Reversible thermochromism in hydrogen-bonded polymers containing polydiacetylenes

Article abstract of DOI:10.1002/anie.200453995

Out of the blue: A reversible temperature-dependent change in the absorption characteristics of spin-coated terephthalic acid/diacetylene polymers is observed. The polymer is prepared by topochemical photopolymerization of the diacetylene side chains of t

Full text of DOI:10.1002/anie.200453995

Angewandte
Chemie
Thermochromism
Reversible Thermochromism in Hydrogen-
Bonded Polymers Containing Polydiacetylenes**
Zhongzhe Yuan, Chan-Woo Lee, and Suck-Hyun Lee*
Numerous diacetylene (DA) derivatives have been synthe-
sized and incorporated into polymerized assemblies for their
potential applications as optoelectronic devices and biosen-
[*] Z. Yuan, C.-W. Lee, S.-H. Lee
Department of Molecular Science and Technology
Ajou University
Suwon 443-749 (Korea)
Fax: (+82)31-219-1592
E-mail: hyja@madang.ajou.ac.kr
[**] We thank the financial support by Korea Science and Engineering
Foundation through the Hyper-Structured Organic Materials
Research Center at the Seoul National University. We thank
Professor Zin at POSTECH for helpful discussions with X-ray
characterization.
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 4197 –4200
DOI: 10.1002/anie.200453995
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Communications
sors.[1] The most extensively studied systems are self-
assembled or surface-attached layers (vesicles and
Langmuir–Blodgett films) containing polydiacetylenes
(PDAs).[2] Much less explored systems include large
molecules containing dyes or other functional units
bridged by a DA group.[3] With these materials sub-
stantial progress has been made in understanding the
nature of the polymerization of DA moiety, the spectral
changes, and the thermochromic transitions of PDAs.
The rationale behind our work is that by choosing the
appropriate substituents the supramolecular polymeric
system will provide a specific order of the reactive
groups and promote a certain degree of flexibility to
accommodate structural changes during the polymer-
ization and subsequent color changes of the PDA. In
the construction of supramolecular polymers hydrogen
bonding plays a prominent role because of its direc-
tionality and versatility. However, hydrogen bonds
between aromatic dicarboxylic acids have rarely been
used to form supramolecular polymers.[4] Recently we
have been able to construct very stable dispersions of
supramolecular polymers from the doubly hydrogen-
bonded compounds of long dialkoxy substituted ter-
ephthalic acids.[5] The phase separation resulting from
the presence of the apolar side-chain groups stabilizes
the terephthalic acid core through the formation of self-
assembled layers of molecules. Herein we report a novel class
of similar supramolecular polymers containing DA moieties
in the side chains. Using these hydrogen-bonded terephthalic
acid DA compounds allows the easy preparation of PDAs
with characteristics, such as thermoreversible color switching,
which differ significantly from those of conventional systems.
The structures of our materials and their synthesis are
schematically shown in Figure 1 and Scheme 1. The powder
Scheme 1. Preparation of the compounds DA1 and DA2. a) I2,
ꢀ
b) HO(CH2)nC CH, c) p-toluenesulfonyl chloride, d) and e) diethyl-2,5-dihy-
droxyterephthalate, f) CH3(CH2)17Br, g) KOH, h) KOH.
Figure 2. Powder X-ray diffractograms of DA1, PDA1, DA2, and PDA2
at 258C. Polymers prepared in dispersion (CHCl3/hexane).
meric properties, such as high solution viscosities. To under-
stand the influence of the DA content and its specific
molecular packing within the side chains on the layer
structure and thermochromism of the PDAs (PDA1 and
PDA2), we examined the polymerization behavior for the two
compounds DA1 and DA2 upon exposure to UV light. The
polymerization of DA groups, which is a topochemical (or
topotactic) polymerization, requires specific molecular posi-
tion and orientation in the solid-state, which gives rise to a lot
of barriers to their production. In our materials the DA
groups could be polymerized either in a simple dispersion
medium of chloroform/hexane or in spin-coated films.
Figure 3 presents the comparison of the UV/Vis spectra for
DA1 in various sample forms before and after UV exposure.
The spectra for DA2 are omitted because the spectra of DA1
and DA2 are nearly identical except the relative intensity
ratio IPDA/IDA. The not-polymerized sample in a CHCl3
Figure 1. Stackingstructures of the compounds a) DA1 and b) DA2.
X-ray diffraction results were not sufficient to allow a
complete indexing of all the reflections but confirmed that
during the photopolymerization process the highly ordered
layer structure is preserved with slightly decreased interlayer
spacings and side-chain crystallinities (Figure 2). These DA
compounds DA1 and DA2 (Scheme 1) are soluble in common
organic solvents, can be deposited by casting or spin coating,
and even before polymerization display some typical poly-
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Chemie
insoluble part (from the soluble monomer part owing to
incomplete polymerization) was also much lower than for the
dispersion specimen (79%). We think that the localized shear
stresses and the accompanying changes in samples and
molecular dimensions produced during the spinning process
drives the films to a more disordered “red” film as a result of
the perturbation of the p-conjugation system. It is also of note
that the topochemical polymerization observed for DA1
provides evidence for the spontaneous association of like–like
side chains, a macroscopic expression of molecular DA
recognition. As this compound has asymmetric side chains
attached to the terephthalic acid units it seems unlikely that
DA chains that are just held in position geometrically will be
able to overcome thermal fluctuations and pack in a stable
enough way for the topochemical reaction to occur in the
liquid state.
To investigate the thermochromic transition of the spin-
coated films, we measured absorption spectra against temper-
ature based on the results of differential scanning calorimetry
(DSC) measurements (Figure 4). These materials exhibit
Figure 3. A) UV/Vis absorption (a,b,c) and transmission spectra (d,e)
of: a) DA1 in CHCl3 solution, b) a dispersion of DA1 in CHCl3/hexane
(0.5% v/v) unexposed and c) exposed to UV irradiation (254 nm,
