Rationally controlled helical organization of a multiple-hydrogen-bonding oligothiophene: Guest-induced transition of helical-to-twisted ribbons

Article abstract of DOI:10.1039/c0cc02225j

A cyanuric acid-functionalized quaterthiophene self-aggregates to form helical ribbons which are transformed into twisted ribbons upon complexing with a complementary bismelamine receptor.

Full text of DOI:10.1039/c0cc02225j

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Rationally controlled helical organization of a multiple-hydrogen-bonding
oligothiophene: guest-induced transition of helical-to-twisted ribbonswz
Shiki Yagai,*a Marina Gushiken,a Takashi Karatsu,a Akihide Kitamuraa and
Yoshihiro Kikkawab
Received 30th June 2010, Accepted 6th September 2010
DOI: 10.1039/c0cc02225j
A cyanuric acid-functionalized quaterthiophene self-aggregates
to form helical ribbons which are transformed into twisted
ribbons upon complexing with a complementary bismelamine
receptor.
UV/Vis and fluorescence spectra of chloroform solutions of
1 (Fig. 1a) show absorption and emission maxima at 414 and
489/513 nm, which is characteristic of the p–p* transition of
the molecularly dissolved quaterthiophenes.8 Upon dissolving
1 in nonpolar solvents such as methylcyclohexane (MCH), the
absorption spectrum undergoes a significant blue-shift to
378 nm whereas the emission band undergoes a red shift to
547 nm. These spectral changes are characteristic of the formation
Nanostructures formed by the self-assembly of p-conjugated
oligomers through noncovalent interactions are attracting
enormous attention as novel materials being developed in
the nanoscale.1 The further development of this research field
requires a precise control of self-assembled nanostructures by
rational molecular designs based on specific noncovalent
interactions.2 Among various self-assembled nanostructures,
helical architectures have attracted special attention in view of
their chirooptical properties, supramolecular chiral chemistry
and supramolecular electronics.1,3 Such helical architectures
can be roughly classified into two different morphologies,
i.e., helical and twisted ribbons.4 Numerous helical p-nano-
structures have been constructed from specifically-designed
p-conjugated oligomers such as oligo(p-phenylenevinylene)s
(OPVs),5 oligo(p-phenyleneethynylene)s,6 oligo(p-phenylene)s7
and oligothiophenes (OTs).8 Ajayaghosh et al. have reported
the formation of helical and twisted ribbons from OPVs that
are mono- or di-substituted with cholesterol.9 We herein
report a rational supramolecular strategy to obtain the two
helical nanostructures of functional p-systems by using
multiple hydrogen-bonding interactions.
of p-stacked OT aggregates with a face-to-face (H-type)
arrangement.8 Temperature-dependent UV/Vis measurement
shows the thermoreversibility of the aggregates (ESIz).
Self-assembled nanostructures of 1 were investigated by
atomic force microscopy (AFM) and transmission electron
microscopy (TEM). Elongated fibrous nanostructures were
observed by AFM and TEM with lengths over several
hundreds of nanometres (ESIz). Remarkably, high-resolution
AFM imaging revealed the formation of helical structures
existing as isolated (i) or bundled (ii) states (Fig. 2a).
The bundled helices exhibit more pronounced helical pitches
compared to isolated ones, which might be attributed to the
flattening of the latter by the adsorption to the surface.
Cross-sectional analysis along the bundled helices reveals the
helical pitch of 16 nm (inset in Fig. 2a), which is significantly
longer than the molecular length of 1 with extended alkyl
chains (ca. 4 nm, ESIz). The average height and width of the
helical structures are 3.4 nm and 16 nm, respectively (Fig. 2b),
after reducing the tip-broadening factor.12 The significant
difference between the two dimensions could be attributed to
Quaterthiophene 1, functionalized on one end by a cyanuric
acid (CA) and on the other end by a tridodecyloxybenzyl
(TDB) tail, was synthesized according to Scheme S1 (ESIz).
Since monosubstituted CA could form hydrogen-bonded
tapes,10 hydrogen bond-directed self-aggregation of 1 may
lead to the formation of flat ribbons. Upon capping
the CA site of 1 with a xylylene-linked bismelamine receptor
BMx,11 the resulting complex may undergo one-dimensional
aggregation directed by p-stacking (Scheme 1).
a Department of Applied Chemistry and Biotechnology,
Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho,
Inage-ku, Chiba 263-8522, Japan. E-mail: yagai@faculty.chiba-u.jp;
Fax: +81-(0)43-290-3039; Tel: +81-(0)43-290-3368
b Photonics Research Institute, National Institute of Advanced
Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba,
Ibaraki 305-8562, Japan
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Synthetic
procedures and characterization data of 1. UV/vis, AFM, 1H NMR,
IR and SEM data of 1 and 1ꢀBMx and 1ꢀ(S)-BMx. See DOI: 10.1039/
c0cc02225j
Scheme 1 Schematic representation of the self-assemblies of 1 and
1ꢀBMx with pictures of their cyclohexane solutions ([1] = 5 ꢁ 10ꢂ4 M)
under UV-light.
c
454 Chem. Commun., 2011, 47, 454–456
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hypsochromic shift of the p–p* transition, and the resulting
bilayer tape is further stabilized by taking helical morphology
(Scheme 1a).
Complexation of 1 with BMx was studied by using
1H NMR spectroscopy in CDCl3, a good solvent for the OT
segment. CDCl3 solution of 1 (c = 5 ꢁ 10ꢂ3 M) showed
well-resolved peaks reflecting the molecularly dissolved state
(ESIz). Upon adding BMx, the signals of all of the thiophene
protons and one of the benzyl protons of the TDB moiety
shifted upfield, indicating the formation of a hydrogen-bonded
complex with suppressed conformational flexibility. The
resonances of the CA imide protons are not observed. Instead,
NH protons of added BMx become sharpest at 1:BMx = 1:1,
after which they become too broad to be observed due to fast
equilibrium between bound and free BMx.13 The binding
constant was estimated to be 25 100 Mꢂ1 by fitting the
shift of the above protons to the 1 : 1 binding isotherm.
The relatively high binding constant in CDCl3 ensures the
stoichiometric complexation in more nonpolar solvents.
A pronounced UV/Vis spectral change is thus observed
when varying amounts of BMx are mixed with 1 in MCH
(Fig. 1b). The absorption band of the p-stacked OT undergoes
a gradual red-shift to 412 nm, and the spectral change levels
off when 1 equiv. of BMx is added. This observation indicates
that BMx quantitatively disrupts the self-aggregates of 1 by
rupturing hydrogen-bonded chains of CA moieties. The
resulting 1 : 1 complex showed fluorescence maxima at 477
and 506 nm. Thus, the UV/Vis and fluorescence spectra of the
1 : 1 complex 1ꢀBMx are similar to those of the molecularly
dissolved 1 in chloroform (Fig. 1a). However, the former is
characterized by the presence of an absorption shoulder at
around 460 nm which disappears upon increasing the
temperature (ESIz). It is suggested that the 1 : 1 complexes
exist as a stacked state in solution, where the OT moieties are
longitudinally displaced by a slipping angle close to the magic
angle 54.71. This is further supported by the observation that
UV/Vis spectra of 1ꢀBMx in MCH do not show significant
Fig. 1 (a) UV/Vis (left axis) and fluorescence (right axis) spectra of 1
(c = 1 ꢁ 10ꢂ5 M) in chloroform (red curve) and in MCH (blue curve).
(b) UV/Vis spectral change of 1 (c = 1 ꢁ 10ꢂ5 M) upon addition of
BMx (c = 0 to 1 ꢁ 10ꢂ5 M). Blue and red curves are the spectra
recorded at 1 : BMx = 1 : 0 and 1 : 1, respectively. Arrows indicate the
changes upon adding BMx. Inset: plot of e at 465 nm versus [BMx]/[1].
Red dashed curve shows the fluorescence spectrum recorded at
1 : BMx = 1 : 1.
changes upon increasing the concentration to 5 ꢁ 10ꢂ4
(gel state, vide infra).
M
Fig. 2 (a) AFM height images of nanostructures formed by 1
(c = 1 ꢁ 10ꢂ4 M) in MCH. For the sample preparation, see ESI.
Inset is a height analysis along the helical ribbon (ii). (b) Cross-
sectional analysis across the fibers (i) and (ii) in (a).
High aspect ratio rodlike nanostructures were observed for
1ꢀBMx and their lengths easily exceed 1 mm (Fig. 3). The width
of the smallest resolved fibers is ca. 20 nm by TEM, and the
height of the nanorods is 8 nm by AFM. This morphological
feature is in great contrast to the deformed helical ribbons
formed by 1, and suggests the formation of twisted ribbons
with a solid interior (Scheme 1b). The nanorods show a
periodic fine structure with pitches of ca. 80 nm, which can
be attributed to periodic bending of twisted ribbons.14
The twisted ribbons of 1ꢀBMx show a remarkable gelation
ability while helical ribbons by 1 afford only viscous liquid at
around 5 ꢁ 10ꢂ3 M in cyclohexane. The minimum gelation
concentrations of 1ꢀBMx in cyclohexane are 0.16 wt%
(5 ꢁ 10ꢂ4 M), classified as super-gelator.15
the formation of a hollow helical ribbon that is susceptible to
deformation through an evaporation process.
In order to obtain insight into the aggregation structure, 1 in
bulk state was investigated by X-ray diffraction (XRD). The
XRD pattern obtained at 150 1C exhibited diffraction peaks at
6.35, 3.18 and 2.13 nm (ESIz), which are ascribable to a
lamellar structure. The layer spacing of 6.35 nm is significantly
longer than the molecular length of 1 (ca. 4 nm), but shorter
than the width of a linear tapelike aggregate (ca. 8 nm) formed
by double hydrogen-bonding between CA moieties
(Scheme 1a).10 Thus, it is suggested that a tilted packing of
such hydrogen-bonded tapes results in the formation of sheets,
which further stack into a lamellar structure. Although such a
densely-packed lamellar structure could not be formed in
diluted solutions, the hydrogen-bonded tapes would be
stabilized by lateral coupling through multipoint p-stacking
interactions of OT segments as shown by the pronounced
We further investigated the twisted organization of 1ꢀBMx
by using a chiral receptor (S)-BMx and circular dichroism
(CD) spectroscopy. Fig. 4a shows the CD spectral change of
1ꢀ(S)-BMx (c = 2 ꢁ 10ꢂ5 M) in MCH upon cooling from 90 to
20 1C. A bisignated CD signal with a negative maximum
at 364 nm and a positive one at 393 nm gradually grows from
c
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hydrogen-bonding interactions to organize functional mole-
cules into desired nanostructures.
This work was partially supported by KAKENHI
(20350061).
Notes and references
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fiber. Inset in (c) shows cross-sectional analysis along the white line in
the simultaneously obtained height images.
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In conclusion, two different types of self-assembled helical
p-nanostructures, i.e., helical and twisted ribbons, have been
rationally constructed upon changing hydrogen-bonding
modes of the building block by the addition of guest. This
study clearly demonstrates the power of complementary
c
456 Chem. Commun., 2011, 47, 454–456
This journal is The Royal Society of Chemistry 2011