Novel cruciform structures as model compounds for coordination induced single molecule switches

Article abstract of DOI:10.2533/chimia.2010.140

We have synthesized various molecular cruciforms consisting of two different crossing π-systems and comprising crosswise arranged thiol- and pyridine-anchor groups. With these model compounds we strive towards the investigation of a new switching concept based on the potential dependent coordination of pyridines to gold electrodes in an electrochemical set-up. Integration of these cruciform molecules between both electrodes of a mechanically controlled break junction in a liquid environment gave insight into their single molecule transport properties. These studies allowed individual transport characteristics to be assigned to the bar subunits of the cruciforms but also revealed the remaining experimental challenges to realize the suggested switching concept. Schweizerische Chemische Gesellschaft.

Full text of DOI:10.2533/chimia.2010.140

140ꢀ CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ
doi:10.2533/chimia.2010.140ꢀꢀ
Laureates: awards and Honors, sCs FaLL Meeting 2009
Chimiaꢀ64ꢀ(2010)ꢀ140–144ꢀ ©ꢀSchweizerischeꢀChemischeꢀGesellschaft
Novel Cruciform Structures as Model
Compounds for Coordination Induced
Single Molecule Switches
SergioꢀGrunder§a,ꢀRomanꢀHuberb,ꢀSongmeiꢀWub,ꢀChristianꢀSchönenbergerb,ꢀMichelꢀCalameb,ꢀ
andꢀMarcelꢀMayor*ac
§SCSꢀMettler-ToledoꢀAwardꢀWinnerꢀ(OralꢀPresentation)
Abstract:ꢀWeꢀhaveꢀsynthesizedꢀvariousꢀmolecularꢀcruciformsꢀconsistingꢀofꢀtwoꢀdifferentꢀcrossingꢀπ-systemsꢀandꢀ
comprisingꢀcrosswiseꢀarrangedꢀthiol-ꢀandꢀpyridine-anchorꢀgroups.ꢀWithꢀtheseꢀmodelꢀcompoundsꢀweꢀstriveꢀto-
wardsꢀtheꢀinvestigationꢀofꢀaꢀnewꢀswitchingꢀconceptꢀbasedꢀonꢀtheꢀpotentialꢀdependentꢀcoordinationꢀofꢀpyridinesꢀtoꢀ
goldꢀelectrodesꢀinꢀanꢀelectrochemicalꢀset-up.ꢀIntegrationꢀofꢀtheseꢀcruciformꢀmoleculesꢀbetweenꢀbothꢀelectrodesꢀ
ofꢀaꢀmechanicallyꢀcontrolledꢀbreakꢀjunctionꢀinꢀaꢀliquidꢀenvironmentꢀgaveꢀinsightꢀintoꢀtheirꢀsingleꢀmoleculeꢀtransportꢀ
properties.ꢀTheseꢀstudiesꢀallowedꢀindividualꢀtransportꢀcharacteristicsꢀtoꢀbeꢀassignedꢀtoꢀtheꢀbarꢀsubunitsꢀofꢀtheꢀ
cruciformsꢀbutꢀalsoꢀrevealedꢀtheꢀremainingꢀexperimentalꢀchallengesꢀtoꢀrealizeꢀtheꢀsuggestedꢀswitchingꢀconcept.
