Chemically controlled conductivity: torsion-angle dependence in a single-molecule biphenyldithiol junction

Article abstract of DOI:10.1002/anie.200903946

Stepwise regulation of the molecular conductance was observed in a series of eight oiphenyldlithiols with fixed torsion angles between the phenyl rings. These compounds were synthesized and their single molecule conductance was investigated in an STM juct

Full text of DOI:10.1002/anie.200903946

Communications
DOI: 10.1002/anie.200903946
Single-Molecule Studies
Chemically Controlled Conductivity: Torsion-Angle Dependence in a
Single-Molecule Biphenyldithiol Junction**
David Vonlanthen, Artem Mishchenko, Mark Elbing, Markus Neuburger, Thomas Wandlowski,*
and Marcel Mayor*
Dedication to Professor Hans Kuhn on the occasion of his 90th birthday
Electron transport through molecules has received great
attention since organic structures have been considered as the
active elements in electronic nanoscale devices.[1–5] In various
experimental setups it is possible to integrate single molecules
in electronic circuits and to measure their conductance.
Examples include mechanically controlled break junctions
(MCBJ)[6,7] and break junctions based on scanning tunneling
microscopy (STM), [8,9] which enabled basic investigations of
correlations between molecular structures and transport
properties on a single-molecule level.[10–14] However, these
studies depend on several parameters: 1) the formation of a
reproducible and stable contact between the molecule and
both metal electrodes, 2) the structure and the conformation
of the bridging molecule, and 3) the algorithms used for the
subsequent processing of the obtained raw data. Since
parameters (1) and (2) can be controlled by synthetic
chemistry to a large extent, synthesis currently plays a
major role in the study of structure–transport correlations
and the design of functional molecules as active components
in electronic circuits.
p systems can be either in the same plane or perpendicular to
each other representing the “on” and “off” states of a
molecular switch, respectively.[18–20] In a recent study, Ven-
kataraman[10] and co-workers reported the interdependence
between the calculated molecular conformation and the
single-molecule conductance of a series of various substituted
biphenyls having terminal amino groups.[10] Despite several
varying parameters such as electron density on the phenyl
rings and steric repulsion of the substituents, a linear
correlation was found between the conductance and the
square of the cosine of the calculated torsion angle between
the planes of the two rings. While these transiently immobi-
lized diamines contribute to understanding the structure–
property relationship, sulfur-functionalized structures form
considerably more stable single-molecule junctions between
gold electrodes.[21] Furthermore, theoretical studies on sulfur-
functionalized biphenyls have gained considerable atten-
tion[19,20] and even molecular junctions comprising biphenyl-
dithiols (BPDT) have been reported.[18] Despite the strong
interest in the correlation between the torsion angle and
transport properties, suitable model compounds enabling the
systematic variation of the torsion angle in biphenyl systems
have not been realized so far. Here we report our new
approach towards biphenyl systems with controlled torsion
angles.
Biphenyl derivatives, which consist of two aromatic rings
ꢀ
connected by a single C C bond, have attracted considerable
interest as model compounds because of their size and ready
availability.[15–17] Furthermore this type of unit has been
considered as a potential conductance switch as the two
As displayed in Figure 1, an alkyl chain of varying length
connected at the 2,2’-positions of the biphenyl system in
compounds 1–5 can be used to adjust the torsion angle F. In
this series of compounds the length of this alkyl chain is the
[*] A. Mishchenko, Prof. Dr. T. Wandlowski
Departement fꢀr Chemie und Biochemie
University Bern
Freiestrasse 3, 3012 Bern (Switzerland)
Fax: (+41)31-631-53-84
E-mail: thomas.wandlowski@dcb.unibe.ch
D. Vonlanthen, Dr. M. Neuburger, Prof. Dr. M. Mayor
Departement fꢀr Chemie
University Basel
St.-Johanns-Ring 19, 4056 Basel (Switzerland)
Fax: (+41)61-267-10-16
E-mail: marcel.mayor@unibas.ch
Dr. M. Elbing, Prof. Dr. M. Mayor
Institute fꢀr Nanotechnologie
Forschungszentrum Karlsruhe GmbH
Postfach 3640, 76021 Karlsruhe (Germany)
Dr. M. Elbing
Elastogran GmbH, Global PU Specialties Research
Elastogranstrasse 60, 49448 Lemfꢁrde (Germany)
[**] This work was supported by the Swiss National Science Foundation,
DFG priority program 1243, the Volkswagen Foundation, FUN-
MOLS, and the Swiss National Center of Competence in Research
“Nanoscale Science”.