23 mWcmÀ2) for 20 min, d) a spin-coated film of DA1 on quartz unex-
posed and e) exposed to UV irradiation for 1 min. B) Reversible colori-
metric transitions of a spin-coated film made from PDA1 as thermal
stimulus was imposed and removed at the indicated temperatures.
solution, in a dispersion of CHCl3/hexane, and as a film show
no absorption peaks in the region from 400 to 720 nm and all
these probes fluoresce when irradiated with UV/Vis light. A
characteristic absorption band of the dispersed phase with
lmax around 370 nm is red-shifted by 25 nm relative to the
corresponding band in solution. A similar absorption band is
also observed for the spin-coated film. This excitonic absorp-
tion shift can be understood as an indication that under the
given experimental conditions supramolecular polymers form
an aggregate by parallel stacking of the aromatic rings of the
terephthalic acid cores and the side chains (Figure 1).[6] The
assembly of the aromatic groups into a rigid core by stacking
is a prerequisite for the formation of the layered architecture
which is required for the DAunits to react topochemically. As
shown in Figure 3, the absorption spectrum for the dispersion
sample has a distinct maximum at 640 nm with a shoulder
around 580 nm after UV irradiation, which is characteristic
for PDAs. When the irradiation time is extended to 20 min,
the absorption intensity of the so-called blue form of the
polymer at 640 nm increased and the maximum only shifted
slightly towards shorter wavelengths. Continuing irradiation
for an hour results in a weak blue-form absorption band,
leaving the red form at 580 nm dominant. The transmission
spectrum for the spin-coated PDA sample is similar but has a
blue-shifted absorption at 600 nm and another at 540 nm
ascribable to the red form, which indicates the formation of a
mixture of red and blue forms. Note that a fractional
monomer conversion after photopolymerization for spin-
coated films (ꢁ 23%) determined by isolating the polymeric
Figure 4. DSC traces of compounds DA1, PDA1, DA2, and PDA2.
structural and thermal behaviors similar to the side-chain
polymers based on a rigid backbone (see Supporting Infor-
mation).[7] Their general architecture can be described in
terms of hydrogen-bonded aromatic rigid backbones sepa-
rated by layers composed of relatively well packed side
chains. The two endothermic transitions observed by DSC
before melting of the whole sample indicate that the proper-
ties of the rigid-backbone layers are considerably different
from the properties of the side-chain layers. The low-temper-
ature transition is attributed to the onset of disordering
processes in the side-chain region, while the layered structure
and thus the ordered arrangement of the backbones is
maintained, the higher transition temperature is associated
with motion of the rigid-core. As illustrated in Figure 3B,
polymer PDA1 shows completely reversible spectroscopic
changes during the thermal cycles. The blue form which gives
rise to the band at 600 nm progressively disappears and the
red form appears as the temperature increases. The absorp-
tions of the red form appear at 540 and 500 nm at 350 K and
then grow in intensity up to the melting temperature of 408 K
even though the film was not fully blue to begin with. Upon
cooling to room temperature the blue form reappears in a
completely reversible manner. This remarkable thermochro-
mic behavior is not usually observed in PDAs. To our
knowledge, only a few examples have been reported to date.[8]
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A more detailed investigation of the spectra reveals that
there appears to be an isosbestic point above the transition
temperature of 352 K. It shows that increasing temperature
merely increases the amount of the red form without passing
through any observable new intermediate form. DSC data
suggest that the spectral change above the transition temper-
ature of 352 K is triggered by strain coming from the side-
chain layers existing in a liquid state. Thermal stimulus gives
the side chains enough energy to impose mechanical strain on
the PDAs by rotation around the single bonds between the
methylene units although such movement of the side chains
must be restricted so that the necessary crystal integrity for
reversibility is maintained. Theoretical calculations revealed
that such a side-group motion could induce a change in the
electronic structure of PDAs.[9] Thus, the origin of this
chromism remains unclear but our data support a mechanism
in which the red form, a form which indicates a diminished
conjugation length, is merely a thermally equilibrated molec-
ular fluctuation of the blue form. The observed reversibility in
PDA2 (see Supporting Information) corroborates that the
layer integrity is retained at higher annealing temperatures. In
fact, polymerization of DA2 produces the unusual rigid
ladder-type bonding in the layer structures (see Figure 1) and
such sterically controlled systems have no degree of freedom
to respond to intralayer strain that causes structural mod-
ifications. Thus up to the backbone melting temperature the
thermochromic transitions for our material are dominated by
the flexibility of the side-chain packing between supramolec-
ular polymers.
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The experimental data presented herein do not allow a
conclusive molecular model to be constructed for the spatial
arrangement of the PDA chains with respect to each other
and the terephthalic acid plane. We can, however, conclude
that the combination of double hydrogen-bonds between the
hard terephthalic acid cores and a molecular recognition of
DA moieties in the side chains, which enables topochemical
polymerization to occur, allowed us to prepare PDAs in
dispersed solutions or even in spin-coated films. The forma-
tion of stable, ultrathin polymer films is crucial for the
envisaged application of PDAs as sensors for biological
detection or coatings for optoelectronic applications. These
materials also have thermochromic properties which allow a
reversible color switching, thus expanding the scope of
potential applications for polydiacetylenes.
Received: February 11, 2004
Revised: May 5, 2004 [Z53995]
Keywords: alkynes · hydrogen bonds · layered compounds ·
.
polymers · thermochromism
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