The synthesis of functionalised diaryltetraynes and their transport properties in single-molecule junctions

Article abstract of DOI:10.1002/chem.201304671

The synthesis and characterisation is described of six diaryltetrayne derivatives [Ar-(C≡C)₄-Ar] with Ar=4-NO₂-C ₆H₄- (NO₂4), 4-NH(Me)C₆H ₄- (NHMe4), 4-NMe₂C₆

Full text of DOI:10.1002/chem.201304671

DOI: 10.1002/chem.201304671
Full Paper
&
Oligoynes
The Synthesis of Functionalised Diaryltetraynes and Their
Transport Properties in Single-Molecule Junctions
Murat Gulcur,[a] Pavel Moreno-Garcꢀa,[b] Xiaotao Zhao,[a] Masoud Baghernejad,[b]
Andrei S. Batsanov,[a] Wenjing Hong,[b] Martin R. Bryce,*[a] and Thomas Wandlowski*[b]
Abstract: The synthesis and characterisation is described of
six diaryltetrayne derivatives [Ar-(CꢀC)4-Ar] with Ar=4-NO2-
C6H4- (NO24), 4-NH(Me)C6H4- (NHMe4), 4-NMe2C6H4- (NMe24),
4-NH2-(2,6-dimethyl)C6H4- (DMeNH24), 5-indolyl (IN4) and
5-benzothienyl (BTh4). X-ray molecular structures are report-
ed for NO24, NHMe4, DMeNH24, IN4 and BTh4. The stability
of the tetraynes has been assessed under ambient laborato-
ry conditions (208C, daylight and in air): NO24 and BTh4 are
stable for at least six months without observable decompo-
sition, whereas NHMe4, NMe24, DMeNH24 and IN4 decom-
pose within a few hours or days. The derivative DMeNH24,
with ortho-methyl groups partially shielding the tetrayne
backbone, is considerably more stable than the parent com-
pound with Ar=4-NH2C6H4 (NH24). The ability of the stable
tetraynes to anchor in AujmoleculejAu junctions is report-
ed. Scanning-tunnelling-microscopy break junction (STM-BJ)
and mechanically controllable break junction (MCBJ) tech-
niques are employed to investigate single-molecule conduc-
tance characteristics.
nents.[12,13] Theoretical studies suggest that polyynes could
show metallic-like charge-transport characteristics when con-
nected between two electrodes.[14,15]
Introduction
The study of carbon-rich compounds is a very active research
topic in organic synthesis and materials chemistry.[1,2] For ex-
ample, sp2 hybridised systems (e.g., graphene, nanotubes, ful-
lerenes) are widely investigated due to their fascinating reac-
tivity and their outstanding electronic and optical properties.[3]
The synthesis of oligoynes encounters problems as the sta-
bility decreases rapidly with the increasing number of the
triple bonds in the backbone.[16] Although a few C8 derivatives
(i.e., Ar-(CꢀC)4-Ar, diaryltetraynes) and longer analogues with
simple aryl end-groups are isolable at room temperature,[17]
the established strategies for stabilising derivatives with nꢂ4
alkyne units are: 1) to covalently attach very bulky aryl or or-
ganometallic end-groups,[18–21] 2) to shield the oligoyne back-
bone within a supramolecular rotaxane-like structure,[22–25] or
3) to encapsulate the polyyne within a carbon nanotube.[26]
The vast majority of synthetic studies on oligo/polyynes have
concerned ways to stabilise longer derivatives.[27] In contrast,
little attention has been given to the synthesis of derivatives
which might be suitable for molecular electronics applications.
This requires functionalisation with terminal substituents which
will anchor the molecules to gold or other electrode materi-
als.[28] The very bulky terminal groups which stabilise longer
oligo/polyynes are not suitable for this purpose, and the tradi-
tional anchoring groups (e.g., amine, thiol) are expected to
make oligoynes unstable in ambient conditions due to inter-
or intramolecular reactions of the anchor group with the oli-
goyne backbone, thereby cross-linking or degrading the mole-
cules.[29]
However, the sp hybridised carbon allotrope (ꢁCꢀC) named
1
carbyne is one of the least studied carbon allotropes due to its
inaccessibility by standard synthetic routes and its insolubili-
ty.[4] Shorter chains of multiple conjugated alkyne units (i.e.,
diynes, oligoynes and polyynes), which possess terminal sub-
stituents to enhance their solubility and stability, have been
studied for many years.[5] They are fundamentally important
molecules for experimental[6–9] and theoretical studies[10,11] of
conjugation and charge-transport through carbon-rich back-
bones.