Figure 1. A) The torsion angle F formed by the planes of the phenyl
rings is adjusted by the length of the bridging chain. B) Compounds
1–8.
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Chemie
only structural variation; the electronic structure of the
biphenyl systems in this family of compounds is kept as
uniform as possible. We note that considerable effects of the
electron-donating character of substituents on the transport
properties of immobilized single molecules have been
reported.[22] Furthermore, the biphenyl conformation is
locked by the intramolecular bridge; the number of CH2
units dictates the torsion angle F and decreases the expected
motion and conformational variation of each molecule
immobilized in the junction considerably. The individual
compounds of this series form stable molecular junctions
through their terminal sulfur anchor groups. Finally, single
crystals of these compounds can be grown and X-ray structure
analyses can be used to determine the torsion angle F.
To follow this strategy we focused on the biphenyls 1–5
with n = 1–5 in which the length of the alkyl bridge and thus
also the torsion angle F increases. In addition, this series of
terminally acetylsulfanyl-functionalized biphenyl systems was
complemented by compounds 6–8, in which the torsion angle
was expected to depend on the substitution pattern.
To obtain a compound with a bridging alkyl chain
containing three carbon atoms, an additional carbon atom
was introduced by the intramolecular cyclization of 11 with
the masked formaldehyde equivalent TosMIC.[24] The result-
ing ketone 12 was obtained in 77% yield. After the Lewis acid
catalyzed reduction of the keto group,[25] the substitution of
both chlorines with methylthiolates followed. The resulting
methylsulfanyl derivative was converted in situ into the
acetylsulfanyl-functionalized target structure 3.[23,26]
Again starting from 11, a copper-mediated Grignard
addition gave the diallylic biphenyl 13. Although the for-
mation of eight-membered rings by ring-closing metathesis
(RCM) was reported to be difficult,[27,28] the cyclization of 13
to give 14 proceeded smoothly, probably because the allyl
chains in 13 are conformationally predisposed. Subsequent
hydrogenation and reaction sequence similar to that de-
scribed for 3 resulted in the replacement of the chlorines by
acetylsulfanyl groups to provide the butylene-bridged BPDT
derivative 4.
The cyclononane structure in 5 was assembled by an
alternative strategy. As shown in Scheme 2, the inter-ring
pentyl chain was established prior to the formation of the
The synthesis of the biphenyldithiols (BPDTs) 1, 2, 6, and
7 has already been published,[23] and the synthetic route to the
tricyclic BPDTs 3 and 4 is displayed in Scheme 1. The key
building block 11 was obtained in three steps. An oxidative
coupling provided the doubly chlorine and carboxylic acid
functionalized biphenyl 9. Its reduction gave diol 10, which
was subsequently transformed to the benzylic dibromide 11 in
a yield of 41% over the three steps.
