Improving the reactivity of phenylacetylene macrocycles toward topochemical polymerization by side chains modification

Article abstract of DOI:10.3762/bjoc.10.167

The synthesis and self-assembly of two new phenylacetylene macrocycle (PAM) organogelators were performed. Polar 2-hydroxyethoxy side chains were incorporated in the inner part of the macrocycles to modify the assembly mode in the gel state. With this modification, it was possible to increase the reactivity of the macrocycles in the xerogel state to form polydiacetylenes (PDAs), leading to a significant enhancement of the polymerization yields. The organogels and the PDAs were characterized using Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Full text of DOI:10.3762/bjoc.10.167

Improving the reactivity of phenylacetylene macrocycles
toward topochemical polymerization by
side chains modification
Simon Rondeau-Gagné‡, Jules Roméo Néabo‡, Maxime Daigle, Katy Cantin
and Jean-François Morin*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1613–1619.
Département de Chimie and Centre de recherche sur les Matériaux
Avancés (CERMA), Université Laval, 1045 avenue de la Médecine,
G1V 0A6 Québec, Québec, Canada
doi:10.3762/bjoc.10.167
Received: 28 January 2014
Accepted: 25 June 2014
Published: 15 July 2014
Email:
Jean-François Morin* - jean-francois.morin@chm.ulaval.ca
This article is part of the Thematic Series "Functionalized
carbon-nanomaterials".
* Corresponding author ‡ Equal contributors
Keywords:
Guest Editor: A. Krueger
carbon nanomaterials; organogels; phenylacetylene macrocycles;
polydiacetylenes; topochemical polymerization
© 2014 Rondeau-Gagné et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The synthesis and self-assembly of two new phenylacetylene macrocycle (PAM) organogelators were performed. Polar 2-hydroxy-
ethoxy side chains were incorporated in the inner part of the macrocycles to modify the assembly mode in the gel state. With this
modification, it was possible to increase the reactivity of the macrocycles in the xerogel state to form polydiacetylenes (PDAs),
leading to a significant enhancement of the polymerization yields. The organogels and the PDAs were characterized using Raman
spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM).
Introduction
The self-assembly of molecular building blocks is an increas- molecular assemblies often suffer from poor stability, meaning
ingly popular method for the preparation of new semicon- that variation of the storage and device operation conditions can
ducting materials. Rational design of building blocks and their perturb the molecular organization, leading to a decline of the
assembly using non-covalent interactions can provide control materials efficiency overtime [7]. In this regard, the covalent
over the size, shape and electronic properties of the resulting immobilization of supramolecular assemblies using physical
nanoarchitectures [1-3]. It is therefore not surprising that supra- stimuli (heat or light) is an interesting way to obtain stable,
molecular interactions are regularly used to afford long-range organized materials [8-11]. For example, self-assembled
molecular organization for organic and molecular electronics butadiyne-containing molecules can be polymerized in a gel or
applications in which high-level of organization is required to crystalline state to yield polydiacetylenes (PDAs) following a
reach good charge transport properties [4-6]. However, supra- 1,4-addition mechanism, thus fixing the initial molecular orga-
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nization through covalent bonds formation [12,13]. In order for the benzamide configuration yields macrocycles with much
this polymerization to proceed, the molecules within the greater gelation properties that allow the formation of PDA-
assembly must be oriented relative to each other following crit- based nanorods. Nonetheless, the polymerization of diyne units
ical parameters, namely a distance of ≤4.9 Å and a tilt angle of within PAMs to give covalent nanorods and nanotubes is very
45° between the monomers [12]. Thus, careful selection of slow and gives low yields of polymerization, generally too low
functional groups onto the molecule is needed to reach these to be determined accurately [38,39]. In order to increase the
requirements [14-20].
efficiency of polymerization, the incorporation of external diyne
chains on the PAMs core have been realized [31]. This design
Recently, many research groups used this strategy to design allowed us to increase the yield of polymerization to 15%.
several types of π-conjugated monomers capable of hydrogen However, the addition of external diyne units is synthetically
bonding to create nanowires [21-23], nanoparticles [24-27], tedious and a more efficient strategy to significantly increase
nanotubes [28-31] and two-dimensional layered materials [32] the yield of the topochemical polymerization is necessary.
from organogels. The key to success was to obtain a good
balance between solubility and gelation properties. Moreover, In order to take a step further toward a better comprehension of
subtle changes in the chemical nature of the building blocks can the self-assembly of PAMs and to increase the polymerization
have a dramatic impact on the self-assembly process. This was yield, we decided to introduce substituents on the inside of the
exemplified in the case of nanotubes obtained from self-assem- macrocycles, more precisely a polar 2-hydroxyethoxy group.
