Topochemical polymerization of a diarylbutadiyne derivative in the gel and solid states

Article abstract of DOI:10.1021/ol200051m

A diarylbutadiyne derivative was synthesized using a few steps and gelified in aromatic solvents. The gel prepared at low concentration is made of micrometers-long nanofibrils as shown by scanning electron microscopy. XRD of the dried gel shows sharp feat

Full text of DOI:10.1021/ol200051m

ORGANIC
LETTERS
2011
Vol. 13, No. 6
1358–1361
Topochemical Polymerization of a
Diarylbutadiyne Derivative in the Gel and
Solid States
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Jules Romeo Neabo, Keshia Imelda Sika Tohoundjona, and Jean-Francois Morin*
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Departement de Chimie and Centre de Recherche sur les Materiaux Avances (CERMA),
1045 Ave. de la Medecine, Universite Laval, Quebec, Canada G1 V 0A6
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Jean-francois.morin@chm.ulaval.ca
Received January 7, 2011
ABSTRACT
A diarylbutadiyne derivative was synthesized using a few steps and gelified in aromatic solvents. The gel prepared at low concentration is made of
micrometers-long nanofibrils as shown by scanning electron microscopy. XRD of the dried gel shows sharp features, revealing a well-organized material.
A topochemical reaction was performed on the dried gel, and a polydiacetylene presenting reversible thermochromism properties was obtained.
The topochemical reaction of butadiyne leading to
polydiacetylene in the crystalline1 or gel state2 is a very
elegant way of preparing very pure, well-defined conju-
gated polymers with unique optical and electrical properties.3
However, to undergo such polymerization in the solid
state, the butadiyne units have to be properly oriented in
˚
relation to each other. A distance of 4.9 A between the two
reactive carbon atoms and an angle of 45° relative to the
butadiyne axis are necessary to perform this reaction
efficiently. Because a topochemical reaction results in a
change of the atomic coordinates inside the crystal, the
nature of the substituents directly attached to the buta-
diyne unit has a significant impact on the polymerization
process.4 Indeed, these substituents should be able to
accommodate structural changes within the crystal lattice,
(1) (a) Wegner, G. Natureforsch., B: Chem. Sci. 1969, 24, 824–832. (b)
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Robitzer, M.; Lere-Porte, J.-P.; Serein-Spirau, F.; Moreau, J. J. E.
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r
10.1021/ol200051m
Published on Web 02/21/2011
2011 American Chemical Society

˚
and this is why almost all of the topochemical reactions of
butadiyne reported so far have been performed on deriva-
tives bearing flexible alkyl chains directly attached to the
butadiyne moiety.
ca. 4.8 A, which is close to the butadiyne distance necessary
to achieve a topochemical reaction.2e,9
The synthesis of derivatives 1 and 2 is described in
Scheme 1. Compound 1 was obtained via six straightfor-
To the best of our knowledge, very few topochemical
polymerizations of butadiyne through irradiation with UV
light have been reported when aryl groups were attached to
both ends of the butadiyne.5 This reaction is very difficult
to achieve in crystals since the phenyl groups are expected
to be too bulky to undergo significant conformational
changes within the crystal. In fact, several other attempts
to achieve this polymerization using UV irradiation
failed,6 and only thermal and γ-ray irradiation polymer-
ization gave a polydiacetylene with relative success.7
Scheme 1. Synthesis of Diarylbutadiynes 1 and 2
Figure 1. Compound 1 and its corresponding polydiacetylene P1.
Because conformational changes are known to be less
demanding in an organogel than in crystals, we hypothe-
size that butadiyne bearing two aryl groups could be more
easilypolymerized within anorganogel. Thisassumption is
supported by the successful topochemical reaction of the
diarylbutadiyne derivatives trapped inside polysaccharide
schizophyllan in solution.8 Herein, we report the efficient
preparation of a polydiacetylene from a diarylbutadiyne
derivative (1, Figure 1) in both gel and solid states using
UV irradiation.
