The ability to modulate the electrical signal passing through devices by switching between two or more well-defined states is essential for the design of logical circuits in data storage and processing.9 Control of chemical reactions via inherent electronic triggers offers a promising switching mechanism at the single-molecule scale, thus opening an avenue for molecular-scale devices with bistability.2 As a ubiquitous type of intramolecular chemical reaction, tautomerization is an ideal mode of operation to realize such a single-molecule switch with bistability. Over the past decade, tautomerization has been demonstrated in the surface-based characterization of tetrapyrrole macrocycles, such as porphyrins and naphthalocyanine4-6,10,11 using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Although distinct states have been observed for these macrocycles upon their absorption on flat surfaces, the resulting pairs of tautomeric states are generally degenerate or quasi-degenerate with a low isomerization barrier12. Due to the low barrier, the interconversion between the tautomeric states is in dynamic balance and spontaneously occurs even under cryogenic conditions, which prevents the potential application of the single-molecule tautomerization in the construction of controllable molecular devices. Furthermore, the in-plane charge transport through the surface-based devices limits the maximal conductance difference between the tautomeric states, and the highest switching ratio is still below ~300%.4,5
Here, we report the reversible switching between two tautomeric states at the single-molecule scale with controlled bistability operated by a moderate bias at room temperature. The mode of operation in our switching device involves two redox-related PESs, one corresponding to the uncharged ground state of the molecular bridge (MB; Fig. 1a, upper panel), and the other one corresponding to a positively charged state of the MB (Fig. 1a, lower panel). On the first PES (PES I, Fig. 1a), the keto form is the thermodynamically stable form with a relatively high barrier associated with an energetically unfavorable four-membered cyclic transition state (Fig. 1a),13 which suppresses any further dynamic and thermal interconversion. Upon charge injection, the second PES becomes accessible (PES II, Fig. 1a) with dramatically reduced tautomerization barrier and reversed thermodynamic driving force. Consequently, the keto form is readily converted into the enol form on the resulting PES II, until an electron is injected back into the MB from one of the contacts. The molecular devices are characterized by the scanning tunnelling microscope break junction (STM-BJ) technique14,15 (Fig. 1b). By adjusting the bias, the charge injection can be reversibly manipulated, and the conductance difference between the switching states of the keto (s bridge) and enol form (p bridge) reaches ~6700%.
To ensure that the charge transport pathway passes through the tautomerization unit, we designed molecule A with the keto-enol unit (red parts in Fig. 2a) between two thioanisole anchors (blue parts in Fig. 2a). The anchors facilitate the connection of the molecular components to the gold electrodes. Molecule A can be synthesized at the gram scale by an electrophilic substitution reaction in one step (see details in the Methods Section). From the NMR characterization of the resulting compound (Extended Data Fig. S7), we observe the exclusive presence of keto form A (Fig. 2a), indicating that this form dominates the tautomeric equilibrium. In the structure of A, the methylene group blocks the conjugation between the phenyl groups of the thioanisole anchor units. Thus, A is expected to exhibit a significantly lower conductivity in the sulfur-to-sulfur pathway than its corresponding tautomeric isomer B, i.e., the isomer with a p-bridge between the two phenyl groups. Such an enol form can also be trapped by methylation, leading to molecule B-OMe as a reference (Fig. 2a).
To investigate the charge transport through single-molecule junctions of A, we dissolved 0.1 mM A in 1,2,4-trichlorobenzene solvent and characterized the resulting solution by STM-BJ experiments at room temperature. As shown in Fig. 2e, the individual conductance traces (conductance vs. stretching distance) characterized at a 0.1 V bias show conductance plateaus (blue traces) between 10-4 and 10-5 G0 (G0 is the quantum conductance, where G0 = 77.6 mS).16 After increasing the bias to a moderate 0.6 V, we observe another conductance plateau with a higher conductance located between 10-2 and 10-3 G0 (red traces). By overlapping more than 3000 such conductance traces, we obtain the two-dimensional (2D) conductance histograms shown in Fig. 2b and 2c (cf. Extended Data Fig. 3 for 2D histograms at intermediate bias voltages). These 2D histograms show similar shapes of conductance plateaus in distinct conductance ranges. We further analyze the stretching distance distributions of the conductance plateaus. As shown in the inset of Fig. 2b and Fig. 2c, the stretching distances of the conductance plateaus are similar at 0.1 and 0.6 V biases. After considering a 0.5 nm snap-back distance after the breaking of the gold-gold atomic contact,17 the calibrated length of the molecular junctions at 0.1 and 0.6 V biases correspond to 0.92 and 0.95 nm, respectively. The consistency of these stretching distances associated with the molecular length suggests that a robust sulfur-to-sulfur charge transport pathway remains intact at different bias voltages.
