Rigid organic nanotubes obtained from phenylene-butadiynylene macrocycles

Article abstract of DOI:10.1039/c3cc43177k

Rigid organic nanotubes were prepared from six-membered phenylene-butadiynylene macrocycles through topochemical polymerization in the xerogel state. All six butadiyne units underwent polymerization, thus creating rigid nanotubes with six polydiacetylene

Full text of DOI:10.1039/c3cc43177k

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Rigid organic nanotubes obtained from
phenylene-butadiynylene macrocycles†
Cite this: DOI: 10.1039/c3cc43177k
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Simon Rondeau-Gagne, Jules Romeo Neabo, Maude Desroches, Isabelle Levesque,
Received 29th April 2013,
Accepted 20th May 2013
Maxime Daigle, Katy Cantin and Jean-François Morin*
DOI: 10.1039/c3cc43177k
www.rsc.org/chemcomm
Rigid organic nanotubes were prepared from six-membered phenylene-
butadiynylene macrocycles through topochemical polymerization in
the xerogel state. All six butadiyne units underwent polymerization,
thus creating rigid nanotubes with six polydiacetylene chains lying
parallel, one relative to each other.
Similar to carbon nanotubes, organic nanotubes show numerous
interesting features for different applications, including host–guest
chemistry and organic electronics.1 However, a reliable and straight-
forward synthetic strategy must be developed before they can be
considered as a serious alternative to other materials. A promising
strategy to prepare them is to assemble macrocyclic building
blocks to obtain supramolecular nanotubes and to link them
covalently to fix the tubular organization.2 Among the strategies
used to link supramolecular organization, topochemical polymeriza-
Fig. 1 Structure of PBM1 and PBM2.
tion of 1,3-butadiynes, yielding polydiacetylene (PDA), is particularly two in the case of PAMs), which could potentially undergo
interesting since no metal catalyst or additive is needed to drive this topochemical polymerization to form PDA. Compared to PAMs,
polymerization.3 However, specific and strict intermolecular struc- PBMs are known to provide stronger p–p interactions owing to
tural parameters are needed for this reaction to occur.4 Recently, we the electron-withdrawing effect of the butadiyne moiety, thus
and others demonstrated that a gel state can be used to obtain such enabling the self-association of macrocycles in different solvents
critical parameters, allowing the preparation of high molecular through gel formation.7 Moreover, PBMs are much larger than PAMs,
weight PDAs from building blocks of different sizes and shapes.5 which could allow the introduction of internal binding functionalities
We have also shown that the incorporation of specific elements of in addition to providing nanotubes with better host ability.8 None-
design allows the control over molecular organization in phenyl- theless, the controlled polymerization of PBM frameworks remains a
acetylene macrocycle (PAM) assembly, allowing topochemical poly- challenging but worthy task considering that it would open the way to
merization between macrocycles in the xerogel state.6 The material the preparation of non-graphitic, semi-conducting nanotubes with
obtained after topochemical polymerization was made of soluble, unique properties. In this regard, very few attempts to cross-link PBMs
PDA-walled conjugated nanorods.
and related butadiyne-containing cyclic molecules through controlled
In order to prepare larger, rigid and more robust PDA-walled topochemical polymerization have been reported but none of them
organic nanotubes, we have chosen to prepare six-membered lead to nanotubes that can be solubilized, handled easily and
phenylene-butadiynylene macrocycles (PBM1 and PBM2, Fig. 1) characterized as unimolecular entities.9
that possess six 1,4-diaryl-1,3-butadiyne units (rather than only
Herein, we report the synthesis, gelation properties and topo-
chemical polymerization in the xerogel state of two phenylene-
butadiynylene macrocycles (PBM1 and PBM2). The resulting
materials were characterized using Raman and UV-visible
spectroscopy and transmission electron microscopy (TEM) to
determine the efficiency of the topochemical polymerization
and to assess the formation of nanotubes. Besides the prepara-
tion of new materials with unique properties, this study permits
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Departement de chimie and Centre de Recherche sur les Materiaux Avances,
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1045 Ave de la Medecine, Pavillon Alexandre-Vachon, Universite Laval, Quebec, QC,
Canada G1V 0A6. E-mail: jean-francois.morin@chm.ulaval.ca;
Fax: +1-418-656-7916; Tel: +1-418-656-2812
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization data for all the new compounds, XRD, SEM, UV-visible and
Raman spectra. See DOI: 10.1039/c3cc43177k
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room temperature. Unexpectedly, gelation was not observed in any
solvents tested except for cyclohexane, which gave a translucent
organogel for PBM1 and PBM2. This result is surprising given that
PAMs bearing the same outer decorations presented good gelation
properties in a large number of solvents.6 We hypothesize that this
significant difference in the self-assembly process between PAMs
and PBMs can be attributed to the better solubility of PBMs.13
The distance and orientation between the macrocycles within the
xerogel state were studied by powder X-ray diffraction (PXRD) analysis.
The PXRD spectrum of PBM1 is presented in ESI† (Fig. S18). A rather
broad peak at 2y = 4.11 (30.4 Å) is observed and can be attributed to a
(100) reflection of a columnar lattice, most likely with a hexagonal
symmetry. Moreover, a broad and intense peak at 2y = 21.51 (4.3 Å) is
observed and can be attributed to liquid-like order between the alkyl
chains. The PXRD spectrum thus suggests that macrocycles are
stacking on top of each other to form tubular assemblies as
observed for similar macrocycles and discotic liquid crystals.8,14
Moreover, the columnar organization of PBMs suggests the
presence of an inner empty cavity within the column although
it was not possible to assess with certainty the presence of such a
cavity based on the sole PXRD analysis.
