Control of Intramolecular Electron Transport Pathways by Varying Localized Electron Distribution

The control of different electron transport pathways by quantum interference (QI) effects offers a unique opportunity for the modulation of electrical properties in molecular electronic devices and materials. In this work, we propose a chemical way to control the intramolecular electron transport pathways by the localization of the highest occupied molecular orbital (HOMO) distribution. The negative charge injection in para -carbazole by deprotonation exhibited a fourfold suppression of single-molecule conductance, while the conductance is almost the same for meta -carbazole before and after deprotonation. The flicker noise analyses and theoretical simulations revealed the localized distribution of HOMO on the para -carbazole center, leading to the appearance of destructive quantum interference (DQI) effect for the control of electron transport pathway. This strategy of reaction-induced orbital localization offers a new strategy for the control of charge transport through molecular devices and materials.


Introduction
The electron transport through single-molecule junctions represents the overall contributions of all transport pathways. [1][2][3][4] In different electron transport pathways, the phase of transmission amplitudes is different, resulting in different quantum interference (QI) patterns to form various conductance states. [5][6][7][8] The QI effects can be regulated through structural modification, such as connection site variation [9][10][11] and side group substitution, [12][13][14][15] which offers the opportunity for the control of specific pathways.
However, because of the strong coupling between the molecular energy level and the Fermi energy level of electrodes, the frontier orbital levels of the molecule are typically overextended, thus inhibiting the formation of destructive quantum interference (DQI). 16,17 To demonstrate an anti-resonance feature from the DQI for the suppression of charge transport, 18 the localization of electron density distribution of frontier orbitals will be a potential option to avoid this strong coupling.
To reduce the electrode-molecule coupling, the localized distribution can be achieved by chemical design via creating the absence of electron density cloud on the connection sites of frontier molecular orbitals. 19 Recent works suggest that the electrode can be decoupled by localizing the lowest unoccupied molecular orbital (LUMO) distribution to form effective DQI to create highly nonlinear I-V features, 19 and also to form the electron-hopping pathway responsible for voltage-induced switching. 20 However, it remains challenging to directly switch between localization and delocalization in single-molecule junctions, the chemical reactions to modify the molecular structure insitu hold a promising way. The chemical reactions, such as isomerization, 21 acidification, 22 photothermal reaction, 23 and redox reaction, 24 provide potentials to change the structure of the molecule and the distribution of electron density cloud. Therefore, regulating the distribution of molecular orbitals through in-situ chemical reactions provides a new strategy for controlling the electron transport pathways.
Herein, we synthesized two series of molecules based on carbazole as the conductive core (Figure 1a), and investigated their single-molecule conductance by the scanning tunneling microscope break junction (STM-BJ) technique. 25,26 We found that the negative charge injection in para-carbazole by deprotonation exhibited a fourfold suppression of single-molecule conductance, while the conductance is almost the same for meta-carbazole before and after deprotonation. Further flicker noise measurements and the theoretical simulations, including density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) and electron transport pathway calculations, demonstrated that the localized distribution of HOMO on the paracarbazole center leads to the appearance of DQI effect and the control of intramolecular electron transport pathways. The carbazole compounds are a class of diphenylamine molecules with an isoelectronic structure, which possesses strong electron-donating and hole transport abilities. 27 The proton on the nitrogen atom can be removed through alkalization to create a localized electron density distribution on the carbazole center, and then recovers by acidification, 28 which acts as an ideal platform for regulating electron transport pathways through an in-situ chemical reaction. 29,30 In addition, to quench the negative charge, methyl trifluoromethanesulfonate (MeOTf) was also used to methylate the carbazole anion derivatives to confirm the formation of the localization further. 31 Compound Para-N-H and Meta-N-H were prepared through the reported procedures (more details in supporting information (SI) section 1). 32,33 To accurately achieve the deprotonation of the carbazole structure, potassium tert-butoxide (t-BuOK) was used as the base to remove the active hydrogen on the nitrogen atom of carbazole, forming a nitrogen anion, which is an extremely hydrogen affinity state. The NMR spectra show that the active hydrogen signal at 11.3 ppm disappears with the addition of base (indicated by the black frame in Figure 1c and 1d), while the chemical shifts of other hydrogens move to high-field, which proves the accomplishment of deprotonation. 34,35 Afterwards, the negative charge can be quenched by methylation with chemical shifts In the investigation of these two states using single-molecule conductance measurement, once high polar DMF as the solvent and a strong base of t-BuOK were used, rendering a strong leakage current in this solution environment between the bare Au tip and the Au substrate. It results in a high electrical background and makes conductance testing impossible (more details in Figure S15a and S15b from SI). Therefore, to realize conductance measurement in such an ionic solution system with high polarity and large tunneling background, the tip coated with hafnium dioxide (HfO2) as the insulating protective layer by Atomic Layer Deposition (ALD) was adopted in the STM-BJ technique. 36 By depositing HfO2 with a high dielectric constant on the Au tips, we achieved intensive tip coating to significantly reduce the leakage current 37, 38 and the electrical background, which resulted in a larger conductance testing window (more details in SI section 4.2).  After alkylation, Meta-N-Me and Meta-N-e exhibit almost the same conductance, which is 10 -4.84 G0 (1.12 nS). However, the conductance of Para-N-Me is significantly higher than that of Para-N-e, which is 10 -4.49 G0 (2.51 nS) and almost close to that of Para-N-H. Therefore, this conductance change must be directly related to the negative charge on nitrogen site. Previous researches have shown that the QI effects can be affected by regulating the charge density of the nitrogen atom on the carbazole ring to change molecular conductance. 40 The charge density of nitrogen atoms can be characterized by the proton binding ability of nitrogen atoms (pKa). Under acidic and neutral conditions, the molecular conductance increases with the increase of pKa.

