Fig. 2 Absorption (a) and fluorescence (b) patterns obtained by irradiating
the frozen mesophase sandwiched between two glass substrates through
nega-type photomask. The pictures were taken without and with excitation
at 365 nm, respectively.
PhDO is a fluorescent mesogen due to the full conjugation in
its rigid part. Its chloroform solution gives a strong greenish
blue emission while in the mesophase the emission is red-
shifted and slightly reduced in intensity due to mesogen
stacking, as reported for similar mesogens.9 During photo-
polymerization in the frozen mesophase, the original pale
yellow color of PhDO gradually turned brown with a remark-
able increase in visible absorption.† Moreover the fluorescence
originating from the conjugated oxadiazole unit was completely
queched after polymerization probably due to complete self-
absorption by the polydiacetylene backbone with strong and
enlarged visible absorption. Though color changes upon
increased absorption are common, fluorescence emission and its
change in diacetylene polymerization is a rare case. Thus these
optical characteristics of PhDO suggest a novel application of
diacetylene polymerization, i.e. image photopatterning. Fig. 2
shows photographs of absorption (a) and fluorescence (b)
patterns obtained by illumination through nega-type photo-
mask, where dark and non-fluorescent parts were photo-
polymerized. The pattern resolution is estimated to be far below
10 mm which is the limitation of our photomask.
Fig. 1 XRD patterns of PhDO representing the triclinic mesophase (a, 80 °C
on cooling) and a possible model for assembly of PhDO in the triclinic
lattice (b). The length of the rigid part (28 Å) was calculated using Mopac
97 (PM3 parameter).
rod–coil molecules, all of them are cubic phases with spherical
micelles or are organized into a non-cubic three-dimensional
lattice with tetragonal symmetry.6–8 To the best of our
knowledge, PhDO is the first example showing a micellar
triclinic mesophase among phasmidic and rod–coil molecules.
Though the reason is not clear at this point, the deviation from
cubic structure seems to imply that the micelle formed by the
double-bent PhDO molecules (see the structure in Scheme 1) is
of non-spherical, anisotropic shape as is illustrated in Fig. 1b.
Photopolymerization of PhDO was investigated with a
sample quenched from the mesophase to room temperature
using a high pressure Hg lamp (polychromic, 14 mW at 365
nm). We noted that the XRD pattern after quenching remained
identical to that of the mesophase, identifying the frozen state of
the mesophase due to the suppressed crystallization by high
viscosity. Interestingly, after exposure for 30 min the polymer-
ized sample retained the XRD pattern of its monomer
mesophase apart from a small reduction of the intra- and
intermicellar distances. The differences of XRD peaks are
smaller than 0.03 nm21 in q spacing.† In the IR spectrum,
absorbances at 2148 and 2222 cm21 from carbon–carbon triple
bonds decreased significantly after irradiation, from which the
conversion was estimated as ca. 33%.† Accordingly, the
obtained products are oligomers with a degree of polymer-
isation of ca. 10, inevitably resulting from the limited numbers
of molecules within the closed micelle. Maintenance of the
supramolecular structure of monomer assembly after polymeri-
zation is a rare case for diacetylene mesogens.4 According to the
proposed micellar structure of PhDO, the reason for the
virtually unchanged supramolecular nanostructure of polymer
may be as follows: topochemical polymerization occurs within
the closed micelle due to the suitable stacking distance of
diacetylenes (4.6 Å), while intermicellar reaction is completely
limited by the isolating effect of flexible tails. Consequently
only a small shrinkage in micelle size is allowed by polymeriza-
tion, which does not lead to changes in the supramolecular
structure. Importantly such a polymerization-induced fixation
of micellar structure without intermicellar connection provides
a simple and potential method to prepare nano-size polymeric
particles; investigations into this are being carried out in our
group.
In summary, a novel phasmidic mesogen, PhDO, was found
to form a micellar triclinic mesophase and proved to be a new
type of fluorescent diacetylene capable of fixing the mesophase
structure by topochemical polymerization. Fluorescence image
patterning by photopolymerization was demonstrated.
We acknowledge CRM-KOSEF for financial support of this
work.
Notes and references
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