bled butadiyne-containing macrocycles, which stack on top of Such a polar group proved to be useful to increase the intermol-
each other in columnar fashion to give supramolecular ecular interactions between PAMs through hydrogen bonding in
nanotubular structures [33-38]. Among other things, we have the solid state [40,41]. We hypothesized that this structural
shown that inversion of the amide group configuration change could provide better control over the molecular organi-
(acetanilide vs benzamide) at two different positions on phenyl- zation of PAMs and, consequently, could lead to higher yield of
acetylene macrocycles (PAMs) leads to significant changes of PDA-walled nanorods through PAMs polymerization. Herein,
the gelation properties and, consequently, on the critical para- we report the synthesis, gelation properties and topochemical
meters needed for polymerization through 1,4-addition reaction polymerization of a new series of PAMs (PAM2 and PAM3,
[38]. In fact, the acetanilide configuration provides macrocy- Figure 1) with polar side chains pointing inside the macrocyclic
cles which can barely self-assemble in organic solvents while scaffold.
Figure 1: Structures of PAM1 to PAM3.
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PAM2 and PAM3 possess some structural dissimilarity. First, column chromatography was not effective due to the low solu-
PAM2 possesses two 2-hydroxyethoxy chains and two amide bility of the macrocycle after deprotection of the hydroxy-
groups in the benzamide configuration while PAM3 contains containing side chains.
four of each. It is important to mention that the alcohol chains
are positioned for synthetic ease. The synthesis and gelation The gelation properties of PAM2 and PAM3 have been studied
properties of PAM1 are reported in the literature [39]. PAM1 and compared to those of PAM1. For each PAM, a precise
was used in this study as a control molecule because of the quantity was mixed with the solvent in a capped glass tube.
absence of 2-hydroxyethoxy chains in its structure, providing a After sonication to breakdown the aggregates, the mixture was
reliable comparison.
heated until a clear solution was obtained. Then, the solution
was allowed to slowly cool at room temperature and the forma-
tion of the gel was confirmed in each case by the tube inversion
Results and Discussion
The synthetic pathway toward PAM2 and PAM3 is shown in method [39]. The gelation properties of PAM1, PAM2 and
Scheme 1. Starting from commercially available 4-hydroxyben- PAM3 in different solvents are summarized in Table S1
zoic acid, iodination was performed to obtain compound 1 in (Supporting Information File 1). PAM2, containing two amides
excellent yield (93%). Then, 2-hydroxyethoxy chains were and two 2-hydroxyethoxy side chains, has exactly the same
installed using K2CO3 to obtain compound 2 in good yield gelation properties as PAM1, demonstrating the absence of the
(61% yield). Amidation using diisopropylcarbodiimide (DIC) influence of the addition of side chains on the gelation prop-
and 6-chlorohydroxybenzotriazole (6-ClHOBt) as coupling erties. In the case of PAM3, however, most of the solvents
agents was then performed, followed by protection of the tested did not lead to gel formation. Thus, only cyclohexane and
hydroxy moiety with TBS in order to facilitate the further syn- toluene resulted in a partial gel state. The presence of four
thetic steps by increasing the solubility of the compound amide functions and four 2-hydroxyethoxy groups lead to a
(57% yield over 2 steps). Then, by using a standard significant decrease of solubility, even in hot solvents, limiting
Castro–Stephens–Sonogashira coupling, TMS protecting groups the formation of organogels.
were installed (73% yield). After basic removal of the alkyne
protective group, the half-macrocycle 5 was obtained by In order to investigate the crystallization temperature of
Castro–Stephens–Sonogashira coupling with previously solvents within the gel and to compare the influence of the
reported 3,5-diiodooctylbenzene [39] in good yields despite of 2-hydroxyethoxy side chains on the thermal stability of the gel,
the possible polymerization side reaction (54% yield). TMS- two gel samples were prepared with PAM1 and PAM2 in
protected alkynes were, then, installed on the half-macrocycle cyclohexane at a 10 mg/mL concentration and subjected to
to afford compound 6 which can be deprotected using potas- differential scanning calorimetry (DSC). The DSC analysis was
sium hydroxide and directly used in modified Eglinton ring carried out at temperatures ranging from 298 to 223 K. In both
closing reaction in pseudo-high dilution to afford hydroxy- cases, a very sharp exotherm attributable to the crystallization
protected PAM. Deprotection was directly performed without of supercooled solvent was observed (see Figure S22 and S23 in
purification using TBAF to afford PAM2 in good yields (88% Supporting Information File 1). For PAM2, the exotherm at
yield). Same approach was used for PAM3. Starting from 279 K is closer to the freezing point of cyclohexane (280 K)
previously synthesized compound 3, the half-macrocycle 9 was than for PAM1, which has an exotherm at 267 K. This result
obtained by standard Castro–Stephens–Sonogashira coupling suggests that the gel of PAM2 might be less organized than that
with dialkyne 8 in good yield (59%). It is noteworthy that com- of PAM1. By heating the gel from 223 to 298 K, an endotherm
pound 8 was obtained from oxidative deprotection of com- was observed in both cases, which is closely related to the
pound 7. After installing TMS-protected alkynes with good melting point of free cyclohexane [39].
