Synthesis of tetrathiafulvalenes suitable for self-assembly applications

Article abstract of DOI:10.1039/b310260b

A series of new tetrathiafulvalenes, with double alkylthiol or alkyldisulfide substitution, have been prepared with a synthetic procedure that allows variation of different substituents. The target compounds 6a-e and 15e-i are sparsely soluble in organic solvents, but TTFs 6d and 15g gave a relatively dense packed monolayer upon exposure to gold surfaces.

Full text of DOI:10.1039/b310260b

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A R T I C L E
Synthesis of tetrathiafulvalenes suitable for self-assembly
applications{
Emma Dahlstedt,a Jonas Hellberg,*a Rodrigo M. Petoral, Jr.b and Kajsa Uvdalb
aDept. of Chemistry, Organic Chemistry, Royal Institute of Technology, SE-100 44
Stockholm, Sweden. E-mail: jhel@kth.se; Fax: 146-8-791 2333; Tel: 146-8-790 8127
bLaboratory of Applied Physics, Department of Physics and Measurement Technology,
Linko¨ping University, SE-581 83 Linko¨ping, Sweden
Received 26th August 2003, Accepted 24th October 2003
First published as an Advance Article on the web 13th November 2003
A series of new tetrathiafulvalenes, with double alkylthiol or alkyldisulfide substitution, have been prepared
with a synthetic procedure that allows variation of different substituents. The target compounds 6a–e and 15e–i
are sparsely soluble in organic solvents, but TTFs 6d and 15g gave a relatively dense packed monolayer upon
exposure to gold surfaces.
Introduction
Tetrathiafulvalenes (TTFs) (1) are the most successful class of
heterocycles in terms of creating highly conducting1 and
superconducting organic crystalline materials.
Good intermolecular overlap between TTF p-stacks gives
high charge carrier mobilities, especially when sulfur sub-
stituents can create a two-dimensional network of contacts,
thereby suppressing Peierls-distortion.2 Many systems have
been studied in detail and valuable insights in the solid state
physics of organic materials have been achieved.3 These
valuable properties have rarely been possible to transfer to
more tractable systems, as in the case of oligothiophenes.4
Langmuir–Blodgett films of tetrathiafulvalenes have been
studied and thin films have been prepared with interesting
results,5 but few electronic devices such as field-effect
transistors have been realized.
The self-assembly methodology, where alkylthiols self-
assembles on an active metal surface, offers an alter-
native way of preparing ordered thin films of organic
materials.6
If successful this would then open up the possibility of
transferring the unique properties of tetrathiafulvalene charge
transfer salts to thin film devices.7
Self-assembly monolayer (SAM) systems with tetrathiaful-
valenes have previously been studied by several groups. TTF-
SAMs were first reported by Yip and Ward8 but these systems
were not particularly electrochemically stable. Bryce and co-
workers studied a TTF with one alkylthiol attached to TTF
substituted with a crown ether i.e. (2),9 which proved to yield
more stable SAMs. A structural variation of 2 has been
presented by Salle´ and co-workers (3),10 as well as by
Eschegoyen and co-workers (4).11
All three variations have been evaluated as electrochemical
probes for different anions. Fujihara et al. have demonstrated
that multiple short chain alkylthiol substituents can create
highly stable SAMs (5).12
Other applications proposed for SAMs are for instance in
tunneling devices.13
In order to be able to create a SAM that could work as an
organic thin film field-effect transistor (OTF-FET) with
{ Electronic supplementary information (ESI) available: Characteriza-
tion data for the new compounds. See http://www.rsc.org/suppdata/jm/
b3/b310260b/
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 1 – 8 5
8 1

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probably enhanced by a long upper alkyl chain, which also may
self-assemble and form a stabilizing layer ‘‘above’’ the TTF
p-stacking layer.
(iii) The two lower sulfur substituents should facilitate a
more extended overlap between adjacent tetrathiafulvalenes,
leading to higher charge carrier mobility.