Oligoynes/polyyne chains (ꢁCꢀC)n have very attractive elec-
tron transport properties due to their almost cylindrical conju-
gation over the alternating single and triple bonds of the one-
dimensional sp-backbone, leading to proposed applications as
molecular wires, switches and non-linear optical compo-
[a] Dr. M. Gulcur, X. Zhao, Dr. A. S. Batsanov, Prof. M. R. Bryce
Department of Chemistry, Durham University
Durham DH1 3LE (UK)
E-mail: m.r.bryce@durham.ac.uk
In this context we have recently reported that diaryltetrayne
derivatives (Ar4) with terminal pyridyl (PY)[30] 4-cyanophenyl
(CN) and 2,3-dihydrobenzo[b]thien-5-yl (BT) end groups can be
isolated, purified and fully characterised under standard labora-
tory conditions (208C, daylight and in air).[31] The transport
properties of the PY4, CN4 and BT4 derivatives in Aujsingle-
[b] Dr. P. Moreno-Garcꢀa, M. Baghernejad, Dr. W. Hong, Prof. T. Wandlowski
Department of Chemistry and Biochemistry
University of Bern, Freiestrasse 3, CH-3012 Bern (Switzerland)
E-mail: thomas.wandlowski@dcb.unibe.ch
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304671.
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moleculejAu junctions were studied by scanning tunnelling
microscopy (STM) and mechanically controllable break junction
(MCBJ) methods.[31] However, the Ar4 analogues with 4-amino-
phenyl (NH2) and 4-(thioacetyl)phenyl (SAc) end-groups rapidly
decompose on attempted isolation and could not be puri-
fied.[31]
observable decomposition. The compound has very low solu-
bility and we could not characterise it by conventional solution
NMR spectroscopy techniques. However, a high resolution
mass spectrum and the X-ray crystal structure confirmed the
assigned structure.
In a previous study, the rapid decomposition under ambient
conditions of 4-aminophenyl end-capped tetrayne NH24 was
reported (Scheme 2).[31] The probable decomposition process is
an intermolecular reaction of the amino group with the
There is a need to explore new oligoynes to establish the
scope of functionalised non-bulky end-groups which can be at-
tached to the oligoyne chain and afford stable derivatives. We
now present the synthesis of new diaryltetraynes with empha-
sis on derivatives which are stable under ambient conditions.
Several new Ar4 derivatives are characterised by spectroscopic
methods and X-ray crystallography; the ability of the stable de-
rivatives to anchor in molecular junctions is assessed and the
electronic properties of Aujsingle-moleculejAu junctions are
reported. This work provides the most comprehensive study to
date on the synthesis of functionalised oligoynes and their ap-
plications in molecular junctions.
Scheme 2. Structures of diaryltetraynes with functional end groups.[31] For
definitions of “stable” and “unstable” see main text.
Results and Discussion
Synthesis
Diaryltetrayne compounds Ar4a–f were synthesised by dimeri-
sation of the trimethylsilyl protected precursor aryldiyne deriv-
ative 3a–f under classical Eglington–Galbraith coupling condi-
tions (Scheme 1).[32] Compounds 3a–d,f were synthesised by
sp-chain to form the observed dark-coloured insoluble prod-
uct. In order to reduce the reactivity of the amine moiety, the
mono- and dimethylated analogues, NHMe4 and NMe24, were
synthesised. In comparison to
NH24, increased stability was ob-
served for N-methylaniline end-
capped tetrayne NHMe4, which
was characterised as soon as
possible after isolation by 1H,
13C NMR, and mass spectroscopy.
Crystals of NHMe4 were grown
in the dark. However, traces of
decomposition products were
seen and only limited stability
under ambient conditions was
observed for NHMe4. The dime-
thylamino analogue NMe24 was
Scheme 1. Synthesis of diaryltetraynes Ar4a–f.
found to be very light sensitive
and attempts to grow crystals of
reaction of the corresponding aryliodide with 1,3-butadiyne-1-
trimethylsilane. Due to the instability of 1,3-butadiyne-1-trime-
thylsilane (2) at high temperatures, compound 3e was syn-
thesised by a different route using 5-alkynylbenzo[b]thiophene
4 and trimethylsilyl acetylene. All steps proceeded in syntheti-
cally viable yields to afford the target diaryltetraynes Ar4.
During the course of our work a similar synthesis of the nitro-
phenyl end-capped tetrayne NO24 was reported by Szafert
et al.[33]
NMe24 were unsuccessful. During the preparation of this
manuscript, the synthesis and X-ray crystal structure of the
pentayne analogue terminated with bulkier 4-(diisopropylami-
no)phenyl groups was reported.[34]
Partially shielding the oligoyne chain by ortho-methyl
groups, compound DMeNH24, considerably increased the sta-
bility compared to the parent compound NH24. DMeNH24 was
1
unambiguously characterised by H and 13C NMR spectroscopy,
mass spectrometry and X-ray crystallography, using single crys-
tals, which were grown in the dark. The NMR spectra of
DMeNH24 provided no evidence for decomposition upon stor-
age in CDCl3 in the NMR tube for six days in the dark after iso-
lation. No precipitation or colour change of the solution was
observed during this time. This sample was then exposed to
Stability of the diaryltetraynes
NO24 is stable under ambient laboratory conditions and a solid
sample can be stored for several months on the shelf with no
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ambient laboratory light; after 2 h the formation of an insolu-
ble brown-black precipitate was observed.
