Modulation of charge transport through single-molecule bilactam junctions by tuning hydrogen bonds

Article abstract of DOI:10.1039/d0cc07423c

Bilactam derivatives with different side groups were synthesized and the twisting angle tuning effect induced by the intramolecular hydrogen bond on the charge transport through their single-molecule junctions was investigated. Molecules with strong intramolecular hydrogen bonds exhibited twice higher conductance because of the reduced dihedral twisting, which was reversible with the addition of hydrogen bond destroying solvent. Our findings reveal that the presence of intramolecular hydrogen bonds promotes the planarization of the molecular structure without additional transmission channels, offering a new strategy for controlling molecular switchesviatuning the molecular twisting.

Full text of DOI:10.1039/d0cc07423c

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Modulation of charge transport through
single-molecule bilactam junctions by tuning
hydrogen bonds†
Cite this: Chem. Commun., 2021,
57, 1935
Received 11th November 2020,
Accepted 12th January 2021
Yaorong Chen,‡a Hua-Chun Wang,‡b Yongxiang Tang,‡a Yu Zhou,a
Longfeng Huang,a Jian Cao,b Chun Tang,a Manxi Zhang,a Jia Shi,a Junyang Liu,
a
ac
Xiancheng Ren,b Yun-Xiang Xu *b and Wenjing Hong
*
DOI: 10.1039/d0cc07423c
rsc.li/chemcomm
Bilactam derivatives with different side groups were synthesized the electronic structure of molecular junctions is inevitably
and the twisting angle tuning effect induced by the intramolecular changed,18,19 leaving challenges in controlling molecular switches
hydrogen bond on the charge transport through their single-molecule via molecular twisting tuning.
junctions was investigated. Molecules with strong intramolecular
To switch the twisting angle of the single-molecule junction
hydrogen bonds exhibited twice higher conductance because of the between different states, a simple and direct way is to lock
reduced dihedral twisting, which was reversible with the addition of down the rotation of molecular building blocks via covalent
hydrogen bond destroying solvent. Our findings reveal that the bonds.16,20–22 In this way, the twisting angle between different
presence of intramolecular hydrogen bonds promotes the planariza- building blocks could be controlled by the bonding formation
tion of the molecular structure without additional transmission to tune the charge transport through molecular devices. How-
channels, offering a new strategy for controlling molecular switches ever, the lock-down strategy based on non-covalent bonds
via tuning the molecular twisting.
remains unexplored due to the challenges in the control of
intramolecular interaction.23 Several studies have suggested
The ultimate goals of molecular electronics are the miniaturiza- that intramolecular hydrogen bonds would improve the planarity
tion of electronic devices and the integration of functional and conjugation of molecules.24–27 The introduction of bilactam
molecular devices into the circuits.1–6 Towards these goals, the units provides a reliable route for forming intramolecular hydro-
functionalization of molecules to construct steerable molecular gen bonds, offering new ideas for overcoming the challenges of
switches, such as redox,7,8 isomerization,9,10 bonding for- controlling the twisting between different states.
mation,11,12 gating,13 etc., remains a critical step. Previous
Herein, we synthesized and investigated the charge trans-
studies have illustrated that the twisting angle in molecular port properties of a series of bilactam derivative molecules with
building blocks effectively changes the intramolecular different twisting angles caused by the ring substituents as
coupling.14,15 An increase of twisting angle between aromatic shown in Fig. 1a. To investigate the influence of the intra-
components leads to a significant decrease in the overlap of molecular interaction on the charge transport, we introduce
p-orbitals, thus resulting in a conductance decrease, which bilactam units into the rotatable group to form the intra-
shows a linear relationship between the conductance and the molecular interaction via a hydrogen bond to suppresses the
square of the cosine of twisting angle.16,17 However, due to freedom of twisting. Combined with the scanning tunneling
the rotation of molecular building blocks and the steric effect, microscope break junction (STM-BJ) technique and density
functional theory (DFT) theoretical calculations, we illustrated
that the intramolecular hydrogen bond modulation gives rise to
the decrease in the planarity of the molecular structure and
a B200% decrease in molecular conductance. In contrast,
a State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM,
College of Chemistry and Chemical Engineering, Xiamen University,
Xiamen 361005, China. E-mail: whong@xmu.edu.cn
b College of Polymer Science & Engineering, State Key Laboratory of Polymer
Materials Engineering, Sichuan University, Chengdu 610065, China.