One of the most important design aspects of compound
1 is the presence of two amide groups, bearing a long alkyl
chain, directly attached to the aryl units. The amide group
iswidelyusedtoproperly orient butadiyne moieties in both
crystal and gel forms since the intermolecular distance
between amide groups of molecules facing each other is
ward steps from commercialy available 4-hydroxybenzoic
acid. The first reaction involves monoiodination using
iodine monochloride (ICl) in hot aqueous sulfuric acid to
yield compound 3 in quantitative yield. Amidation of
compound 3 with dodecylamine using diisopropylcarbo-
diimide (DIC) and 6-chloro-1-hydroxybenzothiazole (Cl-
HOBt) in refluxing tetrahydofuran gives compound 4a in
84% yield. Sonogashira10 coupling with trimethylsilylace-
tylene (TMSA) on 4a in THF using Pd/Cu catalysts and
triethylamine as the base yields compound 5a in 93% yield.
Alkyne deprotection of 5a with tetrabutylammonium
fluoride (TBAF) gives compound 6a in 93% yield. Finally,
Glaser-Hay11 homocoupling of 6a in o-DCB using tetra-
methylethylenediamine (TMEDA), CuCl, and air yields
compound 1 in 85% yield. The synthesis of derivative 2
was achieved by using a similar synthetic pathway starting
from commerically available 3-iodobenzoic acid. All the
products were obtained in good to excellent yields.
(4) Fowler, F. W.; Lauher, J. W. In Carbon-Rich Compounds; Haley,
M. M., Tykwinski, R. R., Eds.; Wiley-Vch Verlag GmbH & Co: Weinheim,
2006.
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Okada, S.; Ohsugi, M.; Masaki, A.; Matsuda, H.; Takaragi, S.;
Nakanishi, H. Mol. Cryst. Liq. Cryst. 1990, 183, 81–90. (c) Matsuo,
H.; Okada, S.; Nakanishi, H.; Matsuda, H.; Takaragi, S. Polym. J. 2002,
34, 825–834. (d) For example with pyridine appended, see ref 9.
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15111–15135.
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513–527. (b) Zhang, Y.; Wang, L.; Wada, T.; Sasabe, H.; Wen, J.
J. Fluor. Chem. 1996, 77, 97–99. (c) Zhang, Y.; Wada, T.; Sasabe, H.;
Wen, J. J. Fluor. Chem. 1996, 77, 27–29.
The gelation properties of the two derivatives were
studied in common organic solvents. First, the organo-
gelator was mixed in a solvent and sonicated for a few
minutes. The solution was then heated near the boiling
(9) Li, Z.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 2009, 131,
634–643.
(10) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 50, 4467–4470.
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J. Org. Chem. 1962, 27, 3320–3321.
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Fujisawa, T.; Kaneko, K.; Sakurai, K.; Shinkai, S. Org. Biomol. Chem.
2005, 3, 4321–4328. (b) Numata, M.; Shinkai, S. Adv. Polym. Sci. 2008,
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Org. Lett., Vol. 13, No. 6, 2011
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point of the solvent and slowly cooled down to room
temperature. Compound 1 showed good gelation ability
in all the aromatic solvents tested (see Table S1 in the
Supporting Information). In fact, aromatic solvents al-
lowed gel formation at low concentration in o-DCB (2.5
mg/mL) and toluene (3.0 mg/mL). Depending on the
solvent used, the gel obtained is translucid, semitranslucid,
or opaque (Figure S1 in the Supporting Information). This
behavior can be attributed to different solublities of com-
pound 1 in aromatic solvents, leading to gels having
various levels of crystallinity. In all the aromatic solvents
tested, repeating the heating/cooling process alwayslead to
similar gels. In nonaromatic solvents, many states are
obtained including a viscous solution, solution, or simply
suspension of insoluble material. In contrast to compound
1, compound 2 was found to be insoluble at room tem-
perature in all the solvents tested. Even after heating a
suspension of compound 2 in different solvents, precipita-
tion rather than gelification occurred. This difference
between both compounds is indicative of the importance
of the hydroxy-ethoxy chain attached to compound 1 for
the assembly process. In addition to increasing the solubi-
lity, they are expected to participate in intermolecular
H-bonding (OH..OH) that supports the formation of a gel.
