## 3 Results and Discussion
The 1D semi-logarithmic conductance histogram retrieved from single-molecule dynamic BJ experiments in the _trans_-TMC molecules exhibits two broad, overlapping conductance features (Figure 2); 1.68 \(\times\) 10\({}^{-4}\) G\({}_{0}\) (low conductance regime, LC) and 3.21 \(\times\) 10\({}^{-4}\) G\({}_{0}\) (high conductance regime, HC), where G\({}_{0}\) = 2\(e^{2}/h\) = 77.47 uS, the \(h\) Planck constant, the \(e\) elementary charge. The HC value is roughly twice of the LC value which might suggest the formation of multiple _trans_-TMC molecular junctions. However, several experimental observations rule out previous scenario: first, all the collected current traces show no correlation, displaying either HC or LC plateaus (Figure 2b), as opposed to current traces of junctions formed by multiple molecules that are characterized by the appearance of sequential plateau features due to consecutive molecular disconnections [47]. Second, junctions formed by multiple molecules also have a characteristic formation yield that decreases with the number of trapped molecules, i.e., the frequency of molecular junction formation decreases from the low conductive (few molecules) to the high conductive junctions (many molecules) [47; 48]. The latter is the behaviour opposite to that observed in Figure 2a where the HC yield (27.7%) is significantly higher than the LC yield (17.4%). As such, we rule out that the HC values are related to a multiple molecule junction scenario and that the detection of the two current signatures must be a consequence of distinct single-molecule properties as discussed below. The nanoscale inter-electrode distance of a molecular junction allows strong electric fields in the gap in the order of 10\({}^{8}\) to 10\({}^{9}\) V/m [5]. Such strong fields have been reported to promote single-molecule reactions of trapped molecules in an STM junction [3]. Given the cis-/trans- isomerization in carotenoids, we explore the possible electric field effect on the isomerization reaction in these systems [25; 49; 50; 51; 52; 53]. The dynamic BJ approach can have detrimental effects when studying molecular configuration dynamics in a molecular junction due to the mechanical stress induced by the STM tip during retraction, which pulls on the molecule and can induce structural modifications [25; 49; 50; 51]. Instead, the static BJ approach (Figure 1b) avoids the aforementioned drawbacks by employing a fixed inter-electrode distances also ensuring a constant gap EEF strength. The static BJ approach allows for a precise oriented EEF along the main junction axis. To this aim, we define three EEF of increasing magnitudes (low, intermediate, and high) by applying V\({}_{\text{bias}}\) values of 7 mV, 65 mV and 130 mV. For a tip-substrate gap of approximately 3 nm (estimated length of the _trans_-TMC), the corresponding gap EEFs are ca. 2.3 \(\times\) 10\({}^{-3}\) V/nm, 2.2 \(\times\) 10\({}^{-2}\) V/nm, and 4.3 \(\times\) 10\({}^{-2}\) V/nm, respectively. Figure 3b-d shows the 2D blinking conductance-map histograms obtained for the three applied EEFs. In the low bias regime of 7 mV, the _trans_-TMC displays HC events only at ca. 3 \(\times\) 10\({}^{-4}\) G\({}_{0}\) (Figure 3b). At intermediate and high V\({}_{\text{bias}}\) of 65 and 130 mV, the _trans_-TMC 2D conductance-maps additionally exhibit a LC feature with  
Figure 2: (a) Semi-logarithmic histogram from dynamic break junction (BJ) experiments for trans-TMC displaying a high conductance peak (HC, green traces) and a low conductance peak (LC, orange traces) fitted with Gaussians. (b) Representative individual current versus gap distance traces. V\({}_{\text{bias}}\) = 20 mV. blinks at ca. \(1.5\times 10^{-4}\) G\({}_{\text{o}}\) (Figure 3c,d). These results are in agreement with the dynamic BJ results and provide further evidence that the observed conductance features originate from two distinct molecular features in the junction.