Scheme 2. Reagents and conditions: a) Acetone, NaOH, EtOH. b) H2,
Pd/C (10%), EtOAc, 1 atm, 52% (over 2 steps).[29] c) Hydrazine
(85%), KOH, triethylene glycol, 190–2008C, 72%.[30] d) Br2, pyridine,
ꢀ108C to RT, 42% (after recrystallization).[31] e) tBuLi, CuCN, LiBr,
methyl-THF, ꢀ608C, then 1,3-dinitrobenzene, 23% for 19. f) BBr3,
CH2Cl2, RT. g) Tf2O, pyridine, 48C to RT. h) tBuSNa, [Pd2(dba)3],
xantphos, p-xylene, 1408C, 52% (over 3 steps). i) BBr3, AcCl, toluene,
61%. dba=dibenzylideneacetone, Tf2O=trifluoromethanesulfonic
anhydride, xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxan-
thene.
biphenyl core. The terminal methoxyphenyl-functionalized
pentane derivative 16 was obtained following an established
protocol.[29,30] Subsequent bromination[31] gave the building
block 17, which was purified by several recrystallizations.
Implementation of the Lipshutz methodology[32] gave the
unwanted dimer 18 as the main product (27%) together with
almost comparable amounts (23%) of the desired pentylene-
bridged biphenyl system 19. In subsequent functional group
transformations the terminal methoxy groups in 19 were
replaced with triflate groups (!21). The tert-butyl-protected
Scheme 1. Reagents and conditions: a) NaNO2, HCl, 08C, then CuSO4,
HO-NH2, NH4OH, H2O, 0 to 708C, 84%; b) NaBH4, BF3·Et2O, THF;
c) PBr3, CH2Cl2, 08C, 49% (over 2 steps); d) TosMIC, NaOH, TBAB,
CH2Cl2/H2O; then HCl, tBME/H2O, 77%. e) PMHS, (C6F5)3B, CH2Cl2,
RT, 61%. f) NaSCH3, DMI; then AcCl, 1108C, 49%. g) CH2CHMgBr,
CuI, CH2Cl2, ꢀ408C to RT, 58%. h) Grubbs’ first generation catalyst,
CH2Cl2, reflux, 88%. i) H2, Pd/C 10%, RT, EtOAc, 95%. j) NaSCH3,
DMI; then AcCl, 1108C, 32%. DMI=1,3-dimethyl-2-imidazolidinone,
PMHS=poly(methylhydrosiloxane), TBAB=tetrabutylammonium bro-
mide, tBME=tert-butylmethyl ether, TosMIC=p-toluenesulfonylmethyl
isocyanide.
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terminal sulfur groups in 22 were introduced in good yields by
a palladium-catalyzed cross-coupling reaction. A final adjust-
ment of the protecting groups[33,34] gave the target structure 5,
as acetyl-protected pentylene-bridged BPDT.
All new compounds and intermediates were fully charac-
terized by 1H and 13C NMR spectroscopy, mass spectrometry,
and elemental analysis. A comprehensive description of the
syntheses of the target structures will be published else-
where.[35]
Of particular interest was the X-ray crystal structure
analysis of the new BPDT structures 1–8 not only to confirm
the identity of the compounds, but also to investigate the
correlation between the length of the bridging alkyl chain and
the resulting torsion angle F. While attempts to crystallize the
fluorene derivative 1 and the unsubstituted BPDT compound
6 failed, we were able to obtain single crystals of the bridged
BPDTs 2–4 from hot cyclohexane and, in the case of 5, from
hot pentane, after storing at 48C.[36] In addition, the solid-state
structure of the dimethylated BPDT 7 has already been
reported, and suitable single crystals from the tetramethyl-
substituted BPDT 8 were also obtained from hot cyclohex-
ane.[35]
The solid-state structures of the BPDTs 2–5 in order of the
increasing length of the bridging alkyl group are shown in
Figure 2. The intramolecular sulfur–sulfur distances and the
torsion angles F are listed in Table 1. Within the series of
compounds the sulfur–sulfur distances are very comparable;
the values between 1.059 nm for 5 and 1.062 nm for 7 indicate
that the alkyl chain length has hardly any effect on the BPDT
backbone. Apparently, steric repulsion arising from the
various alkyl substituents is compensated by adjustment of
Figure 2. X-ray structures of the bridged BPDTs 2, 3, 4, and 5 (from
top to bottom).