(iv) The alkyl thiol chains are rather separated by the TTF-
units, which can create a void in the SAM. This ‘‘space’’
perhaps has to be filled with additional free alkylthiols to
develop a more uniform layer but in turn leading to more
complicated SAM-studies.
The SAM-candidates of type 6 exist in equilibrium between a
cis and trans diastereomer in the presence of acids.14 This can
also make the kinetics of monolayer formation more difficult to
study, but it might be possible to overcome this by performing
the self-assembly under equilibrium conditions (low pH and
elevated temperatures).
We present here the synthesis and full characterization of the
self-assembly candidates, as well as some preliminary results on
film formation.
Fig. 1 Attempted idealised TTF-SAM structure.
Results and discussion
appreciable charge carrier mobility, we should design a system
that is able to form a stable SAM where a defined vertical
region (layer) consists of a relatively densely packed electro-
active p-system, e.g. TTF (Fig. 1).
The distances, both between the p-system and the surface, as
well as ‘‘above’’ the electroactive region, should be possible to
vary by the chosen synthesis. These regions should consist of
alkyl- (sp3) chains, in order to isolate the conducting layer from
the surface.
We envisaged that a tetrathiafulvalene system that could
form a stable SAM should have at least two alkylthiol ‘‘legs’’ as
well as considerable alkyl substitution ‘‘above’’ the TTF-layer.
It was also found desirable to avoid statistic phosphite
couplings in the synthesis of unsymmetrical tetrathiafulvalenes.
We therefore decided to synthesize tetrathiafulvalenes of
type 6, which we anticipated to show several advantages but
also a few drawbacks:
To create structures that would self-assemble, our first
consideration was to choose a rather long alkylthiol chain
that would attach to the substrate and preferably form an
electronically insulating layer between the TTF units and the
surface. In turn we could vary the R-group in 6 freely, with
different alkyl chain lengths, to see if we could achieve more
densely packed structures.
The synthetic sequence, which is described in Scheme 1, is a
modification of a previously published procedure.15
The a-brominated carboxylic acid underwent a substitution
reaction when treated with the sodium salt of N,N-dimethyl-
dithiocarbamic acid, to give 8a–i in 60–93% yield. The
crystallinity varies with the chain length of the R-substituent.
Compounds 8a–i were, without further purification, dissolved
in acetone and treated with acetic anhydride and triethylamine.
The presumed intermediate is a 4-oxyl mesoion, which
underwent a [1,3]-dipolar cycloaddition with simultaneous
release of carbonyl sulfide when carbon disulfide was added to
the solution. The products 9a–i were obtained as yellow
mesoionic precipitates in 40–96% yield. These products are
stable at room temperature and can be stored at ambient
temperature and atmosphere for several years without
significant decomposition. Alkylation of 9a–i at the thiolate
functionality was carried out in acetone with 1,12-dibromo-
dodecane in excess. The reaction proceeded smoothly at reflux
in less than 6 h to yield 4-dodecylthio-1,3-dithiolium salts 10a–i
in 72–99% yield. Reduction of 10a–i with sodium borohydride
in ethanol16 to the corresponding amines 11a–i (61–91% yield)
followed by deamination by treatment with concentrated
(i) The synthesis of 6 should allow relatively straightforward
strategies for satisfying the demand for easy variation of the
two different alkyl chains, as well as being a robust and reliable
synthesis scheme.
(ii) The double alkyl thiols will render SAM stability,
Scheme 1 a: R ~ CH3, b: R ~ C2H5, c: R ~ C3H7, d: R ~ C4H9, e: R ~ C6H13, f: R ~ C10H21, g: R ~ C12H25, h: R ~ 1-naphthyl, i: R ~
2-naphthyl. Reagents and conditions: (i) Me2NCS2Na, EtOH, 0 uC; (ii) Ac2O, Et3N, CS2, rt, acetone; (iii) Br(CH2)12Br, acetone, reflux; (iv) NaBH4,
EtOH, rt; (v) HPF6, H2SO4, 0 uC; (vi) Et3N, CH3CN, rt, N2; (vii) (Me3Si)2S, (n-Bu)4NF, THF, 210u C.
