PERSON]), D.A. ([PERSON]), K.F.D., I.D.-P.; investigation, C.S.Q., A.C.A.; resources, A.C.A., D.A, S.K., I.D.-P.; data curation, A.C.A., D.A. ([PERSON]), K.F.D.; writing--original draft preparation, A.C.A., K.F.D.; writing--review and editing, C.S.Q., A.C.A., D.A. ([PERSON]), K.F.D., S.K., I.D.-P.; visualisation, A.C.A., D.A. ([PERSON]); supervision, A.C.A., K.F.D., D.A. ([PERSON]), S.K., I.D.-P.; project administration, A.C.A., funding acquisition, K.F.D., A.C.A., S.K., I.D.-P. All authors have read and agreed to the published version of the manuscript.  
I.D.-P. thanks the ERC (Fields4 CAT-772391) for financial support. A.C.A. thanks European Union for a H2020-MSCA-IF-2018 Fellowship (TECH-MoDE). K.F.D. is grateful for generous funding through the \"Plus 3\" program of the Boehringer Ingelheim Foundation. S.K. appreciate National Research Foundation of Korea for a research grant (NRF-2020R1A2C1010724). D.A. thanks Powered@NLHPC; this research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02).  
Not applicable.  
All the data and additional information supporting the findings of this study are available from the corresponding authors upon reasonable request.  
The authors declare no conflict of interest.