5-Indolyl end-capped tetrayne IN4 was also light sensitive
and thermally unstable. It was characterised by 1H and
13C NMR and mass spectrometry immediately after purification.
The instability was observed as a colour change of the bright
yellow crystals to dark green when exposed to ambient light
at room temperature. Crystals of IN4 for X-ray analysis were
grown in the dark. In contrast, the 5-benzothienyl analogue
BTh4 is stable for at least six months under ambient condi-
tions.
To enable a comprehensive study to be undertaken of the
length dependence of the single-molecule conductance for
the NO2 and BTh oligoyne series (see below), the correspond-
ing butadiyne and tolane analogues were also synthesised (see
the Supporting Information).
X-ray crystal structures of NO24, NHMe4, DMeNH24, IN4 and
BTh4
Molecules of NO24, DMeNH24 (crystallised as acetone disol-
vate), BTh4 and IN4 have crystallographic Ci symmetry and
therefore parallel arene rings. NHMe4 has no crystallographic
symmetry and its arene rings form a dihedral angle of 438
(Figure 1). The tetrayne rods in NO24, DMeNH24 and IN4 are
practically linear; in the other two compounds they are sub-
stantially bent. In NHMe4 the C(4)ꢁC(7) and C(14)ꢁC(15) bonds
form an angle of 161.48; in BTh4 the central bond C(12)ꢁC(12’)
forms 170.78 angles with C(7)ꢁC(9) and C(7’)ꢁC(9’) bonds. The
molecular lengths are: NO24 O(1)···O(1’) 21.4; NHMe4
N(1)···N(2) 20.0; DMeNH24 N···N’ 20.1; IN4 N···N’ 20.0; and BTh4
S···S’ 20.7 ꢂ.
All five crystal structures comprise arrays of rigorously paral-
lel, albeit offset, molecules related by a crystal lattice transla-
tion a (in NO24) or b (in all other cases). For NHMe4 and
DMeNH24, adjacent molecules in an array are linked by hydro-
gen bond bridges NꢁH···NꢁH···N and NꢁH···O···HꢁNꢁH···O···Hꢁ
N, respectively. In IN4, weak NꢁH···NꢁH···N hydrogen bonds zip
together two adjacent arrays. In NO24, NHMe4, BTh4 and IN4
the shortest C···C distances between tetrayne rods (d) are close
to the normal van der Waals contact (3.5 ꢂ) and the arene
end-groups are stacked in a p–p fashion (Table 1). However,
the slanting angle f is not favourable for topotactic 1,8-poly-
merisation, for which d=3.5 ꢂ and f=218 are optimal.[33] The
molecules of DMeNH24 are intercalated by acetone of crystalli-
sation, resulting in longer d and the absence of any effective
Figure 1. X-ray molecular structures of NO24, NHMe4, DMeNH24, IN4 and
BTh4 showing thermal ellipsoids of 50% probability. Primed atoms are gen-
erated by inversion centres.
overlap of the arene rings. Packing diagrams are shown in the
Supporting Information.
Single-molecule conductance measurements
The transport properties of single oligoyne wires were studied
by STM-BJ[35,36] and complementary MCBJ[37–39] measurements
in solution at room temperature and in the absence of oxygen.
The STM-BJ approach is based on the repeated formation and
breaking of (single) molecule junctions formed between an
atomically sharp gold STM tip and a flat Au(111) substrate, and
the simultaneous monitoring of the current iT or conductance
G=iT/Vbias in function of distance Dz at constant bias voltage,
V
bias. The MCBJ technique relies on the formation and rupture
of molecular junctions between horizontally suspended gold
wires.[37] For further technical details we refer to the Experi-
mental Section and to our previous work.[38–41]
Table 1. Crystal packing parameters.
Figure 2A displays, as an example, typical conductance (G)
versus relative distance (Dz) stretching traces, plotted in
a semi-logarithmic scale, as recorded for BTh4 in TMB/THF.
After formation of the contact between the gold tip and the
Au(111) substrate, the tip was withdrawn with a rate of
58 nmsꢁ1. All curves show initially a step-like decrease of the
conductance from 10G0 up to 1G0 with G0 =2e2 h=77.5 mS
ꢀ [8]
t [ꢂ]
d [ꢂ]
p [ꢂ]
NO24
78
66
58
50
70
3.74
3.99
5.23
4.86
3.86
3.68–3.69
3.62–3.69
4.42–4.51
3.78–3.84
3.62–3.64
3.38
3.41, 3.43
–
3.31
3.51
NHMe4
DMeNH24
IN4
BTh4
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conductance range of a single BTh4 molecule bridging the
gold nanoelectrodes. The 2D histogram in Figure 2C demon-
strates further that the conductance decreases monotonously
with increasing relative displacement (or junction elongation)
Dz. Similar results were obtained in complementary experi-
ments employing the MCBJ technique.
