filtered with a syringe filter and applied on a slide glass. The
ꢁ
The IR study indicated that absorption peaks at 1716 cmꢀ1
(nC]O) and 1250 and 1110 cmꢀ1 (nC–O) newly appeared, while the
carboxyl C]O peaks at 1698 cmꢀ1 disappeared after the esteri-
fication reaction, implying the formation of Poly-AC (Fig. 1(c)
and (d)). The crosslinked polymers afforded transparent, hard
coatings.
solvent was removed at 45 C for 12 h under atmospheric pres-
sure to obtain a film. In a grinding experiment, the film specimen
was vigorously ground in a mortar for 1–4 h at room tempera-
ture. The ground samples were dissolved in NMP (2 mL) and the
resultant solutions were applied on quartz plates to obtain films
for UV-vis and fluorescence spectroscopy. For the 1H NMR
study, DMSO-d6 solutions of the ground samples were used. In
a heating experiment, the film specimens were heated at 85 ꢁC for
In order to investigate the mechanochemical cycloreversion of
the dimeric anthracene moiety upon cracking, Poly-AA (or Poly-
AC) coating specimens were ground into fine particles in
a mortar to create ‘a considerable number of cracks’. It was
found that each ground sample (50 mg) was completely dissolved
in NMP (2 mL) at room temperature to form a clear solution. In
contrast, when the original polymers (50 mg) were put in NMP
(2 mL), a small amount of insoluble portion (10–20 wt%) was
observed. A NMP solution containing the ground sample was
applied on a quartz plate for UV-vis spectroscopy. As shown in
Fig. 2(a), the coating of the ground Poly-AA showed optical
absorption in the range of 325 to 475 nm. In contrast, Poly-AA
itself exhibited only weak absorption above 325 nm (Fig. 2(a)).
Poly-AC showed a similar tendency upon grinding; the ground
Poly-AC showed strong optical absorption (Fig. 2(b)). The
results imply the regeneration of the anthracene moiety upon
grinding of the polymers. This can be supported by the
comparison of optical absorption of dimeric and monomeric
forms of AA: Di-AA shows practically no absorption while AA
shows strong absorption due to its highly conjugated structure
(see Fig. S3(c) and (a)†, respectively). A similar tendency was
observed in UV-vis spectra of AC and Di-AC (see Fig. S3(b) and
(d)†, respectively).
1
4 h in an oven. Samples for UV-vis and H NMR spectroscopy
were prepared in a similar fashion as described above.
Determination of absolute fluorescence quantum yield
A DMF (10 mL) solution containing 5 wt% poly(methyl meth-
acrylate) was prepared. To the solution, 1.0 ꢂ 10ꢀ6 mol of AA,
AC, or ethyl cinnamate was added and completely dissolved. In
the case of their dimers, 5.0 ꢂ 10ꢀ7 mol was used. Each solution
was applied on a quartz plate and the solvent was removed in
vacuo. Absolute quantum yields were measured using a 325 nm
He–Cd laser, a 6 inch diameter integration sphere, and a mono-
chromator with a photomultiplier tube (PMT). Powder and film
samples were used in the case of ground and unground polymers,
respectively.
Crack-sensing test
10 mg of Poly-AA (or Poly-AC) was dissolved in NMP (0.5 mL)
at 50–60 ꢁC, and the solution was applied on a surface of a plastic
ꢁ
substrate. The solvent was removed at 75 C for 12 h to obtain
It has been reported that dimeric anthracene derivatives
cyclorevert to monomeric ones photochemically or thermally.24
In this work all the samples were carefully treated in a dark place,
so there would be no possibility of photochemical cycloreversion
of the dimeric anthracene moiety. In order to rule out the
possibility of the thermal cycloreversion, several control experi-
ments were conducted. Especially, there was concern that local
hot spots may be created by grinding and the thermal cyclo-
reversion may be induced by thermal energy. Benzoyl peroxide
(BPO), a well-known initiator for radical polymerization, was
employed as a kind of ‘temperature indicator’. When heated at
85 ꢁC for 4 h in solution, BPO completely decomposed (see
Fig. S4†). It was reported that BPO decomposes below 100 ꢁC in
the film and the rate and product of decomposition vary
depending on the structure of the matrix material.32 In this study,
a transparent film of Poly-AC (or Poly-AA) containing BPO was
a film, and the plastic was slightly bent to generate cracks in the
film. The cracks formed were observed using the fluorescence
microscope by exposure of the film to UV or white light.
Results and discussion
9-Substituted anthracenes are known to form cyclooctane-type
dimers upon irradiation with light of l > 300 nm under inert
atmosphere.27 It was reported that 9-anthracenecarboxylic acid
(AC) gives mainly a head-to-tail dimer along with a small
amount of a head-to-head dimer.30 NMR spectroscopic studies
showed that the dimer of AC (Di-AC) prepared in this work has
only a head-to-tail structure (see Fig. S1†).31 A dimer of 9-
anthraldehyde (AA), Di-AA, was also prepared by photo-
irradiation of AA and confirmed to have a head-to-tail structure
by NMR spectroscopy (see Fig. S2†). Di-AA and Di-AC were
stable under usual synthetic conditions, and their melting points
ꢁ
prepared and subjected to heating at 85 C for 4 h in an oven.
ꢁ
ꢁ
were measured to be 189 C and 226 C, respectively.
After the thermal treatment of the film, BPO almost decomposed
(see Fig. S5†). We did not perform the analysis of the decom-
position product of BPO because it is beyond the scope of this
work. On the other hand, the Poly-AC (or Poly-AA) film con-
taining BPO was ground vigorously in a mortar for 4 h at room
temperature. Fig. S6† shows that BPO did not decompose during
the grinding. This indicates that the sample temperature does not
rise up to 85 ꢁC during the ꢁgrinding. When a Poly-AC (or Poly-
AA) film was heated at 85 C for 4 h in an oven, practically no
increase in optical absorption was observed (see Fig. S7 and S8†)
while a significant increase in absorbance was induced by
grinding the polymers (Fig. 2). Based on these results from the
control experiments, thermal cycloreversion of the dimeric
Two types of crosslinked polymers, Poly-AA and Poly-AC,
were prepared using Di-AA and Di-AC, respectively (Scheme 2).
Poly-AA was prepared via acetalization between hydroxyl
groups in PVA and formyl groups in Di-AA in the presence of
p-toluenesulfonic acid (p-TSA). The formation of Poly-AA was
confirmed by FT-IR spectroscopy (Fig. 1). After the reaction, the
absorption peak due to the formyl C]O bonds (1727 cmꢀ1
)
disappeared, and new peaks due to the acetal groups appeared at
1234 (nC–O), 1095 (nC–O–C), and 815 cmꢀ1 (skeletal vibration of
acetal ring) (Fig. 1(a) and (b)). On the other hand, Poly-AC was
synthesized through esterification between hydroxyl groups in
PVA and carboxyl groups in Di-AC in the presence of p-TSA.
1382 | J. Mater. Chem., 2012, 22, 1380–1386
This journal is ª The Royal Society of Chemistry 2012
Song, Young-Kyu