8 2
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 1 – 8 5

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Table 1 Electrochemical data for TTFs 13a–ia
E1/V
E2/V
DE ~ (E2 2 E1)/V
13a
13b
13c
13d
13e
13f
13g
13h
13i
0.43
0.42
0.41
0.42
0.42
0.42
0.42
0.49
0.50
0.78
0.78
0.77
0.78
0.77
0.78
0.77
0.78
0.81
0.36
0.36
0.36
0.36
0.36
0.36
0.36
0.29
0.30
a 1 mM (n-Bu)4NClO4 (0.1 M) in CH2Cl2, scan rate 500 mV s21
E vs. SCE.
,
Fig. 2 FAB-MS of 6b.
investigated together with film forming properties. All
measured cyclic voltammograms for compounds 13 exhibit
two reversible one electron transfer processes corresponding to
the successive formation of the cation radical (E1) and dication
(E2) as expected for TTFs. The 1-naphthyl-substituted TTF
13h even showed a third oxidation as illustrated in Fig. 3. This
may be due to oxidation of the naphthalene ring system, no
corresponding reduction was however observed, indicating a
more reactive, unstable oxidized species. As was expected, the
values of the oxidation potentials and the difference DE ~
E2 2 E1 were comparable with previously reported results for
TTF (1), E11/2 ~ 0.33 and E21/2 ~ 0.71.20 What could be noted
was that the naphthyl-substituted TTFs showed a slight
positive shift of E1, compared to the alkyl-substituted TTFs,
but similar potential of E2, that gave a 60–70 mV decrease of
DE.
The film forming ability of TTF-thiols 6d and 15g were
tested. The n-butyl-substituted TTF 6d were immersed in
toluene solution for a month, at room temperature and 70 uC,
in the absence and presence of methanesulfonic acid. The
corresponding dodecyl derivative 15g were also immersed for one
month. The results from ellipsometry and advancing contact
angles are shown in Table 2. Both TTFs gave monolayers of
good and reproducible quality. It is clear from Table 2 that
sulfuric acid and hexafluorophosphoric acid, yielded 1,3-
dithiolium hexafluorophosphates 12a–i. These compounds
are highly hygroscopic and decompose quickly in air and
were therefore reacted without further purification with
triethylamine in dry acetonitrile, which gave TTFs 13a–i in
fair yields after purification. On treating 13 with hexamethyl-
disilathiane and (n-Bu)4NF, according to a recent procedure,17
target compounds 6a–e were obtained. Due to the low
solubility of these sticky products, they could not be purified
by column chromatography and analyses of these compounds
were not trivial. The structures were not suitable for NMR-
characterization due to the low solubility in most organic
solvents. The issue was further complicated by FAB-MS and
MALDI-TOF spectra of products 6b–e, since they showed
predominantly the (M 2 2)1 ion (Fig. 2 shows the FAB-
spectrum of 6b), which would suggest the presence of a disulfide
(inter- or intramolecular) or other insaturations. However,
treatment of 6b with DTT (dithiothreitol) which would have
reduced all disulfide groups, left the mass spectrum unaffected.
The IR spectra showed a weak absorption at 2522 cm21
,
indicating the presence of an S–H bond. Our speculation of the
FAB-MS behavior is that the compounds interact (self-
assembles) with a metal surface in the mass spectrometer,
which can cause an oxidation to the dehydrogenated form.
Moreover, elemental analysis confirmed that bromine had been
substituted with sulfur. For TTFs 13e–i we used another
approach to achieve our ‘‘target’’ molecules.18 Compounds
13e–i were converted to the corresponding disulfides by the
reaction with
a
nucleophilic sulfur transfer agent 1419
according to Scheme 2, yielding target TTFs 15e–i as cyclic
disulfides. The products, which precipitated from DMF, were
filtered off and washed with diethyl ether to give products with
a sticky apperance.