Table 1: Molecular structures and measured properties.
Structure[a]
F [8][b]
d [nm][c]
G/G0
1
2
1.1[38]
–
1.45ꢁ0.1ꢂ10ꢀ4
the inter-ring torsion angle F. This torsion angle was
measured between the planes of the two phenyl rings of the
biphenyl system which were obtained by considering all six
carbon atom positions of the solid-state structure for the least
squares minimization procedure. The obtained torsion angles
F (Table 1) increase continuously with the increasing length
of the alkyl bridge. While the elongation by one CH2 unit on
going from fluorene 1 to dihydrophenanthrene 2 increases the
torsion angle by 15.78, the largest increase in the torsion angle,
almost 288, is observed on going from dihydrophenanthrene 2
to the C3-bridged BPDT system 3. Further extension to the
C4- and C5-bridged derivatives 4 and 5 result in additional
expansions of the angle F by 13.18 and 13.78, respectively.
An almost perpendicular arrangement of the two phenyl
rings was found in the solid-state structures of the dimethyl
and tetramethyl derivatives 7 and 8 (F = 79.78 and 898,
respectively). The rotational barriers of the methyl-substi-
tuted compounds 7 and 8, and in particular of the alkyl-
bridged derivatives 2–5, are expected to be considerably
higher than that of the unsubstituted BPDT 6 (2-3 kcal
molꢀ1).[37] Thus, in spite of the increased structural flexibility
of these compounds in solution, the F values obtained by
16.8
44.7
1.061(2)
1.060(9)
2.19ꢁ0.2ꢂ10ꢀ4
1.30ꢁ0.2ꢂ10ꢀ4
3
4
57.8
71.5
1.060(5)
1.059(4)
6.97ꢁ1.7ꢂ10ꢀ5
1.68ꢁ0.3ꢂ10ꢀ5
5
6
7
36.4[19]
79.7[23]
–
1.72ꢁ0.2ꢂ10ꢀ4
1.29ꢁ0.2ꢂ10ꢀ5
1.06(2)[23]
8
89.0
1.061(2)
9.03ꢁ1.7ꢂ10ꢀ6
[a] Synthesized with R=Ac; immobilized with R=Au electrodes. [b] F is
the torsion angle between the planes of the phenyl rings. [c] d is the
distance between the sulfur atoms.
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solid-state structure analysis remain the best approximation
of the values in solution.
conductance values observed for 1 are considerably lower
than expected. Theoretical calculations did not support an
additional stabilization of the HOMO resulting from the
extended delocalization of the fluorene p system.[41] There-
fore, only the single bridging methylene unit in 1 and the
resulting bend of its biphenyl axis remain as distinctive
structural features which may cause the observed decrease in
conductance.
Of particular interest is the conductance through these
BPDT structures, which vary mainly in the inter-ring torsion
angle F. The BPDT structures 1–8 were investigated between
the gold tip and the gold substrate at the solid–liquid interface
of a scanning tunneling microscope (STM) break junction
setup[9] protected by an argon gas atmosphere. A 250 mm
solution of the BPDT under investigation in mesitylene/
tetrahydrofuran (4:1) was treated with a 125 mm solution of
tetrabutylammonium hydroxide to remove the acetyl protec-
tion groups. The resulting free thiols form covalent gold–
sulfur bonds thus immobilizing the rod-shaped molecule
inside the junction. To distinguish the molecular junctions
from the synthesized molecules 1–8, they will be denoted as
1’–8’. Three different voltages (65, 100, and 180 mV) were
applied between tip and substrate while the junction was
repeatedly opened and closed to establish transient single-
molecule contacts. Several thousand junctions were created,
and the current was recorded as a function of tip distance.
