Colorimetric reversibility of polydiacetylene supramolecules having enhanced hydrogen-bonding under thermal and pH stimuli

Article abstract of DOI:10.1021/ja0299001

To investigate the role of hydrogen-bonding on colorimetric transition of polydiacetylene supramolecules, novel diacetylene derivatives allowing various hydrogen-bonding states were synthesized by coupling carboxy-substituted (ortho-, meta-, and para-) an

Full text of DOI:10.1021/ja0299001

Published on Web 07/03/2003
Colorimetric Reversibility of Polydiacetylene Supramolecules Having
Enhanced Hydrogen-Bonding under Thermal and pH Stimuli
Dong June Ahn,*,† Eun-Hyuk Chae,† Gil Sun Lee,† Hee-Yong Shim,† Tae-Eun Chang,‡
Kwang-Duk Ahn,‡ and Jong-Man Kim*,§
Department of Chemical & Biological Engineering, Korea UniVersity, Seoul 136-701, Korea,
Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea,
and Department of Chemical Engineering, Hanyang UniVersity, Seoul 133-791, Korea
Received December 23, 2002; E-mail: ahn@korea.ac.kr; jmk@hanyang.ac.kr
Self-assembled polydiacetylene supramolecules1-3 in the form
of Langmuir-Schaefer (LS) or Langmuir-Blodgett (LB) films and
liposomes are attractive owing to their functionalizability and
applicability to colorimetric detection systems. Unique “blue-to-
red” colorimetric transition of variously modified polydiacetylene
supramolecules has been utilized to monitor ligand-receptor
binding events involving viruses,4 toxins,5 glucose,6 and ionic
interactions.7 The polydiacetylene-based chemosensors reported to
date, however, function via irreversible fashion. Accordingly, once
the blue-phase shifts to the red-phase upon a given external stimulus,
the backward “red-to-blue” transition does not occur even though
the stimulus is removed afterward from the system. To the best of
our knowledge, two examples of reversible chromism of the
polydiacetylene liposomes have been reported against temperature
with diacetylenic double-chain phosphatidylcholines8 and against
ionic binding with hydrazide-modified single-chain diacetylene
lipids.9 In this Communication, we report the first example of both
thermally stimulated and pH-stimulated reversible polydiacetylene
LS films made of a novel single-chain diacetylene derivative capable
of enhancing the strength of hydrogen-bonding of the resulting
assemblies. The role of enhanced hydrogen-bonding is also
spectroscopically analyzed in situ.
The strategy for the design of the reversible polydiacetylene
supramolecules is as follows. It has been well known that the
polymerized LB/LS films and liposomes prepared with 10,12-
pentacosadiynoic acid (PCDA, 1) do not show reversible chromism
Figure 1. Reversible colorimetric transitions of LS films made from
molecule 4: as thermal stimulus was imposed (a) and removed (b), and as
pH was varied (5.5 f 13.7 f 5.5) (c).
to add another functional group to PCDA is to utilize substituted
anilines. Coupling of an aniline derivative with PCDA should give
an amide moiety and an additional functional group, in this case,
a carboxylic group in the same molecule. Both functional groups
are capable of hydrogen-bonding, and the position of the terminal
carboxylic group can be manipulated by selecting proper substit-
uents. Thereby, diacetylene derivatives containing an unsubstituted
anilide group 2 and carboxy-substituted (ortho-, meta-, and para-)
anilide groups 3-5 were prepared. Monomeric molecules of PCDA
derivatives were assembled into ordered multilayers on a Langmuir
trough, polymerized by exposure to UV light (254 nm, 1 mW/
cm2), and then transferred by the LS method to hydrophobized glass
or CaF2 slides. All the diacetylene derivatives except the compounds
3 and 5 were found to form stable blue-colored LS films.
As expected, the blue-colored LS film made of the molecule 2
was colorimetrically irreversible upon removal of thermal and pH
stimuli after its first transition from blue to red. The purple-colored
LS film prepared with ortho-substituted 3 showed no sign of
reversibility. Interestingly, the LS film obtained with meta-
substituted 4 was found to have a remarkable reversibility (Figure
1). At room temperature, the film prepared with 4 showed typical