Analysing the evolution of a molecular junction upon
stretching we constructed relative displacement (Dz) histo-
grams by calculating the displacement from the relative zero
position at 0.7G0 up to the end of the molecular conductance
region of every individual trace.[38] The lower limit is defined as
one order of magnitude beneath the most probable conduc-
tance GH. Figure 2D displays the characteristic displacement
histograms of BTh4. The plot shows a uniform normal distribu-
tion (G) with a well-defined maximum Dz*=0.64 nm as a mea-
sure of the most-probable plateau lengths of BTh4 molecular
junctions. Stretching traces shorter than 0.5 nm (T) were as-
signed to direct tunnelling through solution without the for-
mation of a molecular junction.[38] This consideration leads to
a junction formation probability of 30% (Table 2).
Figure 2. STM-BJ-based conductance measurements of BTh4 in TMB/THF
(5:1, v/v) as recorded with Vbias =0.065 V and a stretching rate of 58 nmsꢁ1
A) Typical original conductance versus distance traces. B) The 1D conduc-
tance histogram. The noise level is indicated by the asterisk. The small spike
at log(G/G0)ꢅꢁ2 represents an artefact related to the switching of the am-
plifier. C) The 2D conductance histogram generated from 1600 individual
curves without any data selection. D) Characteristic relative dis-
placement length histogram.[38]
.
Finally, we obtained the most probable absolute displace-
Table 2. Single molecule conductance characteristics of molecular junctions formed
being the quantum of conductance.[36] Subsequently,
the conductance decreases abruptly by several
orders in magnitude (“jump out of contact”).[42] The
very end of the gold contacts retract by approximate-
ly (0.5ꢃ0.1) nm, the so-called snap-back distance
Dzcorr (see refs. [31] and [38] for details). Additional
features, such as plateaus, are observed at Gꢄ
10ꢁ3G0, which are attributed to the formation of
(single) molecular junctions. The noise level is
reached at G<10ꢁ6 G0 in the STM-BJ experiments.
Several thousands of individual G versus Dz traces
were recorded and subsequently analysed further by
constructing all-data point histograms (without any
data selection) to extract statistically significant re-
sults. Figure 2B displays the corresponding one-di-
mensional (1D) histogram of BTh4 in a semi-logarith-
mic scale. We observed peaks for the breaking of
Au–Au contacts as marked by integers of G0, and one
well-defined molecular-junction related feature. A
by end-capped tetraynes attached to gold leads.
Molecule
L
Dz*
z*
[nm][c]
G*/G0
JFP
b
Rc
[nm][a] [nm][b]
[%][d] [nm]ꢁ1[e] [kW][f]
BT4
PY4
CN4
NO24[g]
BTh4
2.08
1.76
2.28
2.14
2.08
1.40ꢃ0.10 1.90ꢃ0.10 2.0ꢃ10ꢁ4 100 3.1ꢃ0.2
115ꢃ31
1.40ꢃ0.10 1.90ꢃ0.10 4.0ꢃ10ꢁ5 100 2.9ꢃ0.2
1900ꢃ1100
0.70ꢃ0.10 1.20ꢃ0.10 4.0ꢃ10ꢁ6 45 1.75ꢃ0.1 58000ꢃ20100
0.60ꢃ0.10 1.10ꢃ0.10 1.0ꢃ10ꢁ7 40 5.4ꢃ0.3
0.64ꢃ0.10 1.14ꢃ0.10 1.0ꢃ10ꢁ4 30 2.3ꢃ0.2
980ꢃ500
1050ꢃ650
[a] Molecular length, which is defined as the distance between the centre of the
anchor atom at one end of a fully extended isolated molecule to the centre of the
anchor atom at the other end. The lengths of the molecules were obtained from crys-
tal structure data; [b] the most probable relative junction elongation (displacement);
[c] z*=Dz*+0.5 is the absolute junction elongation (displacement); [d] the junction
formation probability; [e] the attenuation parameter as obtained from the length de-
pendence (L) of families of oligoynes with 4, 2 and 1 triple bond(s) end-capped with
the respective anchors (some of the original data are taken from ref. [31]; [f] contact
resistance as obtained from the semi-logarithmic correlation of conductance G versus
molecular length L; [g] these data are based on the analysis of 600 out of 2000 traces
(for details see the Supporting Information).
Gaussian fit led to G*=10ꢁ4 G0 as the most probable single
molecular junction conductance of BTh4. This value appears to
be independent of the concentration of BTh4 down to 1 mm.
The above analysis was extended by constructing all-data
point two-dimensional (2D) conductance versus displacement
histograms.[43,44] This representation provides direct access to
the evolution of molecular junctions during the formation,
stretching and break-down steps. Figure 2C shows the corre-
sponding data for BTh4. The individual GꢁDz traces were
aligned by setting the common origin to Dz=0 at G=0.7G0.
The data also reveal quantised conductance features at Gꢂ
1G0. They correspond to the breaking of gold–gold contacts.
