Thiol functionalized diphenyl bithiophene for monomolecular bistable layers

Article abstract of DOI:10.1080/10426507.2010.538455

Diphenyl bithiophene derivatives (DPBTs), known for their electrical bistable behavior, have been used in bulk resistive memory cells. In this article, we present the design and synthesis of a ω-alkyl thiol functionalized 3,3′-diphenyl 2,2′-bithiophene; s

Full text of DOI:10.1080/10426507.2010.538455

Phosphorus, Sulfur, and Silicon, 186:1298–1302, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.538455
THIOL FUNCTIONALIZED DIPHENYL BITHIOPHENE
FOR MONOMOLECULAR BISTABLE LAYERS
Eleonora Valeria Canesi1 and Chiara Bertarelli1,2
1Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia,
Milano, Italy
2Dipartimento di Chimica, Materiali e Ing. Chimica “G. Natta”, Politecnico di
Milano, Milano, Italy
Abstract Diphenyl bithiophene derivatives (DPBTs), known for their electrical bistable be-
havior, have been used in bulk resistive memory cells. In this article, we present the design and
synthesis of a ω-alkyl thiol functionalized 3,3ꢁ-diphenyl 2,2ꢁ-bithiophene; such functionaliza-
tion confers the ability to anchor on a metal surface, which may be conveniently applied for
the development of monomolecular bistable devices.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.
Keywords Diphenyl bithiophene; electrical bistability; thiol
INTRODUCTION
In the last few years, DPBT derivatives have been proposed as active molecules for
optoelectronics and organic electronic devices. Indeed, by taking advantage of the versa-
tility of thiophene chemistry,1 the properties of nonlinear optics,2 light emission,3 charge
photogeneration,4 and electrical bistability5,6 have been addressed by suitable molecular
design.
Regarding the last application, the scalability of the device dimensions down to the
molecular level is considered as a challenge, and the ideal limit is represented by a memory
cell based on single bistable molecules.7 To this aim, bistability must arise from a peculiar
molecular feature, rather than from spurious effects related to other device components. As
these phenomena often characterize memory devices,8 the proof of a molecular origin for
bistable systems has been9 and is still a topical subject.
For DPBTs, several findings have been presented to support the active role of the
conjugated molecules in giving the peculiar bistable effect,5,6 and a theoretical model about
the key parameters that govern the bistable behavior for this class of compounds has been
recently proposed.10
Received 10 August 2010; accepted 4 November 2010.
This work was partly funded by Fondazione Cariplo through the project “Nanostructured MATerials for
innovative HYbrid Solar cells—MATHYs” (2010–2011).
Address correspondence to Eleonora Valeria Canesi, Center for Nano Science and Technology @Polimi,
Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy. E-mail: eleonora.canesi@iit.it
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THIOL FUNCTIONALIZED DIPHENYL BITHIOPHENE
1299
A characterization at the molecular scale would definitively confirm the active role
of the conjugated molecule in giving electrical bistability, and besides representing a step
forward on the comprehension of the relationship between the theoretical model and the
experimental findings,11 it opens the way to the development of resistive, reversible, elec-
trically controlled monomolecular-based switching devices.
In this background, a diphenyl bithiophene derivative that is able to anchor onto a
metal surface has been designed, starting from the basic structure of the DPBT molecule
that has shown the best electrical bistable behavior in two terminal devices with a 100-
nanometer thick organic layer.5
RESULTS AND DISCUSSION
Starting from the reference bistable molecule 3,3ꢁ-bis(3,5-di-tert-butyl-4-
methoxyphenyl)-2,2ꢁ-bithiophene5 (labeled as DPBT-a in Figure 1), several possibilities
have been considered to give the target thiol-DPBT.
The position of the thiol functionality was chosen by taking into account the features
of the DPBT molecular structure; indeed, the presence of highly steric demanding sub-
stituted phenyls in the 3-position of thiophenes causes a strong distortion from planarity
of the inter-ring bonds, especially for the bithiophene moiety. As a consequence, in the
neutral compound, the conjugation path turns out to be almost confined to the two phenyl-
thiophenes.3 In the charged species, which is assumed to be related to the ON state of the
memory cells based on DPBT molecules, the structure is less distorted, thus allowing an
enhanced conjugation from one phenyl to the other one through the inner part of the bithio-
phene.6 Accordingly, the thiol functionality was linked to the para-position of phenyl rings,
thus enabling the electrode-to-electrode pathway to match this vertical conjugation. More-
over, due to the trans-conformation that DPBT-a assumes at the solid state,10,12 it appears
that this position is the most convenient for a good intermolecular packing, which can be
exploited to obtain self-assembled monolayers. Indeed, among the widespread number of
contact methods at the molecular scale that have been proposed so far,13,14 self-assembled
monolayers (SAMs) turn out to be simpler and more repeatable to manage with respect to
single molecules, and are considered an interesting alternative for the characterization of
DPBTs.
C6H13
C6H13
CH3
CH3
O
S
O
S
O
S
O
S
t-Bu
t-Bu
S
S
S
S
t-Bu
CH3
O
O
t-Bu
O
O
H12C6
H13C6
H3C
SH
DPBT-a
DPBT-b
DPBT-c
DPBT-SH
Figure 1 Molecular structure of the diphenyl bithiophene derivatives DPBT-a, DPBT-b, DPBT-c, and DPBT-SH.