yields (compound 10, 80% yield), selective removal of TMS
using potassium hydroxide and ring closing reaction under To explore the morphology of the structures created during the
modified Eglinton conditions were performed to afford a TBS gel formation, scanning electron microscopy (SEM) was
protected PAM. Deprotection of TBS using TBAF was then performed on PAM2 only since PAM3 did not present any
realized without purification to afford PAM3 in rather low gelation properties. For SEM analysis, a gel sample was
yield. We hypothesized that this low yield can be due to the allowed to dry at room temperature on a metallic substrate to
increased steric hindrance for compound 10 (2 OTBS groups) form a xerogel. Then, gold was sputtered on the sample prior to
compared to compound 6 (1 OTBS group), impairing the imaging. SEM images of PAM2 are shown in Figure 2. As
macrocyclization reaction and leading to a significant amount of previously observed with PAM1 and other phenylacetylene
oligomerized product. It is noteworthy that purification of macrocycles, a dense network of nanofibrils was formed during
PAM3 was realized by precipitation in methanol. Standard the gelation process [42-48]. These nanofibrils, commonly
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Scheme 1: Synthetic pathway to PAM2 and PAM3.
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Figure 2: Scanning electron microscopy (SEM) images of PAM2 xerogel in cyclohexane (10 mg/mL). Scales are a) 1 μm and b) 0.5 μm.
observed in the gel state, resulted from the strong intermolec- Given the observed polymerization of diyne units, UV–vis
ular interactions that create long-range one-dimensional arrays spectroscopy was performed to confirm the appearance of poly-
of molecules [49,50].
diacetylene. As shown in Figure 3, the UV–vis spectra shows
absorption bands at 600 and 650 nm, associated with the red
and blue bands of the PDA chain, respectively [51]. To deter-
mine whether all the diyne units of PAM2 reacted during irradi-
ation, Raman spectroscopy was performed on the resulting PDA
(Figure 4). As expected, no band associated with the diyne unit
(generally around 2200 cm−1) is present. The final PDA
presented bands at 1470 and 2100 cm−1, which could be res-
pectively associated to alkene and alkyne moieties of PDA. It
should be noted that these two bands are also present in the
Raman spectrum of PAM2 due to partial photo-induced poly-
merization during spectrum acquisition. This demonstrates the
high reactivity of PAM2 under UV irradiation. Further research
on the resulting nanoarchitectures is currently underway to
In order to gain some insights on the molecular packing within
the assembly, X-ray diffraction (XRD) was performed on
PAM2 (see Figure S21 in Supporting Information File 1).
Unlike PAM1, the presence of a columnar arrangement cannot
be established from the diffractogram [31,39]. Instead, only a
broad peak at 2θ = 20° was observed, indicating the presence of
intramolecular liquid-like order between the alkyl chains [2].
The absence of a well-defined pattern suggests that PAM2 does
not organize as well as PAM1 in the gel state and that the
2-hydroxyethoxy chains are unfavourable for the long-range
supramolecular organization.
Despite this unexpected finding, irradiation of a xerogel sample
of PAM2 in cyclohexane (10 mg/mL) was performed for 24 h
under UV light (254 nm). The resulting blue material was puri-
fied using size-exclusion chromatography (SEC) (Bio-Beads
SX-1) to remove all traces of the starting macrocycle. To our
surprise, an increase of the polymerization yields was observed
every time a polymerization was achieved with PAM2. In
several attempts, a maximum yield of 30% (soluble material)
was obtained after purification, representing a two-fold increase
compared to the best result obtained for the same PAM scaf-
fold (≈15% yield). This result suggests that the diyne units in
PAM2 are in a more suitable orientation than PAM1 to undergo
a topochemical polymerization, although gelation properties
and XRD result seem to suggest otherwise. It is also possible
that the apparent yield increase comes from a better solubility of
the PDA obtained from PAM2 compared to that obtained from
PAM1, although quantitative measurements of the solubility
have not been conducted because of the small quantity of ma-
terials prepared.
Figure 3: UV–vis spectrum of PAM2 before (black) and after (red)
polymerization (PDA).
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is currently underway to investigate the precise nature of the
nanoarchitectures formed.
Supporting Information
Supporting Information File 1
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-167-S1.pdf]
Acknowledgements
This work was supported by NSERC through a Discovery
Grant. We thank Rodica Plesu (U. Laval) and Jean-François
Rioux (U. Laval) for their help in polymer characterization, and
Philippe Dufour (U. Laval) for HRMS experiment. Simon
Rondeau-Gagné thanks the NSERC for a PhD scholarship.
Figure 4: Background-corrected Raman spectra of PAM2 (red) and
the blue material obtained after UV irradiation (black).
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In summary, the synthesis and self-assembly of two new
phenylacetylene macrocycles were performed. To improve the
self-assembly properties in the gel state and further yield to
better topochemical polymerization yield, the incorporation of
2-hydroxyethoxy side chains was accomplished. The gelation
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