Chiral molecular tapes from novel tetra(thiafulvalene-crown-ether)- substituted phthalocyanine building blocks

Article abstract of DOI:10.1039/b416034g

A tetra(thiafulvalene-crown-ether) phthalocyanine self-assembles into helical tapes nanometers wide and micrometers long. Formation of these scrolled molecular architectures is a first for phthalocyanine fibres and shows potential as a novel conducting ma

Full text of DOI:10.1039/b416034g

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Chiral molecular tapes from novel tetra(thiafulvalene-crown-
ether)-substituted phthalocyanine building blocks{
Joseph Sly,a Peter Kasa´k,a Elba Gomar-Nadal,b Concepcio´ Rovira,b Luc´ıa Go´rriz,a Pall Thordarson,a
David B. Amabilino,*b Alan E. Rowan*a and Roeland J. M. Nolte*a
Received (in Cambridge, UK) 18th October 2004, Accepted 18th November 2004
First published as an Advance Article on the web 15th December 2004
DOI: 10.1039/b416034g
aryl dibromide 4 into the phthalonitrile precursor 5 did not
proceed as smoothly as could be expected. The isolated yield of 5
(31%) was relatively low in view of the success of the preceding
steps and similar complications with molecules of this type have
been reported in the past.13 However, workable quantities were
obtained and the phthalonitrile 5 was able to be converted
smoothly into the target phthalocyanine 6a.
Molecular modelling experiments14 conducted on Pc 6a
predicted an overall planar geometry for the global energy
minimised structure, with the crown ether units being able to
adopt a boat like conformation. This confirmed the potential for
6a to form stacked aggregates, through the expected Pc–Pc p–p
interactions, as previously observed with other crown-ether Pc
analogues.9 Additional studies, prompted by previous reports of
strong intermolecular TTF–Pc interactions,13 investigated the
effect of a competition between intermolecular Pc–Pc, Pc–TTF
and TTF–TTF p–p interactions. It appeared that if the latter
interactions are fully available, then traditional, columnar Pc
stacking should eventuate. However, should this interaction be
sterically limited (a possibility indicated by molecular modelling)
then Pc–TTF interactions could determine the overall architecture,
one potentially different to normal columnar stacks.
A
tetra(thiafulvalene-crown-ether) phthalocyanine self-
assembles into helical tapes nanometers wide and micrometers
long. Formation of these scrolled molecular architectures is a
first for phthalocyanine fibres and shows potential as a novel
conducting material.
Fundamental to the development of molecular electronic devices is
the efficient construction of effective ‘‘molecular wires’’ and
conductive architectures. These architectures are synthetic mate-
rials of nanometer sized dimensions, capable of directional, long-
range electron-transport. While the development of conducting
polymers has shown the potential role for such ‘‘synthetic metals’’
in emergent technology, challenging issues such as processability
and design flexibility remain.1 The use of molecular self-assembly
to construct functional materials is now well-established and
presents one possible direction in the exploration of readily
accessible conducting materials. The versatility of chemical
synthesis used in forming the individual components for these
self-assembled materials provides for an important control over
both the electronic and structural features of the resulting
materials. Two synthetic components which have attracted above
average interest in this field are phthalocyanines (Pcs)2–4 and
tetrathiafulvalenes (TTFs),5–7 both of which have excellent
conductive properties. Pcs appended with crown-ether moities
substituted with long alkoxy chains readily self-assemble into long
fibres, possessing the multiple potentials of electron conduction,
ion transport and liquid crystallinity.8–11 TTFs have featured
prominently in the area of ‘‘synthetic metal’’ research. In order to
make functional Pc-based fibres, TTF units have been incorpo-
rated into a crown-ether-Pc building block, to give 6, with the aim
of endowing the resulting self-assembled material with potentially
interesting electronic and structural properties.
Aggregation of 6a was observed from UV-Vis studies (Fig. 1) in
which it was determined that, at 1 : 4 v/v of CHCl3 / MeOH, 6a
could be induced into aggregates as shown by the broadening and
blue shifting of the major absorbance bands (Q bands at 663 and
700 nm A 630 nm). More importantly, gelation of 6 was also able
to be induced by the slow addition of dioxane to a chloroform
solution at 5 uC. In its gelated state 6a was observed [by
transmission electron microscopy (TEM)] to be composed of fibres
up to several micrometers in length, which is a length consistent
with the stacking of around 100,000 molecules (Fig. 2). This length
is also in accordance with previous studies on crown-ether
analogues of 6a which lacked the TTF moieties. These analogues
have been previously observed to form fibres and even twisted
bundles of fibres driven by the co-facial, linear stacking behaviour
of the phthalocyanine moieties.9
Phthalocyanine 6a was generated from 4,5-dibromocatechol 18
in a five step procedure (Scheme 1).{ Reaction of 1 with bis-2-
chloroethyl ether followed by halogen exchange provided the 4,5-
dibromobenzo-‘‘half crown’’ structure 2 in good yield. The
coupling of 2 with the bis-protected TTF compound 312 was
effected in a one-pot deprotection / coupling step with the 4,5-
dibromobenzo-TTF-crown-ether 4 being isolated in pure form. It
was found that the penultimate step involving conversion of the
However, further inspection of the fibres formed from the TTF
containing Pc 6a, revealed additional features indicating a
dramatic deviation from the initially expected linear stacking
behaviour. The TEM images reveal the presence of unusual, very
thin bilayer-type structures (Fig. 2a). The width of these long, tape-
like structures formed from 6a is typically of the order of 20 nm.
This width corresponds to approximately 5 times the calculated
width of the individual molecule 6a and is an immediate deviation
from all previously observed crown-ether Pc stacking behaviour.
{ Electronic supplementary information (ESI) available: Synthetic
characterisation, general experimental procedures together with a more
in depth descriptive analysis of the self-association behaviour of 6. See
http://www.rsc.org/suppdata/cc/b4/b416034g/
*amabilino@icmab.es (David B. Amabilino)
a.rowan@science.ru.nl (Alan E. Rowan)
r.nolte@science.ru.nl (Roeland J. M. Nolte)
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Chem. Commun., 2005, 1255–1257 | 1255