absorption spectra having a maximum absorption wavelength at
640 nm and another at 580 nm being relatively small but evident.
During heating from room temperature to 90 °C, the maximal
absorption shifted gradually downward from 640 nm and finally
reached 580 nm with temperature (Figure 1a). This is quite different
from the typical case of the molecule 1, showing a simultaneous
absorption increase at 550 nm with an absorption decrease at 640
nm, without having a gradual peak shift. This indicates that the
average conjugation length of π electrons along the polymer
against temperature and pH. We reasoned that the hydrogen-bonding
of the carboxylic headgroups of polymerized PCDA was not strong
enough for the supramolecules to maintain proper molecular
organizations required for the reversible color switching. Accord-
ingly, adding another functional group to the headgroup capable
of additional hydrogen-bonding would affect the nature of overall
hydrogen-bonding, thus allowing the supramolecules to be in
adequate conditions for the reversible chromism. One obvious way
† Korea University.
‡ KIST.
§ Hanyang University.
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J. AM. CHEM. SOC. 2003, 125, 8976-8977
10.1021/ja0299001 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Schematic of enhanced hydrogen-bonding at terminal carboxyl
and amide-carbonyl groups. Although only intramolecular bondings are
schematically indicated, intermolecular ones are also possible.
back to 25 °C, the strength of hydrogen-bonding was not recovered.
By contrast, molecule 4 maintained the initial strength of hydrogen-
bondings upon the thermal cycle, as indicated by the absence of a
peak shift. Hence, it seems most probable that sturdy, well-
developed double hydrogen-bondings among headgroups of mol-
ecule 4 (Figure 3) provide its supramolecular film with the capability
to recover its initial molecular organization, so that the average
conjugation length of the π electrons returns to the initial value
upon removal of external stimuli. The results on the role of
enhanced hydrogen-bonding in color change should be potentially
useful for designing reversible colorimetric sensors based on
polydiacetylene supramolecules.
Figure 2. Transmission FTIR spectra of LS films of polymerized PCDA
derivatives on CaF2 (a), and in situ external reflection FTIR spectra of LS
films of PCDA (b) and PCDA-mBzA (c) on hydrophobized glass.
Acknowledgment. This study was supported by grants from
the IMT2000 Program (Biomolecular Self-Assembling Nanoma-
terials Center, D.J.A.) and the KOSEF (Center for Ultramicro-
chemical Process System, J.M.K.).
backbone changed gradually from the initially popular 640 nm (blue
phase) to 580 nm (purple phase) for the molecule 4, unlike the
abrupt change from 640 to 550 nm for the molecule 1. During
cooling to 25 °C, the maximal absorption shifted upward to 640
nm, and the original intensity was recovered (Figure 1b). Upon
repeating such thermal cycles, the color of the LS films switched
between blue and purple without losing the initial and final
intensities at 640 and 580 nm, respectively. The LS film of para-
substituted 5 was colored faint blue due to a lesser degree of
topopolymerization and showed an incomplete reversibility.
The LS film of the molecule 4 also showed an interesting feature
against pH change from 5.5 (Figure 1c). When the pH increased
abruptly to 13.7, the film underwent a colorimetric transition. As
the pH was adjusted back to 5.5, the original blue color was
immediately recovered, which was not possible in other cases.
To examine the nature of hydrogen-bonding among the head-
groups, transmission FTIR spectroscopic analyses were executed
for the LS films of molecules 1-5 deposited on CaF2 substrates
(Figure 2a). While molecule 1 had a hydrogen-bonded carbonyl
stretching band at 1696 cm-1, molecule 2 had hydrogen-bonded
Supporting Information Available: Experimental details on the
syntheses of the diacetylene derivatives, the formation of their LS films,
and the spectroscopic analyses (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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