Synthetic Control of Quantum Interference by Regulating Charge on a Single Atom in Heteroaromatic Molecular Junctions.

A key area of activity in contemporary molecular electronics is the chemical control of conductance of molecular junctions and devices. Here we study and modify a range of pyrrolodipyridines (carbazole-like) molecular wires. We are able to change the electrical conductance and quantum interference patterns by chemically regulating the bridging nitrogen atom in the tricyclic ring system. A series of eight different N-substituted pyrrolodipyridines has been synthesized and subjected to single-molecule electrical characterization using an STM break junction. Correlations of these experimental data with theoretical calculations underline the importance of the pyrrolic nitrogen in facilitating conductance across the molecular bridge and controlling quantum interference. The large chemical modulation for the meta-connected series is not apparent for the para-series, showing the competition between (i) meta-connectivity quantum interference phenomena and (ii) the ability of the pyrrolic nitrogen to facilitate conductance, that can be modulated by chemical substitution.

In recent years, quantum interference has been one of the most actively pursued topics in molecular electronics. Quantum interference (QI), which can be either destructive or constructive, results in molecules not following classic electrical circuit rules, and this offers new opportunities to exploit their electrical properties. QI has been demonstrated across a wide range of molecules either as multi- or monolayer films sandwiched between pairs of conductors or as single molecules bridging between nanoelectrode pairs. For example, in the former case, it has been shown through direct two-terminal electrical measurements across self-assembled monolayers that QI is sensitive to chemical changes and conjugation patterns.[1,2] Conjugation has been used as one of the primary ways of controlling quantum interference, with a classic exemplar being para- versus meta-substitution in a central benzene ring in a molecular wire. An example of this recorded at the single molecule level with the break-junction technique is the switch between constructive interference apparent for para-connected oligo(3)-phenylenevinylene to destructive QI for the analogue with meta-connectivity at the central benzene ring.[3] However, QI phenomena can be very sensitive to precise junction details, including even through space interactions, and it has been shown that meta-coupled benzene molecules can even exhibit higher conductance than their para-analogues.[4] This high sensitivity of QI to chemical structure has been used to distinguish two structural isomers within single molecule break junctions.[5] It has been demonstrated that QI can be electrochemically controlled, notably in the case of anthraquinone (AQ) containing molecular bridges and thin films, which can be switched between the cross-conjugated AQ and the linearly conjugated H2AQ state by electrochemical reduction.[6] QI has also been demonstrated in thin films of AQs grafted to a base electrode and sandwiched into a two-terminal device with an evaporated top electrode.[7] It has also been recently shown that electrochemical control can be used to tune the alignment of transport resonances in transmission curves that in turn control QI.[8,9] Other chemical phenomena have also been shown to give rise to significant QI effects. For example, charge transfer complex formation between thiophene molecular wires (donors) and tetracyanoethylene acceptors creates a new resonance in the transmission function near the metal contact Fermi energy.[10,11] This signature of QI gives rise to significant conductance enhancement (constructive QI). Another example of the modulation of QI through changes in noncovalent interactions is the sliding of individual π-stacked dimers in mechanically controlled break junctions.[12] Even simple protonation of a molecule can significantly change QI patterns, for example, protonation of azulene molecular junctions can alleviate destructive interference.[13] It has also been shown that strain introduced into ring structures within molecules impacts charge transport pathways.[14] Changing atom types within aromatic rings is expected be an effective way of modulating QI, and indeed it has been shown that meta-connected five-membered rings can be tuned by swapping between different heteroatoms within the ring.[15] On the other hand, molecular bridges containing fused aromatic ring systems are a veritable playground for investigating QI phenomena since they offer, through chemical synthesis, diverse control of connectivity, aromaticity and conjugation, heteroaromaticity, and substitution. This provides the basis of the present study in which the nitrogen heteroatom within a fused aromatic framework (carbazole) is chemically controlled, with this shown to be an effective way of tuning quantum interference and molecular conductance.