An additional high-density data cloud is observed in 10ꢁ4.5 G0 ꢄ
Gꢄ10ꢁ3.5 G0, centred on 10ꢁ4 G0. This region represents the
ments zH* in an experimental BTh4 molecular junction formed
between a gold STM-tip and an Au(111) surface by adding the
snap-back distance Dzcorr =(0.5ꢃ0.1) nm to the relative dis-
[38]
placement Dz*: zi*=Dz*+Dzcorr
,
which gives (1.14ꢃ
0.10) nm (Table 2). This value is smaller than the molecular
length, L, which is defined as the distance between the centre
of the sulfur anchor atom at one end of a fully extended isolat-
ed molecule to the centre of the anchor atom at the other
end. In other words, the molecular junctions formed with
BTh4 break most often before the molecule is completely ex-
tended.
Next, we investigated the entire family of tetrayne-type mo-
lecular junctions following the methodology described above
for BTh4. Table 2 summarises the characteristic data G*, Dz*, z*
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and L for BT4, PY4, CN4, NO24 and BTh4. The original traces
can be found in the Supporting Information or in ref. [31]. We
note that the low junction conductance value of NO24 was ob-
tained in the MCBJ set-up, which allows more sensitive electri-
cal measurements down to 10ꢁ8.5 G0.[39] We obtained the follow-
ing trend in the most probable molecular junction conductan-
ces:
molecule–electrode interface. Rc is the corresponding contact
resistance. The b value is the attenuation constant, which char-
acterises the electronic coupling in a specific molecular back-
bone as a function of its length. The experimentally deter-
mined b values range between (1.75ꢃ0.1) nmꢁ1 (CN) to (5.4ꢃ
0.3) nmꢁ1 (NO2). The distinctly different values of b demonstrate
that the nature of the anchor group controls the strength of
the electronic coupling to the metal leads, the position of the
energy levels involved in the electron transport across the
single molecule junction as well as their coupling into the wire
backbone. For comparison, we also added the NH2 and SH
end-capped oligoynes despite the limited data set due to the
instability of the tetraynes.[31] The resulting values of the at-
tenuation constant b are comparable with those of other
p-conjugated molecular wires, such as oligo(phenylene-ethyny-
lenes),[41,46–50] oligophenyleneimine[51] and oligo(phenylene-vi-
nylenes).[46] We conclude that stable molecular wires bearing
an oligoyne backbone in combination with proper anchoring
groups may act as promising building blocks in more complex
molecular assemblies combining functional units.
G*(BT4)>G*(BTh4)>G*(PY4)>G*(CN)>G*(NO24)
The junction formation probabilities decrease from 100% for
BT4 and PY4 to values below 50% for the other three deriva-
tives. In combination with the values of the most probable ab-
solute displacements z*, which are considerably smaller than
the molecular length L, we conclude that tetrayne-type molec-
ular junctions terminated with CNꢁ, BThꢁ and NO2ꢁ anchors
are rather unstable and break at an inclined angle with respect
to the surface normal before they are fully extended. The com-
parison with previous experimental observations[31,38] suggests
that these termini are less favourable for the formation of
stable molecular junctions. However, we note that the results
of the current study deviate from recent observations of Zotti
et al.[45] who concluded, based on MCBJ experiments in air
under ambient conditions, that molecular junctions formed
with NO2-capped tolanes are rather stable. This difference may
arise from the nature of the different experimental conditions
in both studies.
Conclusion
We have synthesised new oligoyne derivatives with emphasis
on obtaining stable diaryltetraynes with anchor groups that
are suitable for attaching the molecules to gold electrodes.
The tetraynes Ar4a–f have been synthesised by dimerisation
of the trimethylsilyl-protected precursor aryldiyne derivative
under classical Eglington–Galbraith coupling conditions. Under
ambient laboratory conditions (208C, daylight and in air) the
tetraynes NO24 and BTh4 are stable for at least six months
without observable decomposition, whereas NHMe4, NMe24,
DMeNH24 and IN4 decompose within a few hours or days.
These results, in combination with previous studies,[31] provide
a comprehensive assessment of the range of functional groups
which can be tolerated at the terminal positions of diaryloli-
goynes. The X-ray molecular structures are reported for NO24,
NHMe4, DMeNH24, IN4 and BTh4. STM-BJ and MCBJ tech-
niques have been employed to probe the single-molecule con-
ductance characteristics of the series of oligoynes with termi-
nal 4-nitrophenyl and 5-benzothienyl groups and four, two and
one triple bond(s) in the backbone. We conclude that with
careful choice of the anchor group oligoynes are a viable
family of stable molecular wires for molecular electronics appli-
cations. It remains a challenge to synthesise longer oligoynes
(i.e., with more than four triple bonds) that are stable and con-
tain functional groups that enable the molecules to anchor to
gold electrodes.
We note that no reliable single molecule junction conduc-
tance measurements could be carried out for NHMe4,
DMeNH24, IN4, NH24 and SH4 due to their limited chemical
stability under current experimental conditions. In the case of
NMe24 the two methyl substituents of the nitrogen prevented
the formation of stable molecular contacts, which led to bare
tunnelling traces.