Scheme 1 Synthesis of PBM1 and PBM2.
Knowing that the macrocycles of PBM1 in the xerogel state
us to evaluate the versatility of our general strategy to prepare are stacked in a face-to-face configuration, which is an essential
organic nanotubes with different sizes, shapes and functions. prerequisite for the topochemical polymerization to occur, a thin
The synthetic pathway for PBM1 is depicted in Scheme 1. Starting film of the xerogel obtained from a cyclohexane gel was deposited
from N-dodecyl-3,5-diiodo-benzamide,6a TMS-protected diacetylenes on a glass substrate and subjected to irradiation using UV light
were introduced by Castro–Stephen–Sonogashira coupling reaction (l = 254 nm) for a period of 48 h. Appearance of a dark blue color,
using monodeprotected 1,4-bis(trimethylsilyl)butadiyne10 to afford associated with the formation of PDA, was observed for PBM1 after
compound 1 with good yield. Deprotection of diyne units with irradiation. For PBM2, a slight change in color was observed after
potassium hydroxide was then conducted before introduction of irradiation turning the white xerogel to yellow. This slight color
1-octyl-3,5-diiodobenzene6a using standard Castro–Stephen–Sonoga- change accounts for an incomplete polymerization and/or degrada-
shira coupling to obtain half-macrocycle 2 in rather low yield (28%). tion of the supramolecular network. Surprisingly, the incorporation
It is noteworthy that the low yield is due to the formation of linear of external diynes did not drive the topochemical polymerization,
oligomers, which are common side-products for this kind of as it was the case for the PAM family.6a
statistical coupling. TIPS-protected acetylenes were then installed
The purification of cross-linked PBM1, thereafter called PDA1, was
on the half-macrocycle by standard Castro–Stephen–Sonogashira conducted using size-exclusion chromatography (SEC) (Bio-Beads
coupling to obtain 3 in excellent yield (92%). After removal of TIPS SX-1) and a UV-visible spectrum of PDA1 was recorded in CHCl3
using tetrabutylammonium fluoride (TBAF), the deprotected half- (see Fig. S19 in ESI†). As expected, apparition of a broad absorption
macrocycle was directly used without purification in the ring closing band with lmax at 640 nm was observed, which is characteristic of a
reaction under modified Eglinton conditions to provide PBM1 in PDA backbone. For every 10 mg of PBM1 deposited as a gel, about
relatively low yield (11%).11 This low yield of cyclization can be 1 mg of soluble blue material can be obtained after purification by
attributed to the relative instability of diyne units and the moderate SEC, which gives a yield of about 10%.
solubility of PBM1.9b,12 An identical synthetic approach was used for
PBM2 and it is also depicted in Scheme 1. The yields for each step units, Raman spectroscopy was performed on a xerogel of PBM1
are similar to those obtained for PBM1. and on purified PDA1 and the spectra are shown in Fig. S20 (ESI†).
In order to evaluate the extent of reaction of the butadiyne
In order to obtain proper intermolecular orientation and distance The Raman spectrum of PBM1, recorded on the xerogel, exhibited
between macrocycles prior to the topochemical polymerization, a strong band at 2224 cmÀ1 and a weaker one at 1593 cmÀ1, which
PBM1 and PBM2 were gelified in different organic solvents. The can be attributed to the diyne units. Unexpectedly, these bands
gelation process was chosen over crystal formation to organize disappeared almost completely (conversion >95%) after UV
molecules since it is much less time-consuming. Moreover, we irradiation. This is confirmed by the appearance of new bands
have shown previously that the topochemical polymerization of at 2112 cmÀ1 and 1477 cmÀ1, which can be attributed, respec-
1,4-diarylbutadiyne moieties can be very efficient in the gel state tively, to the stretching modes of alkyne and alkene moieties of
compared to that in the crystalline state since the former accom- the newly formed ene–yne functions. It is important to mention
modates conformational changes better.5c Standard gelation tests that these bands are present, at a lesser extent, in the powder
were performed by dissolving the corresponding PBM in different spectrum of PBM1. This phenomenon is due to partial cross-
solvents (1 to 10 wt per vol%). The resulting suspension was heated linking of the very reactive PBM1 induced by the Raman laser.15
near the boiling point of the solvent and was allowed to cool down at However, the complete disappearance of diyne vibrational bands
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PBMs’ inner butadiyne units to stabilize the supramolecular
architecture. Raman spectroscopy performed on the resulting
1D nanoarchitectures confirmed that all the butadiyne units
disappeared upon irradiation. HRTEM proved that the covalent
nanoarchitectures thus created are rigid and possess an internal
void. Thermal graphitization of this structure to create well-
defined carbon nanotubes is underway.
The authors would like to thank the National Sciences and
Engineering Research Council of Canada (NSERC) and the
Fig. 2 Structure and PDA1 in front and side views. Side groups and hydrogen
atoms have been omitted for clarity. The red and blue carbon atoms represent
the PDA chains and the phenyl groups, respectively.
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Centre Quebecois sur les Materiaux Fonctionnels (CQMF) for
financial support and Jean-François Rioux (ULaval) and
Richard Janvier (ULaval) for their help in characterization.
Notes and references
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butadiynylene macrocycles bearing amide functionalities were
accomplished. PXRD analysis of the resulting xerogels showed
a columnar organization in which the PBMs stacked on top of
each other, allowing the topochemical polymerization of the
experiments was impossible to avoid since sufficient laser power
must be used to overcome the fluorescence background emission
from PBM1. Over three different laser sources were used and none
of them allowed us to solve the problem. Somehow, this proves the
high reactivity of PBM1 under illumination.
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This journal is The Royal Society of Chemistry 2013
Chem. Commun.