The tricyclic scaffold of fluorene-like compounds (Figure ) is an interesting candidate for substitution studies because (i) the two aryl units are locked in a coplanar geometry and therefore no variations in the inter-ring dihedral angle are expected, and (ii) multiple substitutions are possible, as various bridging atoms X, functional groups R, and para-/meta- (2,7-/3,6-) connectivity to the electrodes can be explored by appropriate choice of the synthetic process. A few studies have been performed to probe the effect of chemical substitution on the molecular conductance[17−19] and thermoelectric properties[20] of these tricyclic systems. The results to date suggest that, in the para- (2,7-) connectivity, the variations in conductance observed as X is changed are mostly due to modifications to the overall aromaticity of the tricyclic system,[17] in agreement with previous studies showing an inverse correlation between aromaticity and conductance.[21] However, when the fluorene scaffold is connected to the electrodes in a meta-pattern (3,6-), aromaticity and conductance are no longer correlated, with quantum interference effects[17] and the individual atomic components of the molecular wire[18] having the dominant effect on charge transport. This results, for instance, in meta-carbazoles (X = N) having a significantly higher conductance than the meta-dibenzofurans (X = O)[17] or meta-silafluorenes (X = Si),[18] in contrast with predictions based on the aromaticity of the central 5-membered ring. The peculiar behavior of carbazoles was attributed to a greater ability of the bridging N in facilitating electronic transmission as its lone pair couples the two aryl units, providing an alternative, high efficiency charge transport pathway. These results sparked our interest in studying the anomalous behavior of carbazoles by exploring their rich substitution chemistry, in the hope of shedding more light on the subtle interplay between quantum interference phenomena and chemical structure. We will demonstrate here that, in the series of meta-carbazoles we studied, the QI feature arising from meta-connectivity is shifted away from the Fermi energy of the electrodes and effectively switched off. Furthermore, when the electron density on the pyrrolic N in highly coupled meta-carbazoles is modulated by the presence of electron-donating or electron-withdrawing substituents, the extent of coupling provided by the lone pair changes accordingly, with a remarkable effect on conductance. The latter can therefore be chemically controlled in the meta-connected molecules while remaining roughly unchanged in the para-connectivity.

Figure 1

Structure of fluorenyl compounds. The dashed bonds are the connections to the electrodes (e.g., −C≡C–C6H4–NH2 in González et al.[16]).

We focused our efforts on synthesizing and measuring the conductance of two series of simple tricyclic pyrrolodipyridines: the meta-series 1M–5M and the para-series 1P, 2P, and 5P (Figure ). These are analogues of carbazole, with the contacts to the electrodes embedded in the tricyclic system (pyridyl N), providing a constant high degree of coupling to the electrodes and ensuring relatively high conductance even in the presence of destructive interference phenomena. By focusing on a single class of tricyclic compounds, in this case N-based heterocycles, no significant changes in aromaticity or inter-ring dihedral angle are expected. The compounds were prepared by the sequence shown in Figure a, and full synthetic procedures and characterization are provided in the SI. In brief, we prepared 4,4′-dibromo-3,3′-dipyridine by selective lithiation (lithium diisopropylamine in tetrahydrofuran) of 4-bromopyridine in the 3-position, followed by Ullmann-style coupling with CuCl2. The dihalodipyridine was then subjected to double Buchwald-Hartwig amination[22−24] with an aniline derivative, using Pd2(dba)3·CHCl3 as precatalyst and a bulky biaryl phosphine as ligand,[25] to enforce cyclization to the desired compounds 1M–5M. The para-connected compounds 1P, 2P, and 5P were prepared in the same way, using 3,3′-dibromo-4,4′-dipyridine[26] as starting material.

Figure 2

(a) Synthetic pathway for the synthesis of the N-substituted pyrrolodipyridine, (b) structures of the meta-compounds used in this study, and (c) their para-analogues. Key in (a): (i) lithium diisopropylamide (1 h, −94 °C, THF), CuCl2 (16 h, RT), 27%. (ii) Pd2(dba)3·CHCl3, SPhos, KOtBu, RPhNH2 (overnight, 65–75 °C, toluene). Yield: 17–82%, depending on R (more details in the SI).