Next, we compared the dependencies of the most probable
single junction conductances on molecular length L, as con-
structed from the experimental data of the five tetraynes
(PY4,[31] NO24, BT4,[31] CN4[31] and BTh4) together with data for
the shorter butadiyne and tolane analogues with seven differ-
ent end-groups (Figure 3). Analysing these data with reference
to a simple tunnelling model, which is represented by G*=
Gc*ꢃexp(ꢁbꢃL), leads to the attenuation constant b and to
the contact conductance Gc*. Gc*=1/Rc represents an effective
contact conductance reflecting the electronic coupling at the
Experimental Section
General
All reactions were conducted under a blanket of argon which was
dried by passage through a column of phosphorus pentoxide
unless otherwise stated. All commercial chemicals were used with-
out further purification. Anhydrous toluene, tetrahydrofuran (THF),
Figure 3. Most probable single junction conductance values G* of families of
oligoynes as obtained from the analysis of the 1D and 2D conductance his-
tograms in dependence on the length of the molecule.
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dichloromethane (DCM) and diethyl ether (Et2O) were dried
through an HPLC column on an Innovative Technology Inc. solvent
purification system. Petroleum ether (PE) used for column chroma-
tography was b.p. 40–608C fractions and was used as received.
Column chromatography was carried out using 40–60 mm mesh
silica (Fluorochem). Analytical thin layer chromatography (TLC) was
performed on 20 mm precoated plates of silica gel (Merck, silica
gel 60F254); visualisation was made using ultraviolet light (254 and
365 nm). NMR spectra were recorded on Bruker Avance-400, Varian
VNMRS 700 and Varian Inova 500 spectrometers. Chemical shifts
are reported in ppm relative to CDCl3 (7.26 ppm), [D6]acetone
(2.09 ppm), [D6]DMSO (2.50 ppm) or tetramethylsilane (0.00 ppm).
Melting points were determined in open-ended capillaries using
a Stuart Scientific SMP40 melting point apparatus at a ramping
rate of 28Cminꢁ1. Mass spectra were measured on a Waters Xevo
OTofMS with an ASAP probe. Electron ionisation (EI) mass spectra
were recorded on a Thermoquest Trace or a Thermo-Finnigan DSQ.
Ion analyses were performed on a Dionex 120 Ion Chromatography
detector. Elemental analyses were performed on a CE-400 Elemen-
tal Analyzer. IR spectra were obtained on a PerkinElmer FT-IR Para-
gon 1000 spectrometer. UV/Vis absorption spectra were obtained
on a Unicam UV2 UV/Vis spectrometer. CAUTION! We did not at-
tempt to measure the melting points of the diaryltetrayne deriva-
tives, as on a previous occasion we observed that 1,8-bis(4-cyano-
phenyl)octa-1,3,5,7-tetrayne (CN4) exploded upon heating.[31]
spectra could not be obtained due to the compound’s very low
solubility in standard NMR solvents, including C6D6, which is stated
in ref. [33] as the NMR solvent for this compound. MS (ES): m/z (%):
341.1 [M+ +H]; HR-MS (ASAP+) m/z calcd for C20H8N2O4: 340.0484;
found: 340.0466 [M+]; IR (solid): v˜ =3106, 2202, 1589, 1516, 1340,
1287, 1103, 1014, 851, 745, 682 cmꢁ1; UV/Vis (CH2Cl2): lmax (e)=317
(84600), 334 (96000), 356 (85200), 384 (72400), 416 nm
(44800 dm3 mꢁ1 cmꢁ1). Crystals for X-ray analysis were grown by
slow evaporation of a solution in toluene.
N-Methyl-4-[(trimethylsilyl)buta-1,3-diyn-1-yl]aniline (3b): Gener-
al procedure I: A mixture of 1b (0.500 g, 2.1 mmol), BDTMS (2;
0.525, 4.20 mmol), [Pd(PPh3)4] (0.124 g) and CuI (10 mg) was stirred
at RT for 4 h. Column chromatography (DCM/PE, 1:1 v/v) gave 3b
as
a
yellow, light-sensitive oil (0.445 g, 95% yield). 1H NMR
(400 MHz, CDCl3): d=7.31 (d, J=8.8 Hz, 2H), 6.48 (d, J=8.8 Hz,
2H), 4.01 (s, 1H), 2.83 (s, 3H), 0.22 ppm (s, 9H); 13C NMR (101 MHz,
CDCl3): d=150.24, 134.44, 112.11, 108.54, 89.56, 88.89, 78.86, 72.49,
30.45, ꢁ0.01 ppm; MS (ES): m/z (%): 228.2 [M+ +H, 100%].