Keywords:ꢀMolecularꢀelectronicsꢀ·ꢀMolecularꢀswitchesꢀ·ꢀSingleꢀmoleculeꢀconductance
Introduction
was tackled by teams of synthetic chemists trodes. While one bar comprises termi-
and experimental physicists. However, the nal thiophenols which are known to bind
The ongoing feature size reduction in minute size of molecules restricts the num- strongly to gold electrodes over a large
silicon-based electronic circuitry is fac- ber of electrodes to two and consequently, potential range, the second bar comprises
ing physical and economical challenges, applying the successful terminology of terminal pyridines for which potential de-
resulting in an increasing interest for alter- semiconductor electronics, also the vari- pendent coordination to gold substrates
native strategies. An individual molecule ety of electronic functions realizable in the has been found in electrochemical STM
consisting of a few atoms is probably the single molecule device is restricted. Nev- experiments.[11–14] As displayed in Fig. 1,
smallest possible object that still provides ertheless, monomolecular films and even we therefore suggested to control the co-
the structural variety required to realize dif- single molecules have been addressed with ordination of the pyridine-terminated bar
ferent electronic functions. The integration electrodes, and the electronic response of to the electrode by an electrochemical po-
of single molecules into electronic devices an applied voltage can be measured.[6] tential while the sulfur-terminated bar is
– today referred to as ‘molecular electron- Particularly appealing electronic functions keeping the cruciform positioned between
ics’ – can be considered as the ultimate basedonatwoterminaldevicearerectifica- both electrodes. In case both bars differ in
size reduction in electronic circuitry.[1–5] tion and switching.[4] To switch a molecule their electronic transparencies, the number
Rather driven by scientific curiosity than between two metastable states an external of bars bridging both electrodes should be
by the prospect of smaller and cheaper trigger is required. Various switching mo- reflected in the conductance of the junc-
devices the interdisciplinary challenge of lecular systems responding to for example tion, providing an electrochemically trig-
electronically contacting single molecules light, a chemical or electrochemical poten- gered switch.
tial or a voltage pulse have already been
In this paper we summarize our studies
reported.[7] Interestingly, several switching towards coordination induced single mol-
devices were found empirically and their ecule switching. In particular the structural
switching mechanisms remain the subject design and the synthesis of novel molecu-
of current debate.[8]
lar cruciforms are presented together with
During the last three years we followed single molecule transport investigations in
a systematic approach. We proposed a new a mechanically controlled break junction
switching concept based on the interplay (MCBJ) in a liquid environment.[9,10]
between the integrated molecule and the
*Correspondence: Prof. Dr. M. Mayora
Tel.: +41 61 267 1006
Fax: +41 61 267 1016
E-mail: marcel.mayor@unibas.ch
aDepartment of Chemistry
University of Basel
electrodes, designed and synthesized the
required molecules and investigated their Molecular Design
transport properties.[9,10]
St. Johanns-Ring 19
CH-4056 Basel
The hypothesized switching concept
To investigate the new switching mech-
bDepartment of Physics
University of Basel
relies on a structure consisting of two anism, molecular cruciforms consisting
rigid-rod type π-systems which cross each of thiol and pyridine functionalized bars
other resulting in a molecular cruciform. were required. Considerable knowledge
Both π-systems as bars of the cruciform concerning handling, synthesis and elec-
differ in their electronic transparency and tronic transport investigation of terminally
in their terminal anchor groups to the elec- thiol functionalized rods had already been
Klingelbergstrasse 82
CH-4056 Basel
cInstitute of Nanotechnology
Karlsruhe Institute of Technology