In order to investigate the crystallization temperature of
solvents within the gel and the thermal stability of the gel,
two gel samples were prepared with either toluene or o-
DCB as solvent at a 10 mg/mL concentration and sub-
jected to differential scanning calorimetry (DSC). In o-
DCB, the DSC analysis was carried at temperatures ran-
ging from 298 to 223 K. A very sharp exotherm attribuable
to the crystallization of supercooled solvent was observed
at 232 K (ΔH = 11.0 kJ/mol) (see Figure S2 in the
Supporting Information). This is approximatively 24 K
below the freezing point of free o-DCB (256 K). By heating
the gel from 223 to 298 K, an endotherm was observed at
257.6 K (ΔH = -11.1 kJ/mol), which corresponds to the
melting point of free o-DCB. A similar phenomenon was
observed for a toluene-based gel.
1 is made of micrometers-long 1-D wirelike fibers with
diameters of a few tens of nanometers. Interestingly, most
of the fibers assembled into well-ordered micrometers-
wide bundles, indicating strong interfiber interactions.12
After irradiation for 24 h, the fibrillar structure was
completely lost as a possible consequence of the destruc-
tion of the H-bonding network (Figure 2b). This result is in
agreement with the loss of the gel state observed upon gel
irradiation.
In order to study the intermolecular interactions and
distances within the supramolecular assembly, a gel of
compound 1 (toluene, 10 mg/mL) was deposited onto a
glass slide, slowly dried at room temperature, and sub-
jected to XRD analysis. A recording of XRD spectra was
taken between angles of 2θ = 1° and 15° (see Figure S3 in
the Supporting Information). The diffraction pattern is
˚
characterized by three peaks at 22.9, 11.4, and 7.5 A, which
correspond to a 1:1/2:1/3 ratio. This peak pattern is
indicative of the presence of a layered structure with an
interlayer distance of 2.3 nm.13 To draw a comparison, a
sample of compound 1 was dissolved in MeOH and cast
onto a glass slide substrate. XRD analysis after drying did
not show any crystallographic peaks, thus confirming the
lack of arrangement in the solid state (see Supporting
Information).
Photopolymerization was achieved on a 10 mg/mL
toluene gel of 1 by irradiation at 254 nm with a 120 W
UV lamp at a distance of 2 cm for 6 h. The blue state
generally associated with the highly extended π-delocaliza-
tion along the polydiacetylene backbone14 was never
reached in the gel form. Also, after a dark orange form
was obtained, a loss of the gel state and the appearance of a
red, translucent solution after 6 h of irradiation were
observed (Figure 3). This phenomenon observed by
To gain a better understanding of the nanoscale mor-
phology of the organogel, a small portion of it was slowly
dried under ambient conditions onto a metallic substrate
and subjected to scanning electron microscopy (SEM)
analysis. Asshown inFigure2, theorganogel ofcompound
Figure 3. (a) Compound 1 in the gel form (10 mg/mL in toluene)
at room temperature; (b) the same gel after irradiation at 254 nm
for 4 h; and (c) the gel evaporated on a glass slide and irradiated
for 24 h at 254 nm.
Moreau et al. during the photopolymerization process of
alkyl-substituted butadiyne derivatives could be attributed
to a significant conformational change of the aryl groups
within the gel, thus disturbing the hydrogen bond network.2g
(12) Zhang, P.; Wang, H.; Liu, H.; Li, M. Langmuir 2010, 26, 10183–
10190.
(13) (a) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Langmuir 2000,
16, 7558–7561. (b) Hanabusa, K.; Matsumoto, M.; Kimura, M.;
Kakehi, A.; Shirai, H. J. Colloid Interface Sci. 2000, 224, 231–244.
Figure 2. SEM images of organogel from compound 1 in toluene
(10 mg/mL) before (a) and after (b) irradiation for 24 h at 254
nm. The scale bars are 1 and 0.5 μm for (a) and (b), respectively.