4,4’-(Octa-1,3,5,7-tetrayne-1,8-diyl)bis(N-methylaniline) (NHMe4):
General procedure II: A mixture of 3b (0.200 g, 0.88 mmol), Cu-
(OAc)2·H2O (0.351 g) was stirred for 16 h at RT. Column chromatog-
raphy (DCM/PE, 1:1 v/v) gave NHMe4 as a yellow solid (0.111 g,
81% yield). 1H NMR (400 MHz, CDCl3): d=7.36 (d, J=8.4 Hz, 4H),
6.49 (d, J=8.4 Hz, 4H), 4.07 (s, 2H), 2.86 ppm (s, 6H); 13C NMR
(101 MHz, CDCl3): d=150.56, 135.11, 112.15, 107.76, 79.79, 73.39,
67.53, 64.67, 30.39 ppm; HR-MS (ASAP+): m/z calcd for C22H17N2
[M+ +H]: 309.1371; found: 309.1414 [M+ +H]. Crystals for X-ray
analysis were grown by slow evaporation of a solution in CHCl3.
Synthesis
General procedure I for Sonogashira couplings: The iodoarene
derivative (1 equiv), [Pd(PPh3)4] (5%mmol) and CuI (2%mmol)
were placed in a dry flask and dissolved in THF (7 mL per mmol)
and (iPr)2NH or Et3N (1 mL per mmol), to which the alkyne was
added in small portions in 15 min. The mixture was stirred under
argon at RT until the reaction was complete (judged by TLC). Sol-
vents were removed and the crude was purified by column chro-
matography.
3,5-Dimethyl-4-[(trimethylsilyl)buta-1,3-diyn-1-yl]aniline
(3d):
General procedure I: A mixture of 1d[59] (0.300 g, 1.21 mmol),
BDTMS (2; 0.297 g, 2.43 mmol), [Pd(PPh3)4] (70 mg), (iPr)2NH, CuI
(5 mg), was reacted for 20 min, in a microwave reactor at 1008C.
Column chromatography using DCM/PE (1:1, v/v) as eluent afford-
1
ed 3d as a brownish oil (40 mg, 13%). H NMR (400 MHz, CDCl3):
d=6.32 (s, 2H), 3.76 (s, 2H), 2.34 (s, 6H), 0.23 ppm (s, 9H); 13C NMR
(101 MHz, CDCl3): d=147.17, 144.37, 113.52, 110.84, 90.50, 88.92,
80.05, 76.21, 21.32, 0.03 ppm; MS (ES+): m/z (%): 241.1 ([M]+, 40).
General procedure II for oxidative homo-couplings of TMS-al-
kynes: TMS-alkyne was dissolved in MeOH/pyridine (1:1 v/v) mix-
ture (10 mL per mmol) and Cu(OAc)2·H2O (2 equiv, 0.400 g per
mmol) was added to the mixture in one portion and stirred until
the reaction was complete (judged by TLC). The reaction was
quenched by adding saturated NH4Cl solution (5 mL per mmol)
and diluted with DCM (25 mL per mmol), dried over MgSO4 and
the solvents were removed under reduced pressure. The crude
was purified either by column chromatography or recrystallisation.
4,4’-(Octa-1,3,5,7-tetrayne-1,8-diyl)bis(3,5-dimethylaniline)
(DMeNH24): General procedure II:
A mixture of 3d (40 mg,
0.16 mmol), Cu(OAc)2·H2O (66 mg), was reacted, overnight, at RT.
Column chromatography using DCM as eluent afforded DMeNH24
1
as a yellow solid (18 mg, 65%). H NMR (400 MHz, [D6]acetone): d=
6.41 (s, 4H), 5.27 (s, 4H), 2.29 ppm (s, 12H); 13C NMR (101 MHz,
CDCl3): d=155.72, 150.04, 118.21, 111.92, 84.91, 84.36, 73.54, 70.66,
25.66 ppm; HR-MS (ASAP+): m/z calcd for C24H21N2 [M+H]+:
337.1699; found: 337.1711; UV/Vis (CH2Cl2): lmax (e)=306 (54527),
Compounds 1b,[52] 1c,[53] 1d[54] BDTMS (2),[55] 3c,[56] 4[57] and
NMe24[55] were synthesised according to the literature procedures.
Compounds 1a and 1e were obtained commercially.
325
(78844),
360
(94907),
378
(96986),
411 nm
(63976 dm3 mꢁ1 cmꢁ1). Crystals for X-ray analysis were grown by
Trimethyl[(4-nitrophenyl)buta-1,3-diyn-1-yl]silane (3a): General
procedure I: 4-Iodonitrobenzene (1a; 0.400 g, 1.6 mmol), BDTMS (2;
0.216 g, 1.77 mmol), [Pd(PPh3)4] (92 mg), CuI (6 mg), (iPr)2NH were
allowed to react, overnight, at RT. Column chromatography using
DCM/hexane (1:1, v/v) as eluent afforded 3a as a yellow solid
slow evaporation of a solution in acetone in the dark.