E-mail: yxxu@scu.edu.cn
c Beijing National Laboratory for Molecular Sciences, Beijing 100190, China
† Electronic supplementary information (ESI) available: Synthesis steps and
characterization data of all compounds; details for single-molecule measurement
and theoretical calculations. See DOI: 10.1039/d0cc07423c
‡ These authors contributed equally.
similar molecules without hydrogen bonds remain unchanged.
Such a robust response behavior during the in situ solvent
switching cycle provides a promising method to control the
charge transport through molecules from a chemical prospect.
To investigate charge transport through these molecules, the
single-molecule conductance was measured by the STM-BJ
technique in a solution of 1,2,4-trichlorobenzene (TCB) at room
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Fig. 1 (a) Schematic of the single-molecule junction and the switching
between different states, alkyl chains on N atoms and H atoms except
hydrogen bonds are ignored for better demonstration. (b) Molecular
structures of BTPP, BTPMP and BTPBP. (c) The one-dimensional (1D)
conductance histogram comparisons of target molecules and typical
Fig. 2 Two-dimensional (2D) conductance histogram versus relative displace-
ment and relative distance distributions (inset) of BTPPtcb (red, a), BTPPethanol
(green, b), BTPMPtcb (yellow, c) and BTPBPtcb (blue, d). According to the relative
distance distributions, the conductance ranges are confirmed to be 10ꢀ0.3 G0
B10ꢀ6.0 G0. (e) The optimized molecular structures of BTPP before and after
mixing with ethanol (left), BTPMP and BTPBP (right). And the 1H NMR of BTPP
under different solvent (middle).
individual conductance distance traces (inset) of BTPPtcb, BTPPethanol
,
BTPMPtcb and BTPBPtcb. (d) Reversible on–off conductance switches of
BTPP by solvent regulation, the subscript is the solvent.
calibrated with an additional 0.5 nm Au–Au snap–back
temperature (more experimental details in the ESI†). Three distance.29 Since the lengths of single-molecule junctions are
molecules were employed (Fig. 1b), 4,40-dihexyl-2,20-bis comparable to a fully stretched configuration bridge between
(4-(methylthio)phenyl)-[6,60-bithieno[3,2-b]pyridine]-5,50(4H,40H)- two electrodes, the variation in conductance originates from the
dione (BTPP), 4,40-dihexyl-7,70-dimethyl-2,20-bis(4-(methylthio)- different twisting angle between two building blocks of single–
phenyl)-[6,60-bithieno[3,2-b]pyridine]-5,50(4H,40H)-dione (BTPMP) molecule junctions. To better understand the difference in
and 7,70-dibenzyl-4,40-dihexyl-2,20-bis(4-(methylthio)phenyl)-[6,60- twisting structures, the optimization structures of BTPP, BTPMP
bithieno[3,2-b]pyridine]-5,50(4H,40H)-dione (BTPBP), and the syn- and BTPBP were obtained by Gaussian software, as shown in
thetic routes are described in the ESI.†
Typical individual conductance traces are exhibited in the sequence of BTPP (27.891) o BTPMP (70.291) o BTPBP (79.761).