€
(14) Wenz, G.; Muller, M. A.; Schmidt, M.; Wegner, G. Macro-
molecules 1984, 17, 837–850.
1360
Org. Lett., Vol. 13, No. 6, 2011

Interestingly, when the initial gel was dried onto a glass
slide and then irradiated, the film that was formed rapidly
turned dark blue, which can be attributed to the formation
of a polydiacetylene with an extended conjugation length
(see Figure 3c). Unlike the case of compound 1, the
resulting solid is not soluble in methanol but readily
soluble in THF. After purification of the dark material
using a Soxhlet apparatus in methanol, only the polymer
P1 was recovered (with trace amounts of the starting
material) for a polymerization yield of 75% yield, which
is quite higher than the previous yields reported for similar
topochemical reactions.5 More importantly, size-exclusion
chromatography (SEC) analysis shows a molecular weight
of27.9kDawitha polydispersity index of only1.8, which is
surprinsingly low for a solid-state polymerization.
Figure 4. P1 in THF at (a) 20 °C and (b) 60 °C. (c) UV-vis spectra
of P1 at 20 °C (red line) and at 60 °C (blue line) recorded in THF.
Using the gel rather than a solution of compound 1 to cast
a film onto a glass substrate is of utmost importance to
achieve the topochemical reaction in the solid state. In fact,
when a solution of compound 1 in methanol is cast onto a
glass slide, the film thus formed remains unchanged upon
irradiation, meaning that it does not undergo the topochem-
ical reaction to yield the polydiacetylene structure. This result
is consistent with the XRD observations discussed above.
For a better insight into the topochemical reaction, the
photopolymerization reaction was monitored using Fourier-
transformed Raman spectroscopy (FT-Raman). Before
irradiation, the Raman spectrum of compound 1 shows
two peaks at 2215 and 2095 cm-1 attributed to the
symmetrical and asymmetrical stretching modes of 1,3-
butadiyne, respectively (see Figure S4 in the Supporting
Information). Upon irradiation, the band at 2215 cm-1
completely disappeared suggesting a change in the nature
of the CtC units. The peak between 1400 and 1600 cm-1
generally associated with the formation of the enyne unit
cannot be observed directly since this region of the spec-
trum contains several peaks, mainly attributed to aromatic
ring stretching and bending.15
purple when cooled at room temperature. As a consequence
of this color change, the intensity of the absorption band at
616 nm in the UV-vis spectrum decreased while the band at
565 nm is blue-shifted (14 nm) (Figure 3). In the solid state,
the dark blue film gradually turned orange upon heating
and went back to dark blue upon cooling (see Figure S5 in
the Supporting Information). This rare reversibility prop-
erty for a polydiacetylene opens the way to usage of this new
family of conjugated polymers in sensor devices.3c
In summary, we have demonstrated that a high molecular
weight polydiacetylene with a low polydispersity index can be
obtained in good yield from a butadiyne unit substituted with
sterically demanding phenyl groups using a light-promoted
topochemical reaction. This reaction can be accomplished in
both the gel and solid states. The synthesis of other functional
diarylbutadiyne derivatives is in progress to extend the scope
of this reaction to the synthesis of various electro- and
photoactive diarylbutadiyne derivatives.
Acknowledgment. This work was supported by
NSERC through a Discovery Grant. We thank Richard
Janvier (U. Laval) for his help in SEM experiments,
Rodica Plesu (U. Laval) and Jean-Franc-ois Rioux (U.
Laval) for their help in polymer characterization, and
Philippe Dufour (U. Laval) for HRMS experiments.
As in rare cases for polydiacetylenes,3c P1 shows rever-
sible thermochromism properties in organic solvents
(Figure 4). When P1 was dissolved in THF at room
temperature and heated to 60 °C, the clear dark purple
solution gradually turned reddish. The solution goes back to
Supporting Information Available. Experimental proce-
dures and characterization data for all the new compounds,
DSC, XRD, and Raman spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
(15) Baranovic, B.; Colombo, L.; Furic, K.; Durig, J. R.; Sullivan,
J. F.; Mink, J. J. Mol. Struct. 1986, 144, 53–69.
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