1,8-Bis(indol-5-yl)octa-1,3,5,7-tetrayne (IN4): General procedure I:
5-Iodoindole (1e; 300 mg, 1.23 mmol), CuI (12 mg), [Pd(PPh3)4]
(70 mg), BDTMS (2; 226 mg, 1.85 mmol), THF/Et3N (10 mL) were re-
acted for 24 h at 508C. The solvent was removed by vacuum evap-
oration to yield a product which could not be purified by column
chromatography. General procedure II: This was conducted directly
on the brown crude material. Cu(OAc)2·H2O (320 mg) and MeOH/
pyridine (1:1 v/v) were reacted for 23 h, at RT. Purification by
column chromatography using DCM/hexane (3:7 v/v) as eluent
gave a yellow solid (70 mg, 35% overall yield). M.p.: 1308C (dec.)
1
(0.30 g, 77% yield). M.p.: 110.0–111.08C; H NMR (400 MHz, CDCl3):
d=8.18 (d, J=9.0 Hz, 2H), 7.61 (d, J=9.0 Hz, 2H), 0.24 ppm (s,
9H); 13C NMR (101 MHz, CDCl3): d=147.75, 133.62, 128.58, 123.87,
94.45, 87.12, 79.19, 74.37, ꢁ0.32 ppm; MS (ES+): m/z (%): 244.0
([M+H]+, 100). The data are consistent with the literature.[58]
1,8-Bis(4-nitrophenyl)octa-1,3,5,7-tetrayne (NO24): General proce-
dure II: A mixture of 3a (0.200 g, 0.82 mmol) and Cu(OAc)2·H2O
(0.328 g, 1.64 mmol) was stirred for 12 h at RT. The yellow-green
precipitate was filtered and recrystallised from toluene to give
NO24 as air-stable yellow crystals (0.12 g, 43% yield). Reliable NMR
1
which turned green in ambient conditions within a week. H NMR
(400 MHz, ([D6]DMSO): d=11.51 (s, 2H), 7.94 (s, 2H), 7.47 (m, 2H),
7.43 (d, J=8.4 Hz, 2H), 7.32 (dd, J=8.4, 1.6 Hz, 2H), 6.49 ppm (m,
2H); 13C NMR (101 MHz, [D6]DMSO): d=137.46, 128.33, 128.25,
Chem. Eur. J. 2014, 20, 4653 – 4660
www.chemeurj.org
4658
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
127.57, 126.39, 113.10, 109.12, 102.51, 82.29, 72.35, 66.93,
65.04 ppm; HR-MS (ASAP+) m/z calcd for C24H12N2 [M]+ 328.1000;
found: 328.1014. Crystals for X-ray analysis were grown by slow
evaporation of a solution in dichloromethane/hexane in the dark.
tive closing rate of 0.1 to 5 nmsꢁ1. Typically, we recorded up to
2000 stretching cycles to obtain statistically significant data.
[(Benzothiophen-5-yl)buta-1,3-diyn-1-yl]trimethylsilane (3 f): Tri-
methylsilylacetylene (TMSA; 930 mg, 9.4 mmol), [Pd(PPh3)Cl2]
(33 mg) and CuI (9 mg) was added to a solution of 4 (150 mg,
0.94 mmol) in THF/Et3N (1:1 v/v). Air was bubbled constantly
through the solution until the reaction was complete after 12 h at
RT. Purification by column chromatography using PE as eluent
gave 3 f as a yellow solid (90 mg, 38% yield). M.p.: 91.3–92.48C;
1H NMR (400 MHz, CDCl3): d=7.97 (s, 1H), 7.81 (d, J=8.3 Hz, 1H),
7.48 (d, J=5.5 Hz, 1H), 7.44 (d, J=8.3 Hz, 1H), 7.30 (d, J=5.5 Hz,
1H), 0.26 ppm (s, 9H); 13C NMR (101 MHz, CDCl3): d=140.93,
139.66, 128.47, 128.06, 127.96, 123.87, 122.84, 117.36, 90.67, 88.24,
77.46, 73.98, 0.00 ppm; HR-MS (ASAP+) m/z calcd for C15H15SiS
[M+H]+ 255.0664; found: 255.0662.
Acknowledgements
The work was funded by the EC FP7 ITNs “FUNMOLS” Project
No. 212942 and “MOLESCO” Project No. 606728, the Swiss Na-
tional Science Foundation under contract 200020_144471/
1 and the University of Bern. Z.X. thanks the China Scholarship
Council for the award of a scholarship.
Keywords: cross-coupling · oligoynes · polyynes · single-
molecule conductance · X-ray diffraction
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1608C (decomp.); H NMR (600 MHz, CD2Cl2): d=8.08 (s, 2H), 7.89
(d, J=8.3 Hz, 2H), 7.57 (d, J=5.5 Hz, 2H), 7.51 (d, J=8.3 Hz, 2H),
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Received: November 28, 2013
Revised: January 15, 2014
Published online on March 5, 2014
Chem. Eur. J. 2014, 20, 4653 – 4660
www.chemeurj.org
4660
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim