A successful chemical strategy to induce oligothiophene self-assembly into fibers with tunable shape and function

Article abstract of DOI:10.1021/ja2014949

Functional supramolecular architectures for bottom-up organic nano- and microtechnology are a high priority research topic. We discovered a new recognition algorithm, resulting from the combination of thioalkyl substituents and head-to-head regiochemistry of substitution, to induce the spontaneous self-assembly of sulfur overrich octathiophenes into supramolecular crystalline fibers combining high charge mobility and intense fluorescence. The fibers were grown on various types of surfaces either as superhelices or straight rods depending on molecular structure. Helical fibers directly grown on a field effect transistor displayed efficient charge mobility and intrinsic 'memory effect'. Despite the fact that the oligomers did not have chirality centers, one type of hand-helicity was always predominant in helical fibers, due to the interplay of molecular atropisomerism and supramolecular helicity induced by terminal substituents. Finally, we found that the new sulfur overrich oligothiophenes can easily be prepared in high yields through ultrasound and microwave assistance in green conditions.

Full text of DOI:10.1021/ja2014949

ARTICLE
pubs.acs.org/JACS
A Successful Chemical Strategy To Induce Oligothiophene
Self-Assembly into Fibers with Tunable Shape and Function
†
†
†
†
‡
Francesca Di Maria, Pasquale Olivelli, Massimo Gazzano, Alberto Zanelli, Mariano Biasiucci,
‡
,||
#
‡,§
#
,†
Giuseppe Gigli, Denis Gentili, Pasquale D’Angelo, Massimiliano Cavallini, and Giovanna Barbarella*
†
#
Istitute for Organic Synthesis and Photoreactivity (ISOF-CNR) and Institute for Nanostructured Materials (ISMN-CNR),
National Research Council, Via Gobetti 101, 40129 Bologna, Italy
‡
NNL-CNR Nanoscience Institute c/o Dip. Fisica Ed. G. Marconi, La Sapienza University, Roma, Italy and Electronical
Engineering Department, Tor Vergata University, Via del Politecnico, 00133 Rome, Italy
NNL-CNR Nanoscience Institute and Dip.Ingegneria Innovazione, Universit ꢀa del Salento, Via Arnesano, 73100 Lecce, Italy
S Supporting Information
b
ABSTRACT: Functional supramolecular architectures for bot-
tom-up organic nano- and microtechnology are a high priority
research topic. We discovered a new recognition algorithm,
resulting from the combination of thioalkyl substituents and
head-to-head regiochemistry of substitution, to induce the
spontaneous self-assembly of sulfur overrich octathiophenes
into supramolecular crystalline fibers combining high charge
mobility and intense fluorescence. The fibers were grown on
various types of surfaces either as superhelices or straight rods
depending on molecular structure. Helical fibers directly grown
on a field effect transistor displayed efficient charge mobility and
intrinsic ‘memory effect’. Despite the fact that the oligomers did not have chirality centers, one type of hand-helicity was always
predominant in helical fibers, due to the interplay of molecular atropisomerism and supramolecular helicity induced by terminal
substituents. Finally, we found that the new sulfur overrich oligothiophenes can easily be prepared in high yields through ultrasound
and microwave assistance in green conditions.
’
INTRODUCTION
types of noncovalent intermolecular interactions—hydrogen
bondings, van der Waals interactions, solvophobic association
between aromatic rings promoting extended πꢀπ stacking—have
been exploited. Results from para-hexaphenylenes bearing groups
Molecular self-assembly is a key technology for the design and
bottom-up fabrication of nano- and microdevices based on soft
materials. Supramolecular structures with controlled shapes,
dimensions, and properties are of interest for multicomponent
systems having defined function in devices for flexible
11
capable of multiple hydrogen bondings or dyads based on
alkyl substituted hexa-peri-hexabenzocoronene and perylene
1
2
13
1
ꢀ6
monoimide and amphiphilic rigid macrocycles, variously sub-
14 15
electronics.
In this framework, great attention is devoted to
stituted tetrathiafulavalenes, and oligoanilines have been re-
ported. Recent research has exploited the gelation properties of
functional molecules to induce the formation of electroactive
the formation of functional nano- and microsized fibers self-
organizing on surfaces which could be employed as interconnect-
ing modules in integrated molecular circuits.
The molecules used as building blocks for such supramole-
cular structures must exhibit relevant functional properties such
as fluorescence, conductivity, photoconductivity, and sensitivity
to external stimuli. However, the molecular properties are not
automatically transferred into the self-assembled superstructures,
which represent novel systems from the point of view of the
functional properties. Aggregation modalities and hierarchical
structuring affect light emission, charge transport, and any other
property in a way that is still poorly understood. No well-
established rules are as yet available to relate the molecular
structure to the properties of self-assembled systems, hampering
the rational design of functional supramolecular architectures.
To induce diverse aggregation modalities, a variety of conju-
gated oligomers and polymers have been synthesized and several
7
ꢀ9
16
fibers. Unfortunately, in most cases, small changes in the choice
of the substrate or experimental conditions lead to distinctly
different growth behavior and functional properties which are
difficult to reproduce. Thus, the fibers have to be grown on the
appropriate substrate and the as grown nano- and microfibers have
to be manipulated and mechanically transferred for integration
1
0
17
into a device.
A crucial step forward would be the availability of molecular
systems capable to reproducibly self-assemble on any type of
surface. Toward this goal, we have explored new molecular design
approaches and synthetic patterns to prepare oligothiophenes,
Received: February 17, 2011
Published: April 28, 2011
r 2011 American Chemical Society
8654
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|J. Am. Chem. Soc. 2011, 133, 8654–8661

Journal of the American Chemical Society
Scheme 1. Ultrasound and Microwave Assisted Synthesis of Sulfur-Overrich Thiophene Octamers Starting from Commercial
ARTICLE
a
3
-Bromo-thiophene
a
(
i) BuLi, S8, Et O; (ii) (CH )3COK, Hex-Br, EtOH; (iii) NBS (1 or 2 equiv), DMF; (iv) borolane, NaHCO , Pd(dppf)Cl , THF/H O; (v)
2 3 3 2 2
0
2
-boropinacolate-2,2 -bithiophene or 2-boropinacolate,5-S-hexyl-2,2-bithiophene or 2-boropinacolate,5-hexyl-2,2-bithiophene, NaHCO , Pd-
3
(dppf)Cl
2 2
, THF/H O.
1
8,19
which are among the most efficient conjugated materials.
Owing to the great polarizability of sulfur electrons,
steps, respectively. All the octamers described in Scheme 1 are
symmetric molecules characterized by an inner core made of two
bithiophene subsystems bearing head-to-head β-hexylsulphanyl
substituents and differing for the type of substitution at both
terminal bithiophene subsystems. Efficient monobromination of
mono-, di-, and quaterthiophenes was achieved in a few minutes
with a single addition of NBS in DMF under ultrasound irradia-
tion, while the formation of the dibromo derivative was negligible.
In the absence of ultrasounds, the monobromination of the same
substrates required the slow stepwise addition of NBS in several
hours or days leading to sizable amounts of dibromo derivative.
Using commercial 4,4,5,5-tetramethyl-1,3,2-dioxa-borolane
(TDB) in a one pot borylation-Suzuki coupling reaction in
ethanol/water under microwave irradiation, the doubling of
the oligomer skeleton was obtained in high yield and free of
byproducts, thus, avoiding long and expensive purifications of the
reaction products. The methodology was successfully extended
to the preparation of the regioregular hexadecamer and, using the
dibromobithiophene with TDB, also of the corresponding poly-
mer (not reported).
20ꢀ22
thio-
phene is an easily deformable heterocycle that in the solid state is
rarely present as a regular pentagon but it is deformed, in bond
angles and bond lengths, to adapt its geometry to the
environment. On these grounds, we chose sulfur as the key
element to promote the self-assembly and created new ‘sulfur-
overrich’ thiophene oligomers, so named since they have an extra
sulfur atom per ring directly attached to one β-carbon of the inner
1
9
tetrameric core, to induce ordered self-aggregation via S
S
3
3 3
interactions and weak CH S hydrogen bondings. On the basis
3
3 3
2
0ꢀ23
of literature data, we envisioned that the directionality of
S and CH S interactions, as well as the large polarizability
of sulfur in thioether fragments, would induce the formation of
highly anisotropic supramolecular systems.
We show here that sulfur-overrich octathiophenes do indeed
self-assemble spontaneously on surfaces affording crystalline
fibers combining efficient charge transport with intense fluores-
cence. Minor changes in the molecular structure allow the fine-
tuning of fibers morphology which, although the oligomers do
not bear chiral substituents, may display helical superstructure
with predominance of one type of hand-helicity.
S
3
3 3
3 3 3
The same methodology was also successfully used for the
preparation of head-to-head oligomers and polymers bearing
hexyl instead of S-hexyl substituents, such as octamer 6b
(
Supporting Information). Thus, the combined use of ultra-
’
RESULTS AND DISCUSSION
sound and microwave irradiation provides a very efficient plat-
form for the rapid, low power, low cost, and environmentally
friendly preparation of regioregular head-to-head thiophene
oligomers and polymers.
Fibers Formation and Properties. Of the many newly
synthesized sulfur overrich oligothiophenes, octamers 9 and
11ꢀ13 displayed the most accentuated tendency to form crystal-
line aggregates. The octamers achieved the right balance of size,
conformation, and shape to self-assemble in ordered fibrillar
structures. The formation of crystalline fibers was induced by
A New Platform for the Synthesis of Regioregular Sulfur-
Overrich Thiophene Octamers. Typically, thiophene oligo-
mers are prepared stepwise by successive bromination and
metalation of intermediates using conventional heating methods
for the palladium catalyzed cross-coupling.
a regiospecific synthesis (Scheme 1) based on Suzuki reaction
enabling the high yield synthesis of sulfur overrich regioregular
thiophene octamers, rapidly prepared making use of ultrasound
and microwave irradiation for the bromination and cross-coupling
18,19
Here we describe
24,25
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Journal of the American Chemical Society
ARTICLE
Figure 1. (A) SEM (a and b on SiO
2
), AFM (c, on glass) and AFM profile (d) of fibers of 9; (B) AFM (a, on glass) and AFM profile (b) of fibers of 11;
(C) SEM (a and b on glass) images of fibers of 12.
2
8,29
dissolving at room temperature the octamers in toluene, in which
they are very soluble, depositing solution drops on the surface of
choice, and exposing it to vapors of acetonitrile, in which the
octamers are insoluble. Randomly distributed fibers with high
aspect ratio—from 1:1 to 1:2 height versus width and up to
hundreds micrometers in length—were spontaneously formed
fibers.
In our octamers, no chiral substituents are present and
the S-hexyl groups promote the formation of folded or rod-like
fibers depending on molecular structure, independently of the
nature of the deposition surface. Octamer 9, made of the
repetition of four identical bithiophene head-to-head substituted
subsystems, formed superhelices (helices of helices) and even
double helices of superhelices whose periodicity was size inde-
pendent (Figure 1A and Figure S6). Bigger fibrils (diameter
>1 μm) were formed by several small fibrils wrapped up in
themselves to form a multihelical structure, exhibiting a width
versus height aspect ratio of 1:1 (Figure 1Ad) and length ranging
from 100 nm to 5 μm. On the contrary, octamer 11 formed tapes-
like fibrils (Figure 1Bb and Figure S6) with typical width versus
height aspect ratio of 2:1 and length in the same range as 9, which
appeared atomically flat (Rms Roughness <1 nm) and with
typical terraces of crystals. Finally, octamers 12 and 13, with R,ω
hexyl and S-hexyl chains, respectively, formed a complex network
of rod-like and folded fibers terminated with astonishing
ringlets (Figure 1C and Figures S7ꢀS9). All fibers exhibited
birefringence at optical polarized microscopy regardless of their
specific morphology (helices vs tape-like), extinguishing in
four positions at intervals of 90° by rotating the polarized light
(Figure S7).
(Figure 1 and Figures S7 and S8).
The directional molecular self-assembly affording fibers for-
mation was highly reproducible, while drop casting of the
octamers without exposition to acetonitrile invariably led to
the formation of globular aggregates. The rate of fibers formation
varied from minutes to hours, depending on concentration and
molecular structure, being slower for 9 than for the other
octamers. Contrary to what is generally observed with thiophene
2
6,27
oligomers,
the shape and the formation rate of the fibers from
9
and 11ꢀ13 were independent of the type of surface employed:
glass, ITO, silicon, and gold were employed with similar results.
Generally, the aggregates formed on surfaces are the result of
a balance between moleculeꢀmolecule and moleculeꢀsubstrate
2
6,27
interactions.
Clearly, with octamers 9 and 11ꢀ13, intermolecular interac-
tions are by far more important than moleculeꢀsurface interac-
tions and the supramolecular structures that are formed
correspond to thermodynamically stable states. That the pre-
sence of the β-sulfurs plays an important role in this process is
demonstrated by the fact that octamer 6b of structure equivalent
to 9 but with hexyl instead of S-hexyl chains (see Supporting
Information) was amorphous in the experimental conditions
employed for fibers formation.
The reproducibility of morphology on different substrates
upon deposition from toluene under acetonitrile vapors made
possible to investigate the fibers with a variety of techniques
requiring deposition on glass, ITO, SiO , and gold. Below we
2
report a thorough comparative investigation on the electronic,
optical, and electrical properties of the helical fibers of 9 with
those, rod-like, of 11. Circular dichroism (CD) allowed to check
the chirality of the films; cyclic voltammetry (CV), the electro-
chemical response; confocal microscopy and space resolved
spectroscopy, the light emission properties; Tunneling Atomic
Force Microscopy in torsion mode (Tr-TUNA) and conductive
Atomic Force Microscopy (C-AFM), the charge transport at the
nanoscale. Finally, to test the applicability in devices, the fibers
Remarkably, secondary helical structure was present in most
fibers with the exception of those formed by 11, the only octamer
lacking substituents on terminal bithiophene subsystems. Note
that so far a ‘secondary structure’ has only been assigned to
18
folded polythiophenes in solution containing chiral side chains.
Typically, fibers made of oligothiophenes are rod-like and
strong interactions with surfaces are observed affecting film
2
6,27
morphology.
It is generally assumed that stereocenters in
were directly grown on the SiO surface of a field-effect transistor
2
the side chains are required to obtain chirality in oligothiophene
with gold contacts and FET charge mobilities were measured.
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ARTICLE
Figure 2. (A) Circular Dichroism and UV spectra of fibers of 9 formed on glass. (B) Onset potentials in PC 0.1 M (C H ) NBF of films of fibers and
2
5 4
4
cast films (globular aggregates) of 9 and 11 on ITO, together with the redox potentials in CH
2 2 4 9 4 4
Cl , 0.1 M (C H ) NClO . For comparison, the values for
6
b (the equivalent of 9 with hexyl instead of S-hexyl substituents) are also reported. (C) Second and third voltammograms of the fibers film of 9 showing
charge trapping phenomena.
The helical fibers formed by octamer 9 upon deposition on
glass were both left- and right-handed; however, microscopy
examinations suggested that one or the other type of hand-helicity
was always predominating. CD measurements confirmed that the
films were chiral and that the sign of chirality was random.
Figure 3A displays the CD plots of two different samples of 9,
deposited on glass, that are similar but reversed in sign. Con-
sidering the thickness of the film (1ꢀ2 μm), the signal intensity is
rather high. Since octamer 9 has no chiral substituents, no CD
pattern was detected in solution and the chirality observed in the
fibers was a property related to the supramolecular organization in
the solid state. We explain the presence of the CD signal and its
inversion from one sample to the other by invoking conforma-
30,31
tional atropisomerism
due to restriction of torsional pro-
cesses about inter-ring CꢀC bonds in the solid state, which
deprives the molecule of all symmetry elements rendering it
chiral. Once the first molecule is frozen on the surface in the form
of one or the other enantiomer (depending on the ω interring
angle, þω or ꢀω having equal probabilities), the conformation of
the following ones is prevalently biased toward the helical sense
32,33
preferred by that enantiomer causing chirality amplification.
Presumably, the handedness of the major enantiomer also affects
the sign of supramolecular helicity (see Figure 5B).
Solid state electrochemistry on films of 9 and 11 deposited on
Figure 3. Laser Scanning Confocal Microscopy images and spatially
resolved spectral analysis of fibers of 9 (A) and 11 (B) grown on glass.
ITO afforded the redox potentials shown in Figure 2B, reported
onset
34,35
as E
values.
For comparison, also the corresponding potentials obtained
for films cast from toluene (globular aggregates) and the redox
potentials in solution are reported, together with the values for
which decreases by 0.14 V, and on the reduction potential, which
becomes less negative by more than 0.35 V. Going from solution
to films, an analogous trend was found for both 9 and 11.
Noticeably, the helical fibers displayed the less negative reduction
potential, hence the highest electron affinity. Differences in redox
6
b, the counterpart of 9 with alkyl instead of S-alkyl chains. The
0
comparison of E values in solution between 9 and 6b shows the
remarkable effect of side chain sulfurs on the oxidation potential,
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Journal of the American Chemical Society
ARTICLE
potentials were observed for both octamers when the morphol-
ogy changed from globular aggregates to fibers, in agreement
with results from literature indicating that redox potentials
from 12 and 13, plaids in favor of the formation of J-type (side-
1
7
by-side) rather than H-type (face-to-face) aggregates. To
confirm this point, detailed photophysical studies would be
required which are beyond the aim of the present study. Never-
theless, the similar trend in fluorescence for all fibers suggests
that their intimate molecular organization must be very similar
despite differences in morphology.
34,35
depend on morphology.
Interestingly, the oxidation and
reduction waves of the fibers film of 9, after the first conditioning
doping, showed prepeaks (Figure 2C) indicating charge trapping
phenomena similar to those observed in some polythiophenes
and related to the delayed release of the charge stored during the
Fibers of 9 and 11 grown on ITO were analyzed by Tunneling
Atomic Force Microscopy in torsion mode (Tr-TUNA), a new
scanning probe technique examining simultaneously surface
topography and local current and providing information on
3
5,36
doping process.
All fibers were intensely fluorescent red (Figures S7,S9).
Figure 3 reports laser scanning confocal microscopy (LSCM)
images and spatially resolved photoluminescence (PL) spectra of
fibers of 9 and 11 grown on glass.
The images, with spatial resolution of 500 nm in x-y, reveal the
helical morphology of the fibers of 9 and the rod-like arrange-
ment of the fibers of 11. The LSCM images of 9 show spatial
modulation of fluorescence along the fibers long axis. The space
resolved spectra show that there is no fluorescence emission from
the background and that the maximum emission wavelength of
the fibers, around 650 nm for both octamers, is nearly 100 nm
red-shifted with respect to the values in solution (Figure S6).
The intense fluorescence observed for all fibers, including those
37
nanoscale conductivity within specific domains. Tr-TUNA
images of fibers up to 100 μm long and with an average diameter
of approximatively 300 nm are reported in Figure 4A,B. Tr-
TUNA current mapping shows a direct correlation with the
topography revealing defined conductive domains associated to
the fibers. The variations in conductivity observed for 9 are in
agreement with differences in molecules stacking within the
nanodomains due to helicity, while the large, homogeneous
domains associated to the fibers of 11 are in agreement with a
more uniform stacking of the molecules due to the rod-like
arrangement. It is worth noting that in Figure 4A the wide islands
34
Figure 4. (A) Topography image (left) and hole transportation measurement (right) of helical fibers of 9 grown on ITO obtained using Tr-TUNA
with a þ5 V bias on the tip. (B) The same as panel A for rod-like fibers of 11. (C) Transfer characteristics in saturation regime in air for bottom contact
field effect transistors with the active layers made of helical fibers of 9 (left) and rod-like fibers of 11 (right) directly grown on the device. The red arrow
indicates the clockwise hysteresis observed on the passage from the on state to the off state, indicating an intrinsic ‘memory effect’ of fibers of 9.
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Journal of the American Chemical Society
ARTICLE
Figure 5. (A) X-ray diffraction pattern of the fibers grown on glass; periodic distances reported in nm; conformation of the inner tetrameric core of the
octamers as determined by single crystal X-ray analysis of 11 and model of J-type stacking. (B) A detail of the film from 12 and proposed model for the
supramolecular packing.
with conductivity lower than that of the fibers have topographic
correspondence in amorphous aggregates and impurities whose
medium height ranges between 4 and 50 nm, much lower than
the average height (300 nm) of the fibers. The islands are visible
in topographic images only cutting the upper limit of the height
scale suitable to make visible the topographic features of the
fibers (Figures S12 and S13).
by assembling the randomly oriented fibers into oriented func-
tional arrays by template-directed growth. The FET based on
fibres of 9 shows a peculiar feature of the material represented by
39
the intrinsic ‘memory effect, observed without recurring to
40
polarizable gate dielectrics. The ‘memory effect’ (Figure 4C,
left, and Figure S11) is deduced from the clockwise hysteresis in
drain-source current (red arrow in Figure 4C), indicating that the
device retains the on state in backward gate voltage sweep,
corresponding to the passage from the on to the off state. Such
an effect, modulated by the gate voltage, is more pronounced
in air than in vacuum and is probably related to doping from
oxygen, in agreement with CV data. Remarkably, FETs based on
cast films of 9 and 11 did not show any charge carrier modulation,
despite the highly ordered molecular organization displayed by
thin film X-ray diffraction (not reported), confirming that charge
transport is strongly morphology dependent.
The charge (hole) mobility at the nanoscale of the fibers of 9
and 11 was obtained by measuring the charge transport trough
fibers section by conductive-AFM in contact mode (C-AFM).