Single-molecule conductance was then determined using the scanning tunneling microscope break-junction (STM-BJ) technique,[27] where gold point contacts are continuously formed and broken in a solution of the target molecule (here, in mesitylene/tetrahydrofuran 8:2 v:v), at room temperature and low DC bias (200 mV in this study), by driving a Au tip into contact with a Au substrate and then withdrawing it at constant speed (20 nm s–1). When the point contact is broken, Au-molecule-Au junctions spontaneously form in the nanogap through interaction of the aurophilic pyridyl N termini with undercoordinated Au atoms, and the junction is then stretched until it ruptures. During the whole process, the current flow is recorded as a function of tip–substrate separation, and conductance (current/bias) is calculated in units of G0 (quantum of conductance, 2e2/h ≅ 77.48 μS). A typical break-junction trace shows a series of plateaux at multiple integers of G0, which are due to charge transport through a small number of (or just one) Au atoms, and molecule-dependent plateaux at much smaller conductance values. Junctions are fabricated and ruptured thousands of times, and the corresponding conductance versus elongation traces are analyzed statistically in frequency histograms and two-dimensional maps. Histograms give the most probable conductance value, while the maps correlate charge transport features to the evolution of the junctions from the atomic contact to its final rupture.

The main results are summarized in Figure . The substituent on the pyrrolic N of the meta-family 1M–5M indeed has an effect on molecular conductance and modulates its value by more than one order of magnitude (Figure b), while it has almost no effect on the conductance of the para-analogues 1P, 2P, and 5P (Figure c), also in good agreement with our previous study on planarized 4,4′-dipyridines.[29] It is worth mentioning that the bistable conductance behavior of 4,4′-bipyridine is retained in the para-series, which results in the well-resolved double peaks that can be observed in Figure c. Furthermore, compounds 5M and 5P have an additional possible binding mode through the pyridyl ring attached to the carbazolic N, which contribute to a low-conductance shoulder in both cases (more details in the SI). The overall results therefore suggest a relationship between charge transport efficiency and the electronic state of the bridging pyrrolic N. Chemical insight into molecular conductance can be gained by looking for correlations between molecular conductance and certain parameters classically used by physical organic chemists to characterize the effects of structural change. Examples of this are the correlation between molecular conductance and the Hammett parameter for substituted oligophenylethylene (OPE) molecular wires[30] or the previously mentioned pyridinophane.[31] Here, we expect a relationship between the charge density on the pyrrolic N atom and the electrical behavior of the pyrrolodipyridine junctions. A directly relevant measurable physical parameter that relates to the charge density on the nitrogen atom is the acidity of the protons of the anilinium ions corresponding to the aniline building block used in the synthesis of these pyrrolodipyridines. Plotting the logarithm of conductance versus their pKa (in water) shows a clear correlation (Figure d), with an apparent upper limit of conductance (∼10–3.7G0) that can be attained with these molecular wires. This can be clearly observed in the histograms, where compounds 4M and 5M have very similar conductance, near to the limit.

Figure 3

(a) Structure of 5 M as a single-molecule junction, (b) conductance histograms of compounds 1M–5M and (c) 1P, 2P, and 5P. (d) Logarithmic conductance of 1M–5M and 1P–5P versus pKa, with connecting lines as guide to eyes. We used the pKa of the anilinium ion corresponding to the aniline used in the synthesis of the compound by Buchwald-Hartwig amination. pKa data from the CRC Handbook of Chemistry and Physics.[28] The asterisk in the conductance histograms marks a small artifact introduced by our 4-channel preamplifier transimpedance switch. Conductance data acquired at 200 mV bias and at 10 kSa/s, vertically shifted for clarity in (b) and (c) and normalized as counts/trace (all plots >3000 traces, with no data selection). Key in (a): H = white, C = gray, N = blue, Au = yellow. The error in (d) is σ of the conductance histogram Gaussian fitting. Absolute conductance values and errors can be found in the SI.

The clear correlation between the charge residing on the pyrrolic N and molecular conductance allows us to introduce a simple conceptual model for the interpretation of the data. The lone pair on the pyrrolic N acts as a facilitator of charge transport,[17,18] opening a nonconjugated but efficient alternative conductance pathway (green arrows in Figure c) that attenuates the interference phenomena introduced by the meta-pathway (orange arrows in Figure c) and grants relatively high conductance. The greater the electron density on the pyrrolic N, the larger the extent of aryl–aryl coupling provided by the lone pair and, therefore, the higher the molecular conductance, up to a limit of approximately 10–3.7G0.