One-dimensional (1D) conductance histograms were constructed from more than three thousand traces without data selection at different bias voltages up to 0.6 V. As shown in Fig. 2d, the peak center (determined by Gaussian fitting) of the 1D conductance histograms at 0.6 V is located at 10-2.82 G0, which is ~6700% fold to the conductance at 0.1 V (10-4.65 G0). The 1D conductance histograms at biases ranging from 0.1 to 0.3 V are all very similar, showing an approximately invariant single conductance peak centered around 10-4.6 G0, which we defined as the low-conductance state and labeled 'L'. Increasing the bias beyond 0.4 V, we observe a double peak distribution, with a new peak gradually emerging at around 10-2.8 G0, which we defined as the high-conductance state and labeled 'H'. The 'H' state becomes increasingly prominent as the bias increases and starts to dominate at the bias voltage of 0.6 V. This dramatic conductance switching suggests that the applied bias effectively transforms the nonconductive A to a highly conductive state similar to B. To confirm the transformation, a stabilized enol form of B, B-OMe (Fig. 2a), was characterized at 0.1 V, and the resulting single conductance peak is centered at 10-3.06 G0 (Fig. 2d). Increasing the bias up to 0.6 V does not discernably change the location of this peak (Extended Data Fig. 4). The similarity in the conductance characteristics for B-OMe and the 'H' state suggests that the emergence of the 'H' state at high bias is associated with a conversion from the keto form to the enol form. Owing to a high barrier in the neutral PES (vide infra), we propose the 'H' state is the (positively) charged enol form of B, which can be reached thermally on the charged PES.
Through analysis of the probabilities of the occurrence of the 'H' and 'L' states at different bias voltages, we constructed the scatter diagram shown in Fig. 2f. The probability of the 'H' state begins to rise when the bias exceeds 0.4 V, while the probability of the 'L' state decreases significantly. The two probabilities become almost equal at 0.6 V bias. At a 0.7 V bias, the probability of the 'H' state further increases to 67%. At voltages exceeding 0.7 V, the formed single-molecule junctions become increasingly unstable, which renders characterization of the corresponding probabilities unfeasible. More importantly, we find that the switching between the 'H' and 'L' states is reversible. To assert this, we switched the bias voltages between 0.1 and 0.6 V in an alternating fashion. We collected approximately one thousand conductance traces for three subsequent cycles to determine the most-probable conductance (Extended Data Fig. 5), as shown in Fig. 2g, which demonstrates robust and reversible switching of the single-molecule conductance.
To further reveal the switching between the 'H' and 'L' states, we also recorded the evolution of the current as a function of continuously varied bias for individual junctions. During the break junction process with a 0.1 V bias applied, once we observed a conductance plateau formed between 10-4 and 10-5 G0, the movement of the gold tip was halted, and a bias voltage ranging from -1 to 1 V was scanned (further details are described in the Methods section)18. As shown in Fig. 3a, the so-recorded I-V curves (831 out of 13850 traces) exhibit clear current switching as the bias voltage increases (the other traces exhibit a regular proportional increase in the current with increasing bias). Four representative I-V traces exhibiting this "jumping behavior" are plotted in the 2D I-V histogram shown in Fig. 3a. The sudden nature of the current jump suggests an abrupt electronic transition occurring on the MB.
We further applied transition voltage spectroscopy (TVS)19,20 to gauge the alignment between the Fermi energy of the electrodes and the main eigenchannel, i.e., the transport channel of the molecular junctions, which is the HOMO for molecule A. We selected a number of I-V curves (2542 traces from 3234 traces in Extended Data Fig. 6) not exhibiting the current jump and transformed the most representative I-V curve (yellow line in Fig. 3b) into a TVS spectrum (as shown in Fig. 3c). The resulting spectrum exhibits a minimum, which is called the transition voltage, at a bias of approximately 0.8 V, suggesting that the energy difference between the Fermi level of the electrodes and this transport channel amounts to approximately 0.4 eV. The magnitude of the spacing between the unbiased Fermi level (EF) and the HOMO channel is in line with the observed robustness of the 'L' state in Fig. 2d up to a moderate voltage; the transport regime within this entire bias window, i.e., [EF - eV/2, EF + eV/2], is entirely off-resonant so that the transmission probability of incoming electrons through the MB (and thus the conductance) remains approximately constant, irrespective of the considered bias voltage. This is a clear indication that the sudden emergence of the 'H' state at voltages exceeding 0.4 V is caused by a structural modification of the MB.
