Preparation and Characterization of Poly(diiododiacetylene)
A R T I C L E S
13C MAS NMR. The insolubility of the PIDA cocrystals
makes it difficult to characterize them by solution-phase NMR.
However, magic-angle spinning NMR (MAS NMR) techniques
allow for NMR characterization of the solid samples, particularly
the cocrystals formed with hosts 5 and 6 that cannot be identified
completely by X-ray diffraction. The 13C MAS NMR spectrum
of cocrystal 4·1, characterized previously by X-ray diffraction,
contains peaks at 161.2, 122.6, 39.2, 29.2, 24.6, and 16.7 ppm
(corresponding to the host) and at 80.0 ppm (monomer
ꢀ-carbon), while the signal for the R-carbon of the monomer, a
broad peak in the range of 10-20 ppm,21 is hidden by the host
methylene carbon peaks, as shown in Figure 5A. The spectrum
of polymer cocrystal 4·2 exhibits two new peaks at 110.6 and
80.1 ppm, attributed to the ꢀ (sp) and R (sp2) carbons of PIDA,
respectively (Figure 5B). The polymer R-carbon has a chemical
shift very similar to that of the monomer ꢀ-carbon, but it is
much broader because of the interaction of the R-carbon with
the quadrupolar iodine atom.
Using these 13C NMR spectra as reference, we can determine
the composition of the cocrystals formed with hosts 5 and 6.
The spectrum of the dark blue, week-old cocrystals with host
5, as shown in Figure 5C, does not include clear resonances
corresponding to either monomer or polymer carbons. Instead,
there is a very broad peak or collection of peaks in the range of
70-100 ppm. After heating of the dark cocrystals in vacuum
at 30 °C for 1 month, the NMR spectrum shows a new resonance
at 110 ppm and a broader signal centered at 81 ppm (Figure
5D). The cocrystals with host 5 evidently polymerize very
slowly at room temperature, turning dark gradually. Heating
the samples increases the rate of polymerization, but the crystals
do not regain crystallinity. Figure 5E shows the NMR spectrum
of the gold cocrystals with host 6, with one peak at 109.6 ppm
and another double-peak signal centered at 85.0 ppm. The reason
is not yet clear for this splitting in the peak attributed to the
R-carbon, but this double peak integrates at a ratio of ∼1:1 with
the peak at 110 ppm, corresponding to the ꢀ-carbon, indicating
the formation of fully polymerized PIDA within the cocrystals.
Figure 4. The lighter background shows the structure of the monomer
diiodobutadiyne cocrystal; the bold foreground drawing shows the structure
of the resulting PIDA. The symbols in the figure represent the halogen bond
angle changes between guest and host (blue, monomer; red, polymer; dashed
line, pointing out of the plain). The table below summarizes the changes in
the crystal structure upon polymerization.
appearance from opaque blue to copper-colored and highly
reflective, with a concurrent transformation in the crystal
structure to the previously observed PIDA cocrystal. This single-
crystal-to-single-crystal topochemical polymerization is shown
in Figure 3, while Figure 4 indicates the intermolecular changes
that occur as a result of the polymerization. As the mole fraction
of polymer nears 100%, the crystals take on a characteristic
metal-like appearance.
Within the 4·1 cocrystals, the repeat distance of aligned
monomers is 4.98 Å, the C4-C1′ contact distance is 3.88 Å,
and the molecular tilt angle is 52°. After polymerization, the
repeat distance shortens to 4.94 Å. The topochemical polym-
erization occurs spontaneously at room temperature, requiring
relatively little atomic motion, other than movement of the
carbon atoms that are directly involved in the bonding changes,
as demonstrated in Figure 4.
Efforts to determine the structures of the cocrystals obtained
with host 5 and 6 by X-ray diffraction have been inconclusive.
The mosaicity of the cocrystals prevents complete analysis, but
the diffraction data do provide well-defined unit cells of both
cocrystals, allowing us to determine the ratio of host to guest
in each case. In the case of cocrystal 5·1, assuming a host/
guest ratio of 1:1, the measured non-hydrogen atomic volume
is 19.1 Å3, and the calculated density is 1.94 Mg/m3, both values
that match well with those for previous fully determined
cocrystals. In contrast, the unit cell parameters of cocrystal 6·2
suggest a 1:2 ratio of host and guest within the cocrystal; the
non-hydrogen volume (22.0 Å3) and calculated density (2.02
Mg/m3) for a 1:2 stoichiometry are both consistent with those
observed in cocrystal 4·2 (Table S1, Supporting Information).
Within cocrystal 6·2, the iodine atoms are resolvable, and they
are consistent with the proposed all-planar polymer structure.
Raman Spectroscopy. Raman spectroscopy is a sensitive tool
for identifying the alternating ene-yne structure of poly(diacety-
lenes). Figure 6 shows the progressive change in the Raman
spectrum of cocrystals with host 4 over the course of the
topochemical polymerization of 1. The Raman spectrum of the
initial blue 4·1 cocrystals has a relatively low scattering
intensity, shown as the blue spectrum in Figure 6. The band at
1413 cm-1, corresponding to a carbon-carbon double bond
stretching mode, indicates the presence of some polymer,
consistent with the crystals’ blue color. After staying at room
temperature for 3 days, when the crystals have turned deeper
blue and have an approximately 50% degree of polymerization,
as determined by X-ray diffraction, the Raman scattering
intensity has increased greatly, and three major bands are visible
at962,1415,and2067cm-1,correspondingtothecarbon-carbon
single bond, double bond, and triple bond stretches, respectively
(ν(C-C), ν(CdC), and ν(Ct C)). When the crystals have
polymerized fully, gaining a coppery appearance, the Raman
intensity (red spectrum, Figure 6) increases to about 100 times
that of the initial spectrum. Here, ν(C-C) is found at 971 cm-1
,
ν(CdC) is at 1415 cm-1, and ν(C≡C) is at 2071 cm-1
.
These observations are consistent with the increased concen-
tration of polymer 2 within these cocrystals. The polymer itself
exhibits a very strong Raman spectrum because of the large
polarizability of the PDA backbone. Initially, the cocrystals
contain only small amounts of polymer. As the reaction
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J. AM. CHEM. SOC. VOL. 130, NO. 24, 2008 7705