The electrochemical behavior of TTFs 13a–i has been
investigated by cyclic voltammetry (CV) in dichloromethane
and the data are collected in Table 1.
Due to the low solubility of TTFs 6a–d and 15e–i in
dichloromethane at room temperature no cyclic voltammo-
grams could be taken from solution but this will be further
Fig. 3 Cyclic voltammograms of TTFs 13a, 13h and 13i.
Table 2 Ellipsometric thickness (d), advancing (ha) and receding (hr)
contact angles of water of TTF-thiol molecules on gold
˚
D/A
TTF Sample
ha/u
hr/u
6db
19.6 ¡ 0.3
19.2 ¡ 0.2
15.0 ¡ 0.3
16.0 ¡ 0.4
15.9 ¡ 0.4
18.0 ¡ 0.3
75
69
76
77
74
63
63
55
67
65
49
53
6da,b
15gb
6dc
6dd
6da,e
a In the presence of 0.5 mmol of (10 mol%) MeSO3H. b Incubated for
4 weeks (3 weeks at 70 uC 1 1 week at rt). c Incubated for 1 week at
rt. d Incubated for 48 h at 70 uC. e Incubated for 24 h at 70 uC.
Scheme 2 Reagents and conditions: (i) 14, DMF, 70 uC.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 1 – 8 5
8 3

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longer times and more acidic conditions do not alter the
thickness of the film to any substantial degree. It is also clear
that the film thickness is less than would be anticipated from a
stretched molecular arrangement as in Fig. 1. Interestingly
enough the longer alkyl chain on TTF 15g, does not result in a
higher monolayer thickness.
their incubation, and the collected average value of the refractive
index was later used in a model ‘‘ambient/organic film/gold’’,
assuming an isotropic, transparent organic layer with a refractive
index of n ~ 1.5. The film thickness is calculated as an average of
measurements at five different spots on the sample of each
compound.
The advancing contact angle of water on both type of
monolayers indicates a more hydrophobic than hydrophilic
surface due to the alkyl chains present on the topmost part of
the organic film. These values do not indicate a high surface
hydrophobicity, suggesting that the ring structure may some-
how be on top of the film and partially influences the
hydrophilicity of the surface. The advancing contact angle
value is comparable to that of the HS(CH2)11OCH3 monolayer
(ha ~ 74u) when adsorbed on gold.21 For comparison, the
advancing contact angle value of HS(CH2)16CH3 is known to
be 115u.21 The high hysteresis value (difference between
advancing and receding contact angle) also indicates a more
heterogenous outermost layer at the micron scale.
The thickness and contact angle measurement results suggest
a film formation of 6d and 15g incubated for a month having
densely packed alkylthiol chains on the surface, and with ring
structures and alkyl tails exposed on the topmost part of the
layer. The surface region is not well ordered, with the alkyl
chains to some degree penetrating down into the alkylthiol
region. A more definitive description and characterization of
these monolayers will be possible after IR and XPS-studies
which are now in progress.
Contact angle goniometry
Contact angles were measured with Rame´-Hart NRL 100
goniometer, in air, i.e. without control on the humidity, using
fresh MilliQ water. At least five measurements of advancing
and receding contact angle were done per sample.
Syntheses
NMR spectra were recorded on a Bruker AM 400 (400 MHz)
or Bruker DMX 500 (500 MHz) spectrometer, J values are
given in Hz. Assignment of 1H NMR spectra could not be
performed for the TTFs 6 and 15 due to isomeric mixtures.