The plots current density versus square voltage in a double
logarithmic representation are reported in Figure S14. The plots
showed that 9 and 11 were characterized by different charge
ꢀ
7
ꢀ6
2
mobility values, μholes, of 9.8 ꢁ 10 and 5 ꢁ 10 cm /V s,
3
respectively, in agreement with Tr-TUNA maps.
Finally, since fibers morphology is independent of the sub-
strate employed, fibers of 9 and 11 were directly grown on the
The consistency of the many characterization data reported
above allowed us to elaborate a comprehensive model for fibers
packing, as described below.
SiO substrate of a field effect transistor (FET) to measure the
2
charge mobility along the fibers in the presence of an electric
field. A standard two probe technique was employed to measure
the drain-source current (I ) versus gate voltage (V ) for FET
X-ray Diffraction and Fibers Packing. As shown in
Figure 5A, all X-ray profiles of fibers films are dominated by
the small angle reflection, other reflections being absent or of
very low intensity. The distances indicated by the small angle
reflection grow from 1.64 nm for 11, without substituents on the
terminal bithiophene subsystems, to 2.13 nm for 13, having the
longest terminal substituents. On the basis of these data, our
hypothesis is that all octamers have a linear backbone with nearly
flat conformation and, since the fibers are intensely fluorescent,
DS
GS
devices in bottom configuration (Supporting Information for
details). Typical transfer characteristics in saturation regime,
recorded in air, are reported in Figure 4C. Considering that,
owing to the fibrillar morphology, the test patterns were effec-
tively covered for no more than 50% of their nominal surface,
ꢀ
5
ꢀ4
2
ꢀ1 ꢀ1
μholes for 9 and 11 were 1 ꢁ 10 and 4 ꢁ 10 cm V
s ,
respectively, 2 orders of magnitude higher than the intrinsic
1
7
mobility measured through a single fiber in the absence of
the supramolecular organization is of the J-stacking type. The
β-thioalkyl substitution imposes anti orientation of the thio-
phene rings of the inner core to reduce steric hindrance and there
38
external field, and in the typical range of conductive polymers.
Note that FET charge mobility could be significantly improved
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Journal of the American Chemical Society
ARTICLE
is a parallel order of the π-conjugated cores, with the alkyl side
chains extending almost perpendicularly to the thiophene back-
bone (Figure 5A).
application in molecular based devices for organic electronics,
photovoltaics, and sensing.
In the molecular packing, the J-stacked molecules should be
parallel to one another in columns at the distance obtained from
the X-ray analysis (Figure 5A). We anticipate that these hypoth-
eses are nicely confirmed by single crystal X-ray analysis of 11,
also showing that the crystalline form of this compound is the
same as in powders and fibers (results to be reported in a further
work). In Figure 5A, the geometry of the tetrameric inner core
obtained from single crystal X-ray analysis of 11 is reported. The
geometry is planar with the sulfur atoms on the same plane and
’
ASSOCIATED CONTENT
S
Supporting Information. Full experimental details for
b
the synthesis of the new compounds and their precursors; H-1
and C-13 NMR spectra of octamers 9 and 11ꢀ13 and 6b; optical
microscopy images of octamers 9 and 11ꢀ13 and SEM of 13;
cyclic voltammetry data; conductive AFM; Field Effect Transis-
tors. This material is available free of charge via the Internet at
http://pubs.acs.org.
S
S intermolecular distances largely below the sum of van der
3
3 3
Waals radii (3.60 Å).
’
AUTHOR INFORMATION
The presence of substituents in the outermost rings induces
disorder effects and distorsions at the end points of the molecules
that influence the distance between parallel octathiophene rows,
making it vary with molecular structure. Thus, we can describe
the supramolecular assembly as lamella-type structure in which
the small distortions of the molecular apexes cause a slight
disruption of the compact packing of the molecules simply by
means of a slightly twist of long molecular axes in the short-range.
The morphology is influenced by the packing order of the
molecules and in the long-range ordered helixes and curls appear
as macroscopic effects of stacking faults during the growth
Corresponding Author
barbarella@isof.cnr.it
Present Addresses
Univ. London Imperial Coll. Sci. Technol.; Med., Dept. Chem.,
§
London SW7 2AZ, England.
’ ACKNOWLEDGMENT
This research was supported by projects FIRB RBPR05JH2P_
ITALNANONET (G.G.), FIRB RBIP0642YL-LUCI (G.B.) and
ESF-EURYI DYMOT (M.C. and D.G.).
(Figure 5B).
’
CONCLUSIONS
In summary, we discovered a new recognition algorithm—
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