Fig. 2: Single-molecule conductance characterization.

a Chemical structure of compound 1, which is formally in a tautomeric equilibrium with its enol isomer 2. The staggered equilibrium arrows represent that the keto form dominates this equilibrium. Compound 2-OMe, the methylated analog of enol form 2, is formed by methylation of compound 2. b, c 2D conductance histograms of compound 1 at 0.1 V (b) and 0.6 V (c) bias, with the corresponding stretching distances shown in the insets with the blue (b) and red (c) histograms. The color scales represent the intensity. d 1D conductance histograms of compound 1 characterized at different biases ranging from 0.1 to 0.6 V. The low and high-conductance states are labeled ‘L’ and ‘H’, respectively. The 1D conductance histogram of compound 2-OMe at 0.1 V bias is shown on the upper side. The blue and red lines originate from the 1D conductance histograms of the traces of ‘L’ and ‘H’, respectively, with the blue and red areas obtained through Gaussian fitting. The blue and red areas represent the ‘L’ and ‘H’ states, respectively. e Typical individual conductance traces of compound 1 characterized at 0.1 V (blue lines) and 0.6 V (red lines) biases. f Distribution probabilities of states ‘L’ and ‘H’ plotted against different biases for a constant tip speed (5 nm s⁻¹). The dashed black line represents the threshold, after which the ‘H’ state becomes observable. g Peak centers of the dominant conductance peaks of the corresponding 1D conductance histograms with alternation of the bias between 0.1 and 0.6 V. The error bars correspond to the standard deviation of the Gaussian fitting. The blue and red colors represent the ‘L’ and ‘H’ states, respectively.