Only current traces displaying typical single-molecule current
plateaus were selected by an automated algorithm and
considered (20 ꢁ 10%) for the conductance analysis. The
extracted conductance histograms revealed characteristic
peaks corresponding to the conductance of the single BPDT
junction. A comprehensive technical description of the single-
molecule investigation and the analysis algorithm will be
published elsewhere.[39]
In conclusion, we have described the synthesis and the
structural analysis of a family of BPDTs in which the torsion
angle F is fixed by a bridging alkyl chain. The series is
complemented by three derivatives with various numbers of
methyl substituents in the 2,2’,6,6’-positions to vary F by
steric repulsion. Investigation of the single-molecule conduc-
tance of the series displays a linear correlation with cos2 F of
the inter-ring torsion angle for BPDTs with a divided
p system. A similar trend in increased conductance values
has been reported for various substituted amino-terminated
biphenyls.[10] In contrast to these findings, the planar fluorene
derivative 1 displays reduced conductivity in our case,
pointing at additional, equally important parameters govern-
ing the transport efficiency besides the planarity of the
p system. During the preparation of this manuscript, Haiss
and co-workers reported transport investigations through
several BPDTs.[18] While the reported current values are
surprisingly low, a clear relationship between conductance
and conformation relationship was not found.
The biphenyl unit 7 has already been used as p-system-
dividing subunit in a single-molecule rectifier.[42] Currently we
are integrating the series of biphenyl building blocks with
fixed torsion angles into model compounds to study the
relationship between the structure and nonlinear optical
properties.[43] Furthermore, we are working on potential
switching systems based on the changes of conductivity with
changes in torsion angle.
According to theory, the orbital overlap of adjacent
p systems correlates linearly with cosF and the electron
transmission is proportional to cos2 F, where F is the torsion
angle.[40] Figure 3 displays the plot of the junction conduc-
Received: July 17, 2009
Published online: October 21, 2009
Keywords: biphenyldithiols · conductance · metathesis ·
.
molecular electronics · structure–property relationships
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Angew. Chem. Int. Ed. 2009, 48, 8886 –8890
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Communications
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[39] a) A. Mishchenko, D. Vonlanthen, V. Meded, M. Bꢄrkle, C. Li, I.
Pobelov, A. Bagrets, J. K. Viljas, F. Pauly, F. Evers, M. Mayor, T.
Wandlowski, unpublished results; b) A. Mishchenko, C. Li, I.
Pobelov, T. Wandlowski, unpublished results..
[40] S. Woitellier, J. P. Launay, C. Joachim, Chem. Phys. 1989, 131,
481.
[41] Calculations were performed using Hyperchem Version 7.5. The
HOMO and LUMO energies were calculated by the semi-
empirical methods AM1, PM3, and Extended Hꢄckel in the gas
phase using conformational data from the X-ray structures.
[42] M. Elbing, R. Ochs, M. Koentopp, M. Fischer, C. von Hꢁnisch, F.
Weigend, F. Evers, H. B. Weber, M. Mayor, Proc. Natl. Acad. Sci.
USA 2005, 102, 8815.
[36] The single crystals were measured on a Nonius KappaCCD
diffractometer at 173 K using graphite-monochromated
MoKa radiation with l = 0.71073 ꢂ. Crystal data for 2:
C18H16O2S2, Mr = 328.46, F(000) = 172, colorless plate, 0.13 ꢃ
0.17 ꢃ 0.45 mm3, triclinic, P1, Z = 1, a = 5.35720(10) ꢂ, b =
ꢀ
7.7276(2) ꢂ,
c = 10.5083(3) ꢂ,
a = 105.7574(15)8,
b =
=
101.6766(14)8, g = 101.2325(12)8, V= 395.356(18) ꢂ3, 1calcd
[43] J. Rotzler, D. Vonlanthen, M. Mayor, A. Barsella, A. Boeglin, A.
Fort, unpublished results.
1.379 gcmꢀ3, Vmax = 27.8918. Min./max. transmission 0.94/0.96,
m = 0.340 mmꢀ1. From a total of 3582 reflections, 1880 were
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Angew. Chem. Int. Ed. 2009, 48, 8886 –8890