Mass spectra were recorded on a Finnigan SSQ 7000 or a
Bruker Reflex III MALDI-MS instrument. The starting
materials are commercially available except for 7h and 7i,
which were obtained via bromination of the corresponding
1- and 2-naphthylacetic acids. Piperidinium tetrathiotungstate
(14) was synthesized according to a literature procedure.20
Where stated, the silica gel was deactivated with a 2 M HCl
solution with subsequent drying at rt before use. THF was
distilled from sodium/benzophenone, all other solvents were
either distilled or were HPLC grade.
Conclusions
8: General procedure. 0.10 mol of a-bromocarboxylic acid
was dissolved in 150 ml EtOH and 0.15 mmol of the sodium
salt of N,N-dimethyldithiocarbamic acid was added in portions
while cooling the mixture in an ice-bath. After stirring over
night, the mixture was poured on 200 ml ice–water and
acidified with 25 ml conc. HCl. The product was filtered off and
dissolved in CH2Cl2, washed with water, and dried over
MgSO4. The solvent was evaporated, yielding 8.
The synthesis of a new series of tetrathiafulvalenes with either
double alkylthiol (6a–e) or alkyldisulfide (15e–i) substitution
has been accomplished. A versatile synthetic procedure is
presented which allows a variety of substituents to be present.
TTFs 6d and 15g gave relatively dense packed monolayers on
gold, with inner alkyl chains more organized and outer alkyl-
chains randomly organized with gauche defects.
9: General procedure. 20 mmol of 8 was dissolved in 30 ml
dry acetone and treated with 6 ml Ac2O, 6 ml Et3N and 6 ml
CS2, successively, separated by a couple of minutes. At first a
yellow color developed and as the CS2 was added, the color
darkened while carbonyl sulfide escaped (CAUTION: toxic!)
and after a few minutes orange yellow crystals of the product
precipitated. After 6 h the product was filtered off and washed
with cold acetone, yielding 9.
Experimental
Electrochemical measurments
Cyclic voltammetry was carried out at ambient temperature in
CH2Cl2 at 500 mV s21 with (n-Bu)4NClO4 (0.1 M) as
electrolyte and measured vs. a SCE reference electrode. The
half-wave potential for the ferrocene–ferrocenium couple was
found to be 0.49 V in our system.
10: General procedure. 49 mmol of 9 was refluxed in acetone
with 245 mmol of 1,12-dibromododecane until the yellow color
faded. After cooling to room temperature 500 ml hexane was
added and the flask put in a freezer over night. The white
precipitate formed was filtered off and washed with acetone
and Et2O to remove traces of starting materials, affording 10.
Sample preparation for self-assembly
˚
The substrate used is a 2000 A thick electron beam evaporated-
˚
polycrystalline gold layer on Si(100) with 20–25 A thick Ti as
adhesion promoter. Prior to incubation, the gold samples were
carefully cleaned to remove organic contamination. The
substrates were incubated in the following toluene-based
solutions (200 mM): 6d, 6d–10 mol% MeSO3H and 15g. The
immersion periods were: 24 h, 48 h, 1 week and 1 month. The
samples were kept in the dark and at either room temperature
or heated at 70 uC during the incubation period. Ultrasonic
rinsing with toluene for 5 min and N2 blow-drying of samples
were carried out directly before mounting in measurement
apparatus.
11: General procedure. 3 mmol of 10 was dissolved in 30 ml
99% EtOH and cooled in an ice-bath to 0 uC. NaBH4 was added
over a period of 5 min. After 1 h 30 ml a 10% solution of NH4Cl
and 60 ml Et2O were added and the layers were separated. The
organic phase was then washed with water, dried over MgSO4,
filtered, and evaporated to dryness, yielding 11.
12: General procedure. 11 was added to 7.5 ml ice-cold conc.
H2SO4. After 30 min of stirring the mixture was poured over
25 ml ice containing 1.5 ml 60% HPF6. The product was filtered
off and dissolved in 10 ml CH2Cl2. The filtrate was extracted once
with CH2Cl2 and the two organic phases were combined, dried
over MgSO4, filtered, and evaporated in vacuum to yield 12.