P. O. Box 3640
D-76021 Karlsruhe

Laureates: awards and Honors, sCs FaLL Meeting 2009ꢀ
CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ 141
Fig.ꢀ1.ꢀSketchꢀofꢀtheꢀ
proposedꢀswitchingꢀ
mechanismꢀbasedꢀonꢀ
theꢀelectrochemicalꢀ
potentialꢀdependentꢀ
coordinationꢀofꢀtheꢀ
terminalꢀpyridineꢀ
the sulfur anchor groups were initially ar-
ranged in meta-positions. From earlier in-
vestigations it was known that the subtle
structural variation of moving both anchor
groups from the para-position to the meta-
position reduces the electronic conductivi-
ty by more than one order of magnitude.[16]
Unfortunately, the electronic transparency
of the meta-thiol functionalized OPV rod
turned out to be outside the detectable
range of the MCBJ set-up in a liquid en-
vironment. To enable at least initial in-
vestigations whether these rather bulky
cruciforms can be integrated between both
electrodes of a MCBJ, we also synthesized
the corresponding cruciform 2 with sulfur
anchor groups in para-position. Obvious-
ly, the resulting increased conductivity of
the fixation bar contrasts to some extent
the switching principle, which requires a
considerably better conducting pyridine-
functionalized bar. The conductivity of the
sulfur-bearing fixation bar needs to be low
to suppress electron transport as it repre-
sents the OFF-state of the single molecule
switch, but it has to be above the detection
limit of the integration setup to observe the
formation of the molecular junction. The
superior conductivity of a thiol-function-
alized rod compared with a pyridine func-
tionalized analogue[10] and the require-
ment of having the thiol groups in para-
position[9] increased the molecular design
challenges considerably. In order to tune
the conductance values of the two rod sub-
structures towards each other the position
of the pyridine anchor group was changed
to the OPV bar, as OPVs were found
to be better conducting than OPEs.[22]
To keep an appropriate length ratio of the
two bar substructures, the OPE system
was substituted by an oligophenylene in
the cruciform 3. Oligophenylene rods
with substituents in ortho-positions to the
bridging carbon atoms displayed reduced
conductivities due to the orthogonal ar-
rangement of the π-systems of both phe-
nyl rings.[17] Furthermore, to compensate
the shortness of the oligophenylene bar of
3, naphthyl subunits were considered for
the oligoaryl bar of 4.[10]
subunitsꢀtoꢀtheꢀ
electrodes.ꢀInꢀtheꢀOFF-
stateꢀbothꢀelectrodesꢀ
areꢀonlyꢀbridgedꢀbyꢀ
theꢀthiolꢀfunctionalizedꢀ
barꢀwhichꢀfixesꢀtheꢀ
cruciformꢀinsideꢀtheꢀ
junction.ꢀInꢀtheꢀON-
state,ꢀtheꢀpyridine-
functionalizedꢀbarꢀ
joinsꢀinꢀprovidingꢀanꢀ
additionalꢀelectronicꢀ
transportꢀchannel.
Fig.ꢀ2.ꢀCruciformꢀ
structuresꢀ1ꢀandꢀ2ꢀofꢀ
theꢀfirstꢀgenerationꢀandꢀ
3ꢀandꢀ4ꢀofꢀtheꢀsecondꢀ
generation.
O
N
N
O
S
S
S
S
S
O
N
N
O
1
2
O
N
S
N
O
O
N
S
N
O
S
3
4
Synthesis
The target cruciforms 1–4 were synthe-
sized with acetyl-protected sulfur groups.
The masking of both thiophenols prevents
these bifunctional building blocks from
oxidative polymerization by disulfide for-
mation and the acetyl group turned out to
be easy cleavable in situ prior to the physi-
cal experiments. The synthetic strategy to-
wards 1 and 2 relies on a Wittig reaction to
assemble the OPV unit and a Sonogashira
reaction to assemble the OPE bar. Using
iodine leaving groups the palladium- and
collected within the group.[15–19] Due to the pling via a terminal thiophenol compared
ideal strength of the gold–sulfur bond these to a terminal pyridine subunit, in spite of
rods remained moveable on the electrode the shorter electrode–electrode distance in
surface and displayed rather poor conduc- the later case.[10]
tivities, in part due to the tunnel barriers
The first generation of cruciforms
formed with the gold–sulfur bonds.[20,21] consisted of a sulfur-functionalized oli-
Initially we thus expected an improved gophenylenevinylene (OPV) fixation bar
electronic coupling via pyridine subunits. and a transversal pyridine-functionalized
However, transport studies based on model oligophenyleneethynylene (OPE) bar (Fig.
compounds synthesized within this project 2). To disfavor the electronic transport
revealed the electronically superior cou- through the fixation bar of the cruciform 1,

142ꢀ CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ
Laureates: awards and Honors, sCs FaLL Meeting 2009
boronic acid the phenolic hydroxy group
allows the introduction of a sulfur sub-
PPh3Br
I
Br
S
O
SR
I
I
a
b
c
stituent via a Newman-Kwart rearrange-
ment (NKR).[23] Thus 13 was treated with
dimethylcarbamoylchloride to provide the
O-thiocarbamate 14. The thermally acti-
vated NKR was triggered by heating 14 to
259 °C to give the S-thiocarbamate 15 in
good yields. Hydrolysis of the thiocarba-
mate 15 and subsequent protection of the
phenolic thiol in a radical addition reaction
with vinyl-trimethylsilane led to the ethyl-
TMS protected building block. After treat-
ment of the obtained bromide with n-BuLi
and triisopropylborate the boronic acid 16
was obtained after aqueous workup. In an
Arbusov reaction the bromides of the cen-
tral building block 7 were substituted with
triethylphosphite to give the phosphonate
18. In a HWE reaction 18 and 4-pyridine-
carboxaldehyde provided exclusively the
desired E/E-isomer 19 as first bar of the
cruciform 4 comprising two iodine leaving
groups. The cruciform was assembled in
a Suzuki reaction of 19 with the boronic
acid 16. Finally, the ethyl-TMS protected
terminal sulfur anchor groups were liber-
ated with fluoride ions and de novo pro-
tected in situ by treatment with an excess
acetyl chloride. Thereby also the terminal
pyridine groups were acetylated. However,
the acetylated pyridines were hydrolyzed
selectively in an ethanol chloroform mix-
ture providing the target structure 4 in 58%
yield as a yellow solid. A comprehensive
discussion of the synthesis together with
analytical data of all intermediates and tar-
get structures are reported in ref. [10].
N
I
R = tBu: 11
R = Ac: 12
BrPh3P
Br
h
I
i
5
6
7
g
I
O
O
O
O
N
d
e
f
O
S
RS
S
N
S
F
1
8
9
10
S
O
TMS
S
OH
O
N
S
j
k
l, m, n
RS
N
B(OH)2
16
Br
Br
14
Br
15
q
13
N
O
P
OEt
OEt
Br
I
I
I
o
p
N
SR
I
I
R = ethylTMS: 20
R = Ac:
I
EtO
EtO
Br
P
O
r
4
N
7
18
19
Schemeꢀ1:ꢀa)ꢀI2,ꢀHIO3,ꢀAcOH,ꢀH2SO4,ꢀ85ꢀ°C,ꢀ79%;ꢀb)ꢀBr2,ꢀBrCH2CH2Br,ꢀ135ꢀ°C,ꢀ20%,ꢀc)ꢀPPh3,ꢀDMF,ꢀ
85ꢀ°C,ꢀ100%;ꢀd)ꢀ1,2-propanediol,ꢀPTSA,ꢀtoluene,ꢀ98ꢀ°C,ꢀ84%;ꢀe)ꢀNaStBu,ꢀDMI,ꢀ150ꢀ°C,ꢀ66%;ꢀf)ꢀ
HCl,ꢀdioxane,ꢀr.t.,ꢀ92%;ꢀg)ꢀ1.)ꢀNaOH,ꢀDCM,ꢀr.t.,ꢀ2.)ꢀI2,ꢀtoluene,ꢀ111ꢀ°C,ꢀ83%;ꢀh)ꢀBBr3,ꢀAcCl,ꢀtoluene,ꢀ
r.t.,ꢀ87%;ꢀi)ꢀ4-ethynylpyridine,ꢀPd(PPh3)4,ꢀCuI,ꢀiPr2NH,ꢀ45ꢀ°C,ꢀ51%;ꢀj)ꢀdimethylcarbamoylchloride,ꢀ
NaH,ꢀDMF,ꢀ80ꢀ°C,ꢀquant.;ꢀk)ꢀphenylether,ꢀ259ꢀ°C,ꢀ89%;ꢀl)ꢀKOH,ꢀMeOH,ꢀ80ꢀ°C,ꢀ97%;ꢀm)ꢀvinylꢀ
trimethylsilane,ꢀAIBN,ꢀ100ꢀ°C,ꢀ96%;ꢀn)ꢀn-BuLi,ꢀB(OiPr)3,ꢀTHF,ꢀ–78ꢀ°Cꢀꢀr.t.,ꢀ76%;ꢀo)ꢀP(OEt)3,ꢀ150ꢀ
°C,ꢀ91%;ꢀp)ꢀ4-pyridineꢀcarboxaldehyde,ꢀNaH,ꢀTHF,ꢀ50ꢀ°C,ꢀ67%;ꢀq)ꢀPd(PPh3)4,ꢀK2CO3,ꢀtoluene,ꢀ
ethanol,ꢀ80ꢀ°C,ꢀ85%;ꢀr)ꢀTBAF,ꢀAcCl,ꢀTHF,ꢀr.t.,ꢀ58%.