1300
E. V. CANESI AND C. BERTARELLI
Concerning the nature of the thiol-DPBT spacer, the alkyl chain is the simplest and
most widely used spacer in complex molecular structures. The choice of its length is the
result of a compromise among good flexibility, which allows molecules to self-aggregate,
and entropy effect, which induces disorder into the film. On the contrary, the possible
insulating effect has not been considered as an issue, if limited to reasonable values, as we
are not interested in the absolute conductivity of the matter, but in its variation upon voltage
application; furthermore, a weak electronic interaction between the conjugated core and
the metal contacts is desirable to reduce the interfacial effects, thus enhancing the peculiar
molecular features. An alkyl chain as long as six carbon atoms represents the arrangement
that takes into account all these issues; nevertheless, this parameter can be simply varied in
the synthetic route, so that its influence can be actually analyzed.
In order to maintain the symmetry that characterizes the molecules of this series (see
DPBT-a, DPBT-b, and DPBT-c in Figure 1), two thiol functionalities should be introduced
onto the two phenyl rings; however, a single functionalization has been preferred, as dithiols
are known for showing a poorer repeatability of monolayer deposition15 and can enable
molecules to link both the thiols on the same surface in a bridge-like configuration16
(although this possibility is hardly feasible for the hindered structure under study).
Since the steric hindrance of the tert-butyl groups in the meta-position plays a relevant
role in protecting the para- position of phenols, to gain a better accessibility to the hydroxyl
group, a tert-butyl free analogous molecule is here addressed. Good solubility in common
organic solvents is guaranteed by the alkyl chains in para.
The synthesis of the target molecule DPBT-SH, reported in Scheme 1, consists in the
coupling of two differently functionalized 2-bromo-3-phenyl thiophenes through Kumada
coupling, following the same route that has been recently proposed for the synthesis of
DPBT-a.12 Due to its reactivity, the unprotected thiol cannot be introduced in the first steps
of synthesis; moreover, protecting groups of thiols turn out to be fairly stable to the reaction
environments during bromination and Kumada coupling, or, if stronger, their cleavage
also involves other parts of the molecule.17 As a result, the substitution of phenol with a
ω-bromo alkyl chain has been preferred, being a stable functionalization18 that can be
simply converted into a thiol group with thiourea.19
Therefore, 2-bromo-3-(4-hexyloxyphenyl)-thiophene (5) and 2-bromo-3-(4-(ω-
bromo)hexyloxyphenyl)-thiophene (9) are the two precursors of DPBT-SH; the first
one is obtained through a Suzuki coupling between 3-thiophene boronic acid and
4-bromo-hexyloxy benzene, and subsequent bromination with NBS in chloroform/acetic
acid. As for the latter, an analogous Suzuki coupling, namely between 3-thiophene boronic
acid and 4-bromo phenol, is carried out to yield 3-(4-hydroxyphenyl)-thiophene (7), which
is followed by the functionalization of the free phenol with 1,6-dibromohexane and the
bromination in 2- with NBS in chloroform/acetic acid. The two precursors are then coupled
by Kumada route yielding 3-(4-hexyloxyphenyl)-3ꢁ-(4-(ω-bromo)hexyloxyphenyl)-2,2ꢁ-
bithiophene (10). The target molecule is finally obtained by the reaction of 10 with thiourea
in absolute ethanol.
Concurrently with molecule DPBT-SH, other DPBT derivatives have been prepared,
namely DPBT-b and DPBT-c (shown in Figure 1). By representing a stepwise varia-
tion of the chemical structure from the original bistable molecule DPBT-a to the thiol-
functionalized DPBT, due to the removal of tert-butyl groups and the addition of alkyl
chains in para, respectively, their characterization in bulk devices may indicate whether
the modification of these substituents has an effect on the bistable properties of the or-
ganic matter. DPBT-c is simply obtained as a byproduct of the Kumada coupling to yield