inset of Fig. 1c, where the peak at G0 (quantum conductance, Notably, for BTPP, the length of O–H (B2.14 Å) indicates the
Fig. 2e. The dihedral angle of the building blocks follows a
G0 = 2e2/h) illustrates the formation of Au–Au atomic point formation of hydrogen bonds, which corresponds to the lowest
contact.28 The individual conductance plateaus of target mole- potential energy structure in Fig. S10 (ESI†).30–32 For comparison,
cules are determined to be 10ꢀ4.24ꢁ0.38 G0 (4.46 nS) for BTPP, the dihedral of BTPP mixed with ethanol is 36.721, which is
10ꢀ4.59ꢁ0.38 G0 (1.99 nS) for BTPPethanol, 10ꢀ5.18ꢁ0.42 G0 (0.51 nS) comparable to the degree of the corresponding crystalline char-
for BTPMP and 10ꢀ5.33ꢁ0.33 G0 (0.36 nS) for BTPBP, respectively acterized by X-ray diffraction.33 The displacement (from d 8.60 to
by Gaussian fitting. The conductance results follow an increas- d 8.57, Fig. 2e) in 1H NMR indicates that the hydrogen bonds are
ing sequence of BTPBP o BTPMP o BTPP as the decrease in broken by introducing a hydrogen bond destroying agent, and
the twisting angle, and the conductance of BTPP is superior to therefore the smaller dihedral angle of BTPP is due to the
those of BTPMP and BTPBP owing to the existence of H-bonds confinement of intramolecular hydrogen bonds and bare steric
and lack of steric groups. More importantly, after being treated hindrance. In contrast, the intramolecular hydrogen bonds of
with ethanol, the conductances of BTPMP and BTPBP remain BTPMP and BTPBP are unable to form (see Fig. S2–S4, ESI†) due
unchanged (see Fig. S7, ESI†). While the conductance of the to large steric hindrance, and thus there is no change in
BTPP junction was reduced by B200% without changing the conductance after introducing ethanol. Furthermore, the minor
length, indicating that the high conductivity of BTPP benefits dihedral angle difference between BTPMP and BTPBP indicates
from the existence of intramolecular hydrogen bonds. Further- that the increase of the steric-hindrance of the ring substitution
more, after adding ethanol, the solution was vaporized and weakly promotes the further increase of dihedral angle.
then pure TCB was added again, the conductance of BTPP was
To further investigate the intramolecular coupling, flicker
reversed back, and remarkably, the cycle by changing the noise measurements were performed.34 According to the above-
polarity of the solvent can be achieved repeatedly, showing that mentioned results, the molecular junction elongation was
it is feasible to control the hydrogen bond by solvent to switch paused for 150 ms to extract the conductance signals within
the conductance, as shown in Fig. 1d (see more details in the the period for flicker noise analysis (the typical traces are
ESI†).
shown in Fig. 3a, see the ESI,† for more details).35 Due to the
The two-dimensional (2D) conductance and displacement volatilization of ethanol, the flicker noise of BTPPethanol is
histograms of target molecules are constructed from 3000 inevitably affected. Therefore, we only discuss the situation
stretching traces without data selection. As shown in Fig. 2a–d, with pure TCB solvent. We found that the scale of the noise
the intensity clouds of single-molecule junctions are observed power of BTPP is G1.4 due to the not fully planar conjugated
with a similar distance of B2.3 nm located at B1.8 nm and configuration (Fig. 3b), indicating that the charge transport
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symmetry leads to non-completely destructive quantum inter-
ference, and thus the transmission is obviously inhibited.
To further reveal this phenomenon from the perspective of
molecular orbitals, Molecular Projected Self-consistent Hamilto-
nian (MPSH) analysis using the junction configuration were all
employed to plot the energies and spatial distribution of the
frontier orbitals (more details in Fig. S12, ESI†).18,39 The order of
HOMO–LUMO gap is in accordance with the above conductance
measurement results (BTPPtcb o BTPPethanol o BTPMPtcb
o
BTPBPtcb). That is, a larger gap is generally associated with a low
conductance of molecular junction. As shown in Fig. 3f, the
electronic structures of target molecules tend to be localized as
the dihedral angle increases. The separation of the frontier
molecular orbitals is ascribed to the weak coupling of building
blocks caused by the orthogonality of the p-orbitals onto the two
connected rings, which suppresses the charge transport through
the bond, making the transport through space become dominated
and leading to lower conductance of the molecular junction. The
through-space conjugation between the oxygen atom and the
Fig. 3 (a) The typical hover conductance traces of target molecules. The
noise power versus averaged conductance for (b) BTPP, (c) BTPMP and
(d) BTPBP. (e) DFT calculations of transmission analysis. The calculation
transmission function of BTPPtcb (red), BTPPethanol (green), BTPMPtcb
(yellow) and BTPBPtcb (blue) versus EF (eV). (f) The spatial distribution of
orbital levels of BTPPtcb, BTPPethanol, BTPMPtcb and BTPBPtcb (from up to
down) related to the Fermi energy with an isovalue of 0.05.