copper-catalyzed Sonogashira coupling orine atom was substituted by tert-butyl-
reaction proceeds at moderate tempera- thiolate. The aldehyde 10 was obtained by
tures (45 °C) allowing the presence of the cleaving the acetal under acidic conditions.
acetyl-protected thiophenols, which were The aldehyde 10 and the bis-phosphonium
thus introduced in advance. The OPV sub- salt 7 formed together the desired ethenyl-
structure in target structure 3 and 4 were bridged aryl structure as a mixture of the
assembled using a Horner-Wadsworth- E/Z-isomers by treatment with aqueous
Emmons (HWE) reaction while the oligo- sodium hydroxide. A subsequent addition/
aryl bar was obtained by a Suzuki reaction. elimination sequence with iodine in reflux-
As the acetyl protection groups would not ing toluene provided the E/E-isomer 13 in
survive the basic conditions of the Suzuki 83% yield. Transprotection of the termi-
reaction, they were introduced in the last nal sulfur anchor groups was achieved by
step. Ethyl-trimethylsilane (ethyl-TMS) treatment with boron tribromide followed
protection groups were used to mask the by acetyl chloride to yield in the acetyl-
reactivity of the thiophenols during the as- protected building block 12. To assemble
sembly of the cruciforms instead. Due to the target structure 1, both iodines of 12
the limited space and the modular and re- were substituted by 4-ethynylpyridine in a
petitive nature of their assembly, only the Sonogashira reaction. Purification by pre-
synthesis of the cruciforms 1 and 4 as rep- cipitation and recrystallization provided
resentative examples will be summarized the cruciform 1 as a yellow solid in 51%
Single Molecule Transport Studies
The electronic transport properties
of the novel cruciform structures 1–4
were investigated on a single molecule
level in a MCBJ setup in a liquid environ-
ment.[9,10,24] The MCBJ relies on a micro-
fabricatedgoldstructure(BinFig. 3)which
is bent in a three-point bending mechanism
(A in Fig. 3) such that, at one point, the
gold structure breaks apart to form two
atomically sharp electrodes. While adjust-
ing the pushing rod distance z, the distance
between the two electrodes can be adjusted
with a resolution better than an Ångstrom,
making the MCBJ a well-suited tool to
investigate single molecules. Details con-
cerning the experimental setup can be
found in refs [9, 10, 22, 24]. To get infor-
mation about the junction, the conductance
between the two electrodes was examined
as a function of the pushing rod distance z
resulting in typical conductance traces (C
in Fig. 3). After breaking the last gold con-
tact, an abrupt down-jump followed by an
exponential decay curve is observed when
only solvent molecules are present. How-
in the following (Scheme 1).
yield. A similar sequence of reaction steps
Starting with para-xylene (5), aromatic provided the cruciform 2. Synthetic pro-
iodination followed by bromination of the tocols together with the analytical data of
methyl groups provided the suitably func- all intermediates and target structures are
tionalized compound 6 as precursor of the reported in ref. [9].