THIOL FUNCTIONALIZED DIPHENYL BITHIOPHENE
1301
O
R
O
R
B(OH)2
, Pd(0)
Br
O
NBS
S
R
1. R=C6H13
2. R=CH3
S
Br
S
3. R=C6H13, yield: 86%
4. R=CH3, yield: 91%
5. R=C6H13, yield: >98%
6. R=CH3, yield: 96%
OH
O
C6H12Br
O
C6H12Br
B(OH)2
, Pd(0)
S
, KOH
NBS
BrC6H12Br
Br
OH
62%
>98%
51%
Br
S
7
S
8
S
9
C6H13
R
O
S
O
S
S
Br
1) Mg
2) 9 (6), Ni(dppp)Cl2
thiourea
S
S
61%
O
R
O
O
5. R=C6H13
6. R=CH3
H12C6
R1
SH
10. R=C6H13, R1=C6H12Br, yield: 38%
DPBT-b. R=R1=CH 3, yield: 65%
DPBT-c. R=R1=C 6H13
DPBT-SH
Scheme 1 Synthetic route for DPBT-SH and for DPBT derivatives in general.
DPBT-SH, while DPBT-b is synthesized by following the same synthetic route of DPBT-
a, starting from 2-bromo-3-(4-methoxyphenyl)-thiophene (6). As for DPBT-b, it should
be noted that a solubility in chloroform of some tens of mg/ml, necessary to deposit
100-nanometer thick films, is gained even without alkyl substituents, probably due to the
strongly distorted geometry of the molecule at the ground state. Indeed, the analogous
species bearing the phenols in the 5 and 5ꢁ positions of the thienylene core (thus being less
steric demanding) shows a very poor solubility.
CONCLUSIONS
A thiol functionalized 3,3ꢁ-diphenyl-2,2ꢁ-bithiophene has been designed, and a suit-
able synthetic route has been proposed. The ability of DPBT-SH to anchor onto a metal

1302
E. V. CANESI AND C. BERTARELLI
surface will allow its electrical investigation at the molecular level, aimed at confirming if
the switching behavior of DPBT molecules in bulk devices is maintained also at the molec-
ular scale, with a significant improvement in terms of morphology control and scaling down
with respect to the memory cells previously developed.
EXPERIMENTAL
All steps were carried out under a dry, oxygen-free, argon atmosphere. Unless oth-
erwise specified, all reagents, catalysts, and reagent-grade solvents were commercial. Dry
solvents were commercial, and further maintained over molecular sieves.
1H NMR and 13C NMR measurements have been performed by using a Bruker ARX
400 (TMS as internal standard), mass spectroscopy has been carried out with a Bruker
Esquire 3000 plus, and melting points have been obtained through differential scanning
calorimetry (DSC Seiko 6200).
Details about the synthetic procedures and the compounds characterization are in-
cluded in the Supplementary Materials (available online).
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