Results
However, in this experiment, after deprotonation, Para-N-e and Meta-N-e actually have a strong proton binding ability with large pKa. We found that under alkaline conditions, the conductance of meta-carbazole after deprotonation moderately changed, and the regulation effect of pKa reached to saturation, while the conductance of para-  Table S1 from SI), as shown in Table 1  To further investigate the electrical transport properties before and after deprotonation, the flicker noise measurement was performed. According to the conductance results above, the molecular junction elongation was hovered for 200 ms to extract the conductance signals within the period for flicker noise analysis. 42,43 We find that the scale of the noise power of Para-N-H and Para-N-Me is G 1.5 as shown in Figure 3b and 3d, indicating that the charge transport through Para-N-H and Para-N-Me is synchronously dominated by through-bond and through-space transport. In contrast, for  (Figure 4a-4b), respectively, and the emission spectra show obvious redshift (Figure 4d-4e), indicating the shrinkage of energy gaps. 28,44 After methylation, the UV absorption peaks and the fluorescence emission peaks of Para-N-Me and Meta-N-Me are basically the same as those of Para-N-e and Meta-N-e. 45 To further reveal the changes in molecular energy levels before and after deprotonation, density functional theory (DFT) calculations were employed to perform theoretical studies. 46 Frontier molecular orbitals, which are directly correlated to the charge transport, were calculated for all the investigated molecules.
Overall, the energy of HOMOs and LUMOs increase with the injection of the negative charge as shown in Figure 4c. However, the increase of LUMO energy is less than the increase of HOMO energy, which results in narrower HOMO-LUMO gaps. The calculated energy level changes are consistent with the results of macroscopic spectroscopic measurements.   Figure   S20 from SI). Furthermore, the weak coupling leads to a decrease in the broadening of HOMO formant and a decrease in the resonance energy, resulting in a low and sharp HOMO formant as shown in Figure 5a. In addition, the localized HOMO and the delocalized LUMO even fulfill the theory of Fano resonance that a localized state interacts with a continuum. 18 In contrast, as shown in Figure 5b,  To investigate the effect of the introduction of negative charge on the intramolecular electron transport pathway, we calculated the electron transport pathway of Para-N-H and Para-N-e as shown in Figure 6a-6b and Table S2 in SI. 50,51 For Para-N-H, electrons are mainly transmitted through the C-C pathway (more details in Table S2, at the energy position of -1.4 eV, the ratio of current in the C-C pathway reached 71.36%), such a para-connection pathway does not have DQI effect, 6 thus a higher conductance is revealed, which is also verified on the transmission curve (Figure 5a, purple line).
However, for Para-N-e, deprotonation leads to the localization of HOMO orbitals, which further causes the ring current in carbazole center (Figure 6b). and Meta-N-e is G 1.1 before and after deprotonation, which means that their electrical transport mode remains through-bond tunneling (more details in Figure S19 from SI).
The flicker noise remains unchanged before and after deprotonation for meta-carbazole, which to some extent proved that the change of flicker noise before and after deprotonation for para-carbazole is due to the change of intramolecular electron transport pathway.

Conclusion
In conclusion, the charge transport through single-molecule junctions of para-carbazole and meta-carbazole before and after deprotonation was investigated using the STM-BJ technique. The results demonstrate that the injection of negative charge via the deprotonation of para-carbazole leads to a reversible fourfold suppression of conductance. The combined theoretical calculations and the flicker noise analysis revealed that the injection of negative charge results in the localization of HOMO orbitals in para-carbazole, leading to the reversed ring current and eventually blocking the intramolecular electrical transport pathways because of DQI. In comparison, the conductance and main electron transport pathway of meta-carbazole remain unchanged before and after deprotonation. Our findings provide an in-situ strategy for designing reversible molecular switches to manipulate charge transport through the singlemolecule junction, which is achieved via blocking the intramolecular transport pathways of electrons by localized electron distribution.

Materials
The molecules were synthesized according to the previous report (For more details, see supporting information (SI) section 1). 32,33,54 For more details, see supporting information (SI) section 1. The gold substrates were prepared by coating 200 nm Au film on silicon wafers. Gold wire (99.99%, 0.25 mm diameter) was purchased from Beijing Jiaming Platinum Nonferrous Metal Co, Ltd. for the fabrication of the gold tip. The hafnium dioxide coated gold tip was prepared by Atomic Layer Deposition (ALD) (For more details, see SI section 4.2).

The STM-BJ measurement
The conductance of molecular junctions was measured using the lab-built scanning tunneling microscope break junction (STM-BJ) technique under ambient conditions ( Figure S11). 55,56 In the STM-BJ measurement, the distance between the gold tip and the substrate is controlled by a stepper motor and a piezo stack. The bias voltage is applied between the tip and substrate, and the current is used as feedback to control the movement of the gold tip. During the repeating opening (tip retracting) and closing (tip approaching) cycles, the conductance versus displacement traces are collected, and the traces of the opening cycles are used for further analysis.
The flicker noise measurement.
The flicker noise measurement carried out according to the previous studies. 42,52 We suspended the tip during the retracting process for 200 ms after the formation of molecular junctions. The noise spectrums were obtained by the fast Fourier transform. We calculated the noise power according to the noise spectrum integration from 100 Hz to 1000 Hz and plotted the noise power against the average conductance.