central phenyl ring. Substitution of the ben-
The bifunctional naphthalene build-
zylic bromides with triphenylphosphine ing block 16 having a masked thiol group
gave the bis-phosphonium salt 7 as starting and a boronic acid in 2- and 6-position
material of the Wittig reaction. The alde- respectively was required for the assem-
hyde 10 as second precursor of the Wittig bly of the cruciform 4. As a suitable com-
reaction was obtained from commercially mercial precursor 6-bromonaphthalen-2-ol
available 3-fluorobenzaldehyde (8). After (13) was considered. While the bromine
protecting the aldehyde 8 as acetal the flu- substituent is ideally suited to introduce a

Laureates: awards and Honors, sCs FaLL Meeting 2009ꢀ
CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ 143
Fig.ꢀ3.ꢀA)ꢀSchematicꢀrepresentationꢀofꢀtheꢀMCBJꢀsetupꢀwithꢀtheꢀunderlyingꢀ
threeꢀpointꢀbendingꢀmechanism.ꢀB)ꢀScanningꢀelectronꢀmicroscopyꢀ(SEM)ꢀ
imageꢀofꢀtheꢀmicrofabricatedꢀgoldꢀstructure.ꢀC)ꢀTheꢀconductanceꢀbetweenꢀ
theꢀtwoꢀelectrodesꢀisꢀmeasuredꢀasꢀaꢀfunctionꢀofꢀtheꢀdistanceꢀzꢀresultingꢀ
inꢀconductanceꢀtraces.ꢀWhenꢀonlyꢀsolventꢀmoleculesꢀareꢀpresentꢀ(greyꢀ
curves)ꢀexponentialꢀdecayꢀcurvesꢀareꢀobtained.ꢀConductanceꢀplateausꢀareꢀ
observedꢀinꢀtheꢀpresenceꢀofꢀmoleculesꢀableꢀtoꢀbridgeꢀbothꢀelectrodesꢀ(likeꢀ
2,ꢀblackꢀpeak)ꢀattributedꢀtoꢀtheꢀformationꢀofꢀsingleꢀmoleculeꢀjunctions.ꢀ
Aꢀstatisticalꢀanalysisꢀofꢀtheꢀcurrentꢀtracesꢀisꢀperformedꢀwhileꢀformingꢀ
histograms.
When only solvent molecules are present,
no peak in the logG histogram is observed
besides the peak at 1 G0, which is attributed
to the conductance of the last gold contact
(77.5 μS). To investigate the cruciforms
1–4, these were dissolved in tetrahydrofu-
ran (THF)/mesitylene (1:4) and applied to
the MCBJ setup with or without deprotec-
tion with tetrabutylammonium hydroxide
(TBAOH). Typical histograms obtained
for the cruciforms 2–4 are displayed in
Fig. 4. Surprisingly, no molecular signa-
ture was observed in the histogram of the
Fig.ꢀ4.ꢀLogGꢀ
histogramsꢀofꢀtheꢀ
cruciformsꢀ2–4ꢀandꢀ
ofꢀtheꢀsolventꢀmixtureꢀ
THF/mesityleneꢀ(1:4).ꢀ
Forꢀtheꢀcruciformsꢀ
3ꢀandꢀ4ꢀhistogramsꢀ
2
3
cruciform 1. Presumably the considerable
loss of conjugation due to its sulfur anchor
groups in meta-positions resulted in trans-
port currents below the detection threshold
of the setup. To strengthen this hypothesis
2 was investigated as control structure with
increased conductivity through the sulfur-
terminated bar. And indeed, a peak in the
logG histogram at 2.2 × 10–4 G was ob-
served pointing at single molecu0les bridg-
ing both gold electrodes via their thiol an-
chor groups.[9] While the single molecule
traces demonstrate that even rather bulky
molecules like the cruciform 2 can form
molecular junctions, the superior conduc-
tance of the sulfur-functionalized bar did
not allow detection of conduction altera-
tions arising from the coordination of the
pyridine bar. To improve the transport
characteristics the pyridines were placed
on the OPV bar in 3 and 4. Interestingly,
two peaks were observed in the logG his-
tograms of both compounds 3 and 4 when
no deprotection agent was added. Com-
parison of the electronic signatures of the
cruciforms 3 and 4 with that of the parent
bar structures allowed the peaks to be at-
tributed. While for both cases the higher
conductance peak was ascribed to the sul-
fur-functionalized fixation bar, the lower
conductance peak arises from the pyridine-
functionalized bar.[10] The observation of
two peaks in the histograms is assumed to
arise from part of the molecules binding