To further elucidate the switching mechanism, we performed DFT calculations on a model system consisting of the bare molecule A in the absence of Au clusters and thiomethyl-groups (Fig. 4a; cf. Section S2 of the Supporting Information for an in-depth discussion of the effect of the anchor groups). First, the PES associated with the neutral species was considered. In accordance with the experimental observations, we observe that at the B3LYP/def2-TZVP level of theory (full computational details can be found in the Methods Section), the enol form lies 7.5 kcal mol-1 higher in energy than the keto form. This energy difference persists even in the presence of an electric field corresponding to the field exerted by the approximate threshold voltage for switching in the junction (FZ = + 0.05 V/Å; cf. Fig. 4a). This finding constitutes an unequivocal confirmation that tautomeric interconversion cannot occur on the (uncharged) ground-state PES (in blue): even if a kinetically accessible reaction pathway connecting the keto and enol form could be identified, the population of the enol form would always remain negligible at room temperature.
Concerning the kinetics of the tautomeric interconversion process, our calculations indicate that the barrier separating the keto and enol forms is unsurmountable; the direct tautomerization mechanism, i.e., the H-atom shuttling from the carbon to the oxygen center (Fig. 4a), corresponds to a potential energy hill of 60.1 kcal mol-1. With the help of a control experiment in the glovebox (Extended Data Fig. 2a), which enabled control over the presence of trace amounts of H2O, we could rule out any alternative water-assisted tautomerization mechanism (Extended Data Fig. 2b-e and Fig. 9).21-23 The isotope effect is also proven to be very weak (Extended Data Fig. 1), which rules out a proton tunnelling mechanism. Meanwhile, no other source of acids or bases is present in the reaction medium to catalyze the process, assuming that the enol form is thermodynamically and kinetically inaccessible on the ground-state PES is reasonable. Furthermore, since the experiment is performed in the absence of a light source, any mechanism involving a transition – upon absorption of a photon – to the PES of an (equally uncharged) excited state can also be ruled out.24-26
Our calculations suggest that removing an electron from the neutral model system dramatically impacts the kinetics and thermodynamics associated with the keto-enol tautomerization. In the positively charged system (Fig. 4a in red), the enol form becomes the thermodynamically stable form, with a driving force of 22.0 kcal mol-1. This thermodynamic reversal from keto to enol preference upon oxidation of tautomeric compounds is well documented in the literature.27-30 Associated with the switch in the thermodynamic preference, the energy hill separating both states is remarkably reduced upon removal of an electron: the barrier associated with this reaction pathway decreases to 22.9 kcal mol-1 for the cationic species (Fig. 4a), which can be readily crossed at room temperature. The root cause of the dramatic change in the reaction profile upon charge injection is straightforwardly revealed by a qualitative valence bond analysis (cf. Section S3 in the Supporting Information). Thus, our computational analysis suggests that only a redox-based mechanism is viable for the tautomerization of the MB. Calculating the transmission spectra for the candidate OFF and ON state associated with this proposed switching mechanism (cf. Fig. 4c), i.e., the uncharged keto-species and the positively charged enol-species respectively, leads to a conductance ratio in the transmission spectrum region corresponding to the bias window that is qualitatively in line with the experimentally obtained value (~ 100:1), which further strengthens this hypothesis.
At this point, it is important to note that our experimental results indicate that the charging event does not require a crossing between the chemical potential of one of the contacts and the transport channel associated with the HOMO of the MB (cf. the TVS in Fig. 3c), in contrast to what could be expected from a pure Landauer-Büttiker framework. Fig. 2f also indicates that there is no fixed threshold bias triggering the switch; beyond a bias of 0.4 V, the proportion of 'H' traces gradually increases. This realization suggests that the charge injection that triggers the tautomerization reaction is a thermal transition (cf. Marcus theory of electron transfer31): as the chemical potential of (one of) the contacts is lowered through the application of the bias, the thermal barrier for the charge transfer process can be expected to decrease gradually, which concomitantly increases the probability of such an event occurring (Fig. 4b; cf. Section S4 in the Supporting Information). As soon as the PES of the charged species is reached through this thermal transition, the associated thermodynamic driving force will steer the MB from its insulating keto form to conducting enol form, and hence the observed conductance switching is obtained. Note that this finding of the role of the molecular states in the transport properties of the junction underscores the intimate relationship and interplay between the Landauer-Büttiker and Marcus frameworks to describe molecular conductance.32,33
In summary, we presented a bias-triggered keto-enol tautomerization switch with controlled bistability in a single-molecule junction, and the switching can be reversibly operated between two defined states even at room temperature. The charge injection from the molecular bridge to the electrodes plays a pivotal role in modifying the PES, which leads to effective regulation of the tautomeric equilibrium. The switching between the two PESs controls both the thermodynamic driving forces and the kinetic barriers, ensuring robust bistability of the resulting devices. The presented two-terminal device structure breaks the limitation of the previously considered surface-based devices, leading to a ~6700% conductance difference between the tautomeric states, which is more than one order of magnitude higher than the conductance difference previously obtained for in-plane molecular devices. Moreover, the functional unit of the molecular device is ubiquitous and can be straightforwardly synthesized, showing the potential to achieve robust, reversible, and scalable switching devices through the control of chemical reactivity at the single-molecule scale.