Figure 4

(a) Conceptual model for the interpretation of the data. The meta-charge transport pathway (orange arrows) is inefficient due to quantum interference effects. An alternative para-pathway (green arrows) is provided by the bridging atom, and its efficiency is modulated by the electron density on the pyrrolic N. (b) Transmission curves for compound 1M–5M and (c) magnification of the area between −0.25 and 0.25 eV. The DFT-predicted Fermi energy is represented as a dotted gray line.

While this simple model is enough for a qualitative interpretation of the data, molecular circuits do not behave like classic electrical networks, where the total conductance is the sum of the individual contributions. Multiple pathways in a single-molecule junction contribute additional quantum interference effects, and therefore, a rigorous DFT analysis is needed to better characterize the behavior of the compounds used in this study. We therefore used the transport code Gollum[32] to calculate the transmission coefficient T(E) for electrons of energy E passing from one electrode to the other via the molecule. We then introduced a scissor operator[33−35] using the optical bandgap of the compounds to account for the inability of DFT-LDA accurately to predict the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies.[36,37] The conductance can be calculated from the transmission curves as G = G0T(EF), where T(EF) is the value of the transmission coefficient at the DFT-predicted Fermi energy of the metallic electrodes. The absolute position of the Fermi level of the electrodes cannot be predicted with full accuracy as it is dependent on the local shape of the electrodes and the surrounding nanoenvironment, which constantly change during an STM-BJ experiment. Therefore, molecular conductance cannot be exactly calculated, but information about it can be inferred from the behavior of T(E) within the HOMO–LUMO bandgap, where EF generally lies. All the meta-compounds showed indeed signatures of destructive quantum interference in the calculated T(E) profile, which result in a “dip” in the transmission curve (more details and T(E) curves magnification in the SI). The DQI “dip” is however heavily shifted toward the nonconducting orbital and not as sharp as observed, for instance, in simple meta-connected aryls. These effects are due to the bridging N atom that couples the two pyridyl rings[17,18] and generates a strong asymmetry in the behavior of T(E) in the HOMO–LUMO gap. Substituents on the pyrrolic N do not significantly change the energy position of the interference feature, but they control the value of the transmission coefficient at E – EF = 0 (near the DFT-predicted Fermi level of the electrodes, Figure b). The same calculations were performed on the para-compounds 1P, 2P, and 5P, and the behavior of T(E) in the HOMO–LUMO gap was found to be insensitive to the nature of the substituent on the pyrrolic N (see SI for additional details). To provide further insights on the mechanisms of conductance modulation and to contribute to the theoretical framework that explains the correlation of molecular conductance with pKa, we calculated the net charge gain on the pyrrolic N in compounds 1M–5M by three different methods. Plotting these values against molecular conductance and pKa shows a clear mutual dependence, with the implication that control of the charge on the bridging atom is the dominant mechanism. As can be observed in Table S1 and Figure S27, SI, the changes in net electron gain are only minute (<10%), but they contribute to a substantial change in the charge transport properties of the molecular junction.

In conclusion, we demonstrated here that it is possible to chemically control the conductance of a molecular wire by appropriate choice of substituents and electrode connectivity pattern. The key phenomenon here is the competition between a quantum interference feature introduced by a meta-connectivity pattern, that suppresses conductance, and the presence of an alternative, high-conductance pathway through a single atom (the pyrrolic N), whose electron density can be modulated by appropriate chemical substitution. Control over the effect of quantum interference features at the EF of the metallic electrodes is important for the development of functional molecular devices such as efficient insulators[38] and thermoelectric converters,[39] and our results demonstrate an effective way to exert control by regulating the charge on just a single atomic component of the molecular wire. This represents an expansion of the portfolio of methods currently used to influence quantum interference phenomena, with a purely synthetic approach that complements the use of electrochemical/electrostatic potential to change the energy alignment with the electrodes EF,[8,9,40] or the use of light,[41] potential,[6,42−44] and acid–base interactions[5,13] to trigger changes in the structure and degree of conjugation of the molecular wire.