via the pyridine anchor groups and part of
the molecules binding via spontaneously
deprotected thiol anchor groups. If these
cruciforms were deprotected prior to ex-
position to the setup offering the free thiol
for binding, one distinct peak is observed
in the histogram arising from the sulfur-
terminated bar dominating the conduc-
withꢀandꢀwithoutꢀ
theꢀadditionꢀofꢀaꢀ
deprotectionꢀagentꢀ
(TBAOH)ꢀareꢀdisplayed.ꢀ
3
deprotected
4
4
deprotected
solvent
-6
-5
-4
-3
-2
-1
0
Log(G/G0)
es of these sensitive junctions, numerous
junctions were analyzed statistically.[25]
Therefore hundreds of conductance traces
were measured and collected in histo-
grams showing peaks at the positions of
the most abundant conductance plateaus.
ever, if molecules bearing suitable anchor
groups to bridge both electrodes are ex-
posed to the setup, the conductance traces
show plateaus which are attributed to the
formation of molecule junctions. To bal-
ance fluctuations in the conductance trac-

144ꢀ CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ
Laureates: awards and Honors, sCs FaLL Meeting 2009
[1] C. Joachim, J. K. Gimzewski, A. Aviram,
tance characteristics. The conductance of
the pyridine-functionalized rod substruc-
ture moved closer to the conductance of
the fixation bar by elongating the sulfur-
terminated bar from 3 to 4, but still remains
below the threshold of the fixation bar.
Nature 2000, 408, 541.
[2] J. R. Heath, M. A. Ratner, Phys. Today 2003,
56, 43.
[3] N. J. Tao, Nat. Nanotechnol. 2006, 1, 173.
[4] R. L. Carroll, C. B. Gorman, Angew. Chem., Int.
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[5] N. Weibel, S. Grunder, M. Mayor, Org. Biomol.
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[6] H. Haick, D. Cahen, Prog. Surf. Sci. 2008, 83,
217.
[7] ‘Molecular Switches’, Ed. B. L. Feringa, Wiley-
VCH, Weinheim, 2001.
Summary and Conclusion
Our efforts towards a new switching
concept based on the interplay between
molecule and electrode are summarized.
The proposed switching mechanism relies
on the electrochemical potential dependent
coordination of pyridine anchor groups to
gold electrodes. Different cruciform struc-
tures consisting of a terminally sulfur-
functionalized fixation bar and a switching
bar comprising terminal pyridine groups
were synthesized and fully characterized.
Single molecule transport properties of
these cruciforms were investigated in a
mechanically controlled break junction
(MCBJ) setup in a liquid environment.
A full in situ electrochemical opera-
tion of the cruciform switch remains to be
demonstrated. Also, the inspection of the
transport properties of the cruciforms to-
gether with model compounds consisting
of single bars as their parent substructures
exhibit that further tuning of the electronic
transport properties is required. Neverthe-
less, it was possible to integrate the rather
bulky cruciform molecules bearing the
required functions for the proposed coor-
dination-induced switching mechanism on
a single molecule level. Furthermore, we
were able to demonstrate that both bars of
the cruciforms 3 and 4 were able to bridge
the gap between both electrodes.
[8] E. Lörtscher, J. W. Ciszek, J. Tour, H. Riel,
Small 2006, 2, 973.
[9] S. Grunder, R. Huber, V. Horhoiu, M. T.
González, C. Schönenberger, M. Calame, M.
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Acknowledgment
Financial support by the Swiss National
Science Foundation (SNSF), the NCCR na-
noscale science and the University of Basel is
gratefully acknowledged.
Received: December 24, 2009