Thermal reaction of a columnar assembled diacetylene macrocycle

Article abstract of DOI:10.1021/ja9107066

Reported is a macrocyclic diacetylene that assembled into columns to afford porous crystals. Heating this assembly initiated a topochemical polymerization of the preorganized diacetylene units to give covalent conjugated polydiacetylenes. These stable con

Full text of DOI:10.1021/ja9107066

Published on Web 03/25/2010
Thermal Reaction of a Columnar Assembled Diacetylene Macrocycle
Yuewen Xu, Mark D. Smith, Michael F. Geer, Perry J. Pellechia, Julius C. Brown, Arief C. Wibowo,
and Linda S. Shimizu*
Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208
Received January 4, 2010; E-mail: shimizul@chem.sc.edu
Conjugated polymers have demonstrated utility in organic solar
cells,1 chemical sensors,2 and optoelectronics.3 Microporous materi-
als such as metal organic frameworks (MOFs) and zeolites show
practical applications in gas storage,4 separation,5 catalysis,6 and
as confined environments for reactions.7 MOFs and covalently
linked organic frameworks8 utilize transition metal ions or organic
monomeric units to afford porous materials with high surface areas
and excellent thermal stabilities. An alternate strategy for forming
porous materials relies on the columnar assembly of molecular
building blocks directed by noncovalent interactions. We sought
to combine the attributes of a conjugated polymer with porous
materials. Herein, we report a diacetylene macrocycle that was
readily synthesized and self-assembled into columns to afford
porous crystals (Figure 1). Heating initiated a topochemical
polymerization of the preorganized diacetylene units to give
covalent conjugated polydiacetylenes (PDAs). These stable con-
jugated materials maintained permanent porosity as evidenced by
their type I gas adsorption isotherms with CO2 (g). Conjugated
porous PDAs could have applications for sensing and in electronics.
served to stabilize the column. However, the exact pattern could
not be determined due to disorder of the water molecules. Two
sets of diacetylenes were aligned along the column on each side.
This columnar assembly adopted a monomer repeat distance dm at
4.98(6) Å and positioned neighboring C1-C4 distance R1,4 at
3.58(1) Å, close to the structural parameters preferred for diacety-
lene polymerizations.10 The shortest intercolumnar distance between
diacetylene units is ∼6.2 Å (Figure 2b).
Figure 2. X-ray crystal structure of macrocycle: (a) A single tubular array
oriented by N-H---O hydrogen bonds; H2O molecules were omitted for
clarity. b) Packing diagram of the porous structure looking down the tube
axis.
Thermally induced transitions of 1 were studied by differential
scanning calorimetry (DSC) (Figure 3a). Assembled 1 showed an
irreversible endothermic transition at ∼90 °C, which corresponded
to a loss of water as observed by TGA (SI). Irreversible exothermic
reactions commenced at ∼180 °C within a reasonable range for
the reaction of diacetylenes. These multiple exotherms suggested
that the two sets of diacetylene units did not react simultaneously.
The integration of the peaks gave a polymerization enthalpy of 250
kJ/mol, nearly twice the value measured for polymerizations of
crystalline diacetylenes (∼150-165 kJ/mol),11 which was likely
due to the presence of two diacetylene units within each macrocycle.
The heated crystals turned dark purple/brown and were insoluble,
suggesting the formation of PDAs.11 Cooling and reheating showed
no phase transitions in this range (inset). Batches of crystals (30
mg) were heated at 180 °C for 12 h and afforded similar dark
insoluble crystals. UV-irradiation also initiated the reaction but to
a less extent (∼15%) based on recovery of the soluble monomer.
The UV-vis absorption spectra after heating showed that the sharp
monomer absorbance λmax ) 256 nm had disappeared, replaced by
a broad absorbance in the visible range, consistent with an increased
conjugation length in PDAs (SI). The presence of different length
oligomers, the aggregation of PDAs in the solid state, or cross-
links could account for the broad UV-vis spectrum.12 Cross-links
are less likely due to the large separation of diacetylene units in
neighboring columns of assembled 1.
Figure 1. Diacetylene macrocycle 1 self-assembled into columns via amide
hydrogen bonds. Upon heating the diacetylene units may undergo a
topochemical polymerization to yield polydiacetylenes.
Supramolecular interactions have been employed to construct
networks of diacetylenes. Relatively few diacetylenes whose
structures have been elucidated by X-ray undergo polymerization
to afford PDAs. Topochemical polymerization of diacetylene
requires a precise prealignment of reacting units in the crystalline
phase with a monomer repeat distance (dm) of ∼4.9 Å, a neighbor-
ing C1-C4 distance (R1,4) of 3.5 Å, and an orientation angle of
45°.10 Although acyclic diacetylenes have been polymerized, few
systems with supramolecularly assembled macrocyclic diacetylenes
have been investigated.11
Diacetylene 1 was synthesized in two steps from isophthaloyl
chloride and propargylamine (Supporting Information, SI). The key
step was an Eglinton acetylene-acetylene coupling reaction.12
Recrystallization yielded white needle crystals, and X-ray analysis
revealed the desired macrocycle. Macrocycle 1 stacked in columns
with each individual cycle tilted at an angle of 41.6° with respect
to the column axis (Figure 2a). The macrocycles assembled by
amide-amide hydrogen bonds into tubular structures with N-H---O
distances of 2.81 Å. Water molecules in the channel were also
involved in hydrogen bonding with carbonyl oxygen atoms, which
Powder X-ray diffraction (PXRD) was used to probe the structure
of 1 after heating because the crystals were no longer of quality
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5334 J. AM. CHEM. SOC. 2010, 132, 5334–5335
10.1021/ja9107066  2010 American Chemical Society

C O M M U N I C A T I O N S
adsorption isotherm with CO2 (g) at 0 °C was observed for the
heated material, which displayed a slightly larger surface area (480
m2/g). These results indicated that the polymer retained accessible
channels and suggested that the reaction occurs down individual
columns.
In summary, we synthesized a diacetylene macrocycle that
assembled into columnar structures and underwent a thermal
reaction to give oligo- or polydiacetylenes. Both the assembled
monomer and the covalent polymer are crystalline materials that
displayed permanent porosity. The covalent polymer is a robust,
insoluble crystalline material. We are currently investigating if
assembled 1 and heat-treated 1 exhibit changes in their optoelec-
tronic properties or conductivity upon guest absorption or envi-
ronmental perturbations, as PDAs have great potential as sensing
materials.
Figure 3. Effects of heating on host 1: (a) DSC plots of 1 (heating rate 10
°C/min). Inset shows cooling (20 °C/min) and reheating. (b) PXRD patterns
of assembled 1 and heat-treated 1.
Acknowledgment. The authors gratefully acknowledge support
for this work from the NSF (CHE-0718171). We thank Christopher
Williams in the Dept. of Chem. Eng. for Raman spectra.
Supporting Information Available: Synthesis and characterization
of host 1 including crystal data (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 4. (a) Raman spectra (excitation at 632 nm) of single crystal samples
of assembled 1 (bottom) and heat-treated 1 (top). (b) CO2 gas adsorption
isotherms at 0 °C of assembled 1 and heat-treated 1.
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