Ellipsometry
Single-wavelength ellipsometry was performed using an auto-
matic Rudolph Research AutoEL ellipsometer with He–Ne laser
light source, l ~ 632.8 nm, at an angle of incidence of 70u. The
freshly cleaned gold sample substrates were measured prior to
8 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 1 – 8 5

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3
4
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13: General procedure. 1 equiv. of 12 was dissolved in dry
CH3CN and Et3N (1.2 equiv.) was added dropwise to the
stirred solution. At first a red brown color was observed which
rapidly faded as oily droplets of the red product, 13 appeared.
After 15 min the solvent was decanted and the product was
washed with EtOH and dried in vacuum. The product was
purified by chromatography on a deactivated silica gel column,
where 13 was obtained after evaporation of solvent.
5
6
G. G. Abashev, E. V. Shklyaeva and V. S. Russkikh, Russ. J. Org.
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6: General procedure. To a stirred solution of 13 (1 equiv.) in
freshly distilled THF (0.5 M) cooled in an ice-bath to 210 uC,
hexamethyldisilathiane (2.4 equiv.) was added dropwise,
followed by a 1.1 M solution of (n-Bu)4NF in THF (2.2 equiv.).
The reaction was monitored by TLC as the mixture was
allowed to warm to room temperature and the color darkened.
Once all starting 13 had reacted, the reaction mixture was
diluted with Et2O and washed with saturated aqueous
ammonium chloride. The organic phase was dried over
MgSO4 and the solvent evaporated, yielding 6 as a sticky
orange product. An analytically pure sample could be obtained
by washing the product with Et2O. Attempts to purify the
product by column chromatography have not been successful.
(a) A. Ulman, An Introduction to Utrathin Films, Academic Press
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7
8
9
J. H. Scho¨n, Adv. Mater., 2002, 14, 323.
C. M. Yip and M. D. Ward, Langmuir, 1994, 10, 549.
A. J. Moore, L. M. Goldenberg, M. R. Bryce, M. C. Petty,
J. Moloney, J. A. K. Howard, M. J. Joyce and S. N. Port, J. Org.
Chem., 2000, 65, 8269.
10 G. Trippe´, M. Oc¸afrain, M. Besbes, V. Monroche, J. Lyskawa,
F. Le Derf, M. Salle´, J. Becher, B. Colonna and L. Echegoyen,
New J. Chem., 2002, 26, 1320.
15: General procedure. 13 (1 equiv.) was dissolved in dry DMF
(0.2 M) and treated with 1.25 equiv. of piperidinium tetra-
thiotungstate (14). The yellow reaction mixture darkened during
the addition and turned orange during heating (70 uC) over night.
The solvent was decanted and the sticky red precipitate was
washed with Et2O, followed by THF. The residual solvent
was evaporated, yielding 15 as a sticky red material that was
characterized by either HRMS or elemental analysis.
11 (a) S. Liu, H. Liu, K. Bandyopadhyay, Z. Gao and L. Echegoyen,
´
J. Org. Chem., 2000, 65, 3292; (b) M. A. Herranz, B. Colonna and
L. Echegoyen, PNAS, 2002, 99, 5040.
12 H. Fujihara, H. Nakai, M. Yoshihara and T. Maeshima, Chem.
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13 M. Dorogi, J. Gomez, R. Osifchin, R. P. Andres and
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Acknowledgements
16 E. Klingsberg, J. Am. Chem. Soc., 1964, 86, 5290.
17 J. Hu and M. A. Fox, J. Org. Chem., 1999, 64, 4959.
18 Only starting material could be recovered from the reaction of
either 13f or 13g with hexamethyldisilathiane.
19 P. Dhar, N. Chidambaram and S. Chandrasekaran, J. Org. Chem.,
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Financial support from the Swedish Research Council as well
as the Foundation for Strategic Research is gratefully
acknowledged. The assistance of Anna Herland and Andreas
Woldegiorgis is also gratefully acknowledged.
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