through BTPP is primarily dominated by through-bond coupling hydrogen atom does not appear in the frontier orbitals, indicating
and partially through-space coupling. In contrast, for BTPMP, that the intramolecular H-bond does not act as an additional
the noise power of G1.6 (Fig. 3c) indicates the presence conduction channel for this twisting structure but it only limits
of primary through-space coupling and partial through-bond the twisting angles. Our experimental and theoretical results
coupling. While for BTPBP, the noise power scales as G2.0 manifest that the dihedral angle between the molecular building
(Fig. 3d), which indicates that the through-space coupling is
dominant. Notably, the decrease in conductance of these mole-
cular junctions is associated with an increase in the predomi-
blocks can be adjusted by intramolecular interactions, providing
an effective chemical control strategy.
In conclusion, we investigated the charge transport through
nant proportion of through-space coupling, suggesting that the bilactam derivatives with different dihedral angles using the
increase of twisting angle promotes the transformation of charge STM-BJ technique. Our results demonstrate the control of the
transport through the molecular junction from through-bond to intramolecular hydrogen bond, which results in a dihedral
through-space.
change of 8.831, could elaborately tune the charge transport
To further understand the role of the hydrogen bond on through the molecules, and the conductance switching could
the intramolecular coupling, the transmission spectra of the
single–molecule junctions at zero bias were carried out.36,37
be reversibly achieved for cycling. Combined with theoretical
calculations, the localization of the electronic structure originated
The results were obtained in DFT with the non-equilibrium from the twisting structure which hinders charge transport,
Green’s function (NEGF) method by using the Quantum ATK resulting in the transition from through-bond transmission to
software package shown in Fig. 3e (more details in the ESI†). through-space transmission, as revealed by the flicker noise
The intramolecular hydrogen bond in BTPP diminishes the analysis. In comparison, the presence of intramolecular hydrogen
dihedral angle of the centre molecular blocks, which reduced bonds does not provide a conductive channel but brings a change
the twisting of the molecule, thus improving the conduction in the twisting between molecular building blocks. Our findings
through the p system. More interestingly, the absolute value of
the HOMO (highest occupied molecular orbital, ꢀ0.48 eV) and
provide a strategy for the design of reversible molecular switches
to manipulate charge transport through the single-molecule
the LUMO (lowest unoccupied molecular orbital, 1.24 eV) of junction via the control of coupling between different molecular
BTPP is smaller than that of BTPBP (HOMO of ꢀ0.64 eV and building blocks.
LUMO of 1.46 eV). We found that the twisting angles of the
This work was supported by the Natural Science Foundation of
molecule do not change the dominated transmission energy China (No. 21673195, 21722305, 21703188, 21933012, 31871877,
level but only broaden the HOMO–LUMO gap confirmed by the U1705254, and 51403135), the National Key R&D Program of China
UV-Vis spectrum (Fig. S1, ESI†), which is also accompanied by a (2017YFA0204902), Sichuan Science and Technology Program
significant difference in the transmission near the Fermi
energy as shown in Fig. 3e. More importantly, compared with
(Grant No. 2019YJ0123), Fundamental Research Funds for the
Central Universities (No. 20720200068), and State Key Laboratory
the independently distributed energy levels of the planar of Polymer Materials Engineering (Grant No. sklpme 2014-3-16).
configuration, the HOMO and HOMOꢀ1 (or LUMO and
LUMO+1) energy levels are quasi-degenerate in the orthogonal
configuration.38 Since the energy levels of target molecules tend
to be quasi-degenerate as the increase in twisting angle, the
Conflicts of interest
superposition of two degenerate orbitals with an opposite There are no conflicts to declare.
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