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Scheme 1 i Bis(2-chloroethyl)ether, K2CO3, D then NaI, acetone, D (two steps, 68% overall). ii CsOH?H2O, MeOH, DMF, RT, 53%. iii CuCN, DMF,
D, 31%. iv Hexanol, n-butyllithium, D, 11%. (6b Formed from CuCl2 dimethylaminoethanol, D and the diiminoisoindoline derivative of 5, 9% overall
conversion from 5.)
the presence of any of the expected, columnar type Pc fibres in
these TEM images. From these observations and initial modelling
experiments, it is suggested that an unusual combination of
multiple TTF–TTF and TTF–Pc interactions can dominate (over
Pc–Pc interactions) in the self-association of Pc 6a leading to the
eventual formation of scrolled molecular architectures (Fig. 3 and
Fig. 4). The observed equal distribution of both left and right
helices is as expected, given the absence of any chiral bias to the
structure of 6a.
Cyclic voltametry (CV) studies on 6b showed the redox
properties expected for a combination of Pc and TTF moieties.
K
The first (E1 ) and second (E2 ) TTF oxidations were
K
K
slightly raised (E1 5 0.75, E2 5 1.25 E/V versus Ag/AgI,
K
Fig. 1 UV-Vis spectra of 6a (CHCl3) with the effect of increasing MeOH
K
K
compared to E1 5 0.65, E2 5 1.01 E/V for 3) obscuring
the expected, weaker Pc oxidation. These oxidation levels are
in agreement with other reported Pc–TTF structures which
addition (shown inserted) indicating induced aggegation of the molecule.
Fig. 2 TEM images of the gelated state of 6a showing a) thin Pc tapes
(left ) and b) an equal distribution of both left and right helical structures,
nanometers wide and micrometeres in length (right). Scale bar 5 200 nm.
Fig. 3 Molecular models14 (Pc moiety in green, crown-ether in red, TTF
in yellow, alkyl chains in grey) showing the difference between the two
modes of stacking of 6a with schematic of resulting bulk architecture: a)
TTF–TTF and Pc–Pc interactions dominate resulting in columnar stacks
(not observed). b) Pc–TTF and TTF–TTF interactions dominate resulting
in initial bilayer type structures which scroll into helical tapes (observed).
Of even greater interest is that, concurrently observed with these
tapes, is a second type of fibre immediately identifiable as
possessing a three-dimensional detail to its structure. These fibres
appear helical in nature, micrometers in length and have a width of
approximately 15–25 nm (Fig. 2b). It was not possible to identify
1256 | Chem. Commun., 2005, 1255–1257
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cation radical species of the TTF moieties. The formation of a
TCNQ2 / TTF+ charge-transfer complex was confirmed by the
FT-IR spectrum which showed the nitrile stretch of the TCNQ
radical anion (observed at 2179 cm21 compared to the neutral
state of 2223 cm21). Doping of 6b in CH2Cl2 with I2 produced a
similar effect in the UV-Vis spectrum as with the TCNQ example
in DMSO. In this case, an observed lowering of the Q-band
absorbances suggests increased aggregation and may indicate a
preferred state for the multiple-oxidised species.
Currently, the incorporation of chirality into the basic structure
of 6a, investigations into subsequent controlling effects upon the
direction of scroll formation and detailed electrochemical evalua-
tion of these novel, self-assembled structures are underway and will
be reported in due course.
We thank the European TMR Phthalocyanine Network
(HPRN-CT-2000-00020), DGI Spain (BQU2003-00760) and
COST (Action D19 WG 004/01) for support of this research.
Joseph Sly,a Peter Kasa´k,a Elba Gomar-Nadal,b Concepcio´ Rovira,b
Luc´ıa Go´rriz,a Pall Thordarson,a David B. Amabilino,*b
Alan E. Rowan*a and Roeland J. M. Nolte*a
aRadboud University Nijmegen, Department of Organic Chemistry, NSR
Center, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands.
E-mail: a.rowan@science.ru.nl; r.nolte@science.ru.nl;
Fax: +31 24 3652929; Tel: +31 24 3652143
bInstitut de Cie`ncia de Materials de Barcelona (CSIC), Campus
Universitari, 08193, Bellaterra, Spain. E-mail: amabilino@icmab.es;
Fax: +34 935 805 729; Tel: +34 935 801 853
Notes and references
1 A. G. MacDiarmid, Angew. Chem. Int. Ed., 2001, 40, 2581.
2 Phthalocyanines, Properties and Applications, ed. C. C. Leznoff and
A. B. P. Lever, VCH, New York, 1989–1993.
3 M. Hanack and M. Lang, Adv. Mater., 1994, 6, 819.
4 M. J. Cook, J. Mater. Chem., 1996, 6, 677.
5 M. R. Bryce, Adv. Mater., 1999, 11, 11.
6 J. L. Segura and N. Martin, Angew. Chem. Int. Ed., 2001, 40, 1372.
7 J. Becher, J. O. Jeppesen and K. Nielen, Synth. Met., 2003, 133–134,
309.
8 C. F. van Nostrum, S. J. Picken, A.-J. Schouten and R. J. M. Nolte,
J. Am. Chem. Soc., 1995, 117, 9957.
9 C. F. Van Nostrum, Adv. Mater., 1996, 8, 1027.
Fig. 4 (a) Proposed arrangement of Pc 6 monomers within the observed
thin tape-like structures (colours are as described in Fig. 3). Within the
tape, Pc–TTF interactions (b) are the dominant intermolecular interactions
within the tape which form offset columns (c) which run lengthwise within
the tape. These columns are associated within the tape through weaker
TTF–TTF interactions (d). These interactions define the tape width (e).
Estimation based on modeling and an observed width of 20 nm suggests
approximately five rows of columns within the tapes. The number of
columns within the tape is dependent upon the exact offset relationships
shown in (b) and (d).
10 H. Engelkamp, S. Middelbeek and R. J. M. Nolte, Science, 1999, 284,
785.
11 P. Samori, H. Engelkamp, P. de Witte, A. E. Rowan, R. J. M. Nolte
and J. P. Rabe, Angew. Chem. Int. Ed., 2001, 40, 2348.
12 Compound 3 was formed from the coupling of 4,5-bis(cyano-
ethylthio)-1,3-dithiol-2-one and 4,5-bis(dodecylthio)-1,3-dithiole-2-
thione in the presence of P(OMe)3 at reflux for 3 h. See also:
N. Svenstrup, K. M. Rasmussen, T. Kruse and J. Becher, Synthesis,
1994, 809; P. Wu, G. Saito, K. Imeda, Z. Shi, T. Mori, T. Enoki and
H. Inokuchi, Chem. Lett., 1986, 441; K. B. Simonsen and J. Becher,
Synlett, 1997, 1213.
13 C. Wang, M. R. Bryce, A. S. Batsanov, C. F. Stanely, A. Beeby and
J. A. K. Howard, J. Chem. Soc., Perkin Trans. 2, 1997, 1671.
14 Molecular modeling was conducted using Spartan 04 for
windows (‘Wavefunctions Inc.’ California, 2004) using the MMFF
force field.
showed little change in the TTF potentials when linked to the Pc
macrocycle.13
To investigate the potential of these structures to act as a
conducting architecture, doping studies on 6b were conducted
using 7,7,8,8-tetra-cyanoquinodimethane (TCNQ) in DMSO. This
produced a new absorption band in the UV-Vis spectrum at
750 nm which corresponds to the SOMO–LUMO transition of the
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1255–1257 | 1257