Tuning of thermochromic properties of polydiacetylene toward universal temperature sensing materials through amido hydrogen bonding

Article abstract of DOI:10.1021/ma902282c

Mono- and diamides derivatives of 10,12-pentacosadiynoic acid (PCDA) were synthesized from condensation of PCDA with various aliphatic and aromatic diamines. Polydiacetylenes of the amidoPCDA derivatives were prepared by photopolymerization of their molec

Full text of DOI:10.1021/ma902282c

716 Macromolecules 2010, 43, 716–724
DOI: 10.1021/ma902282c
Tuning of Thermochromic Properties of Polydiacetylene toward Universal
Temperature Sensing Materials through Amido Hydrogen Bonding
Sumrit Wacharasindhu,† Suriyakamon Montha,‡ Jasuma Boonyiseng,‡
Anupat Potisatityuenyong,† Chaiwat Phollookin,† Gamolwan Tumcharern,§ and
Mongkol Sukwattanasinitt*,†
†Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University,
Bangkok 10330, Thailand, ‡Program of Petrochemistry and Polymer Science, Faculty of Science,
Chulalongkorn University, Bangkok, Thailand, and §Thailand National Nanotechnology Center,
National Science and Technology Development Agency, Patumthanee 12120, Thailand
Received October 18, 2009; Revised Manuscript Received December 9, 2009
ABSTRACT: Mono- and diamides derivatives of 10,12-pentacosadiynoic acid (PCDA) were synthesized
from condensation of PCDA with various aliphatic and aromatic diamines. Polydiacetylenes of the amido-
PCDA derivatives were prepared by photopolymerization of their molecular assembly homogeneously
dispersed in aqueous media. Thermochromic properties of the resulting polydiacetylene sol were studied by
temperature variable UV-vis spectrometry along with photographic recording. The color transition
temperatures and thermochromic reversibility of the polymers are varied depended on the number of
amide groups and the structure of the aliphatic and aromatic linkers. The phenylenediamide and poly-
methylenediamide PCDA derivatives give polydiacetylenes with complete thermochromic reversibility, while
the polydiacetylenes obtained from 1,2-cyclohexylene and glycolic chain diamide derivatives exhibited
irreversible thermochromism, whereas the polymers attained from the aromatic monoamide analogues are
partially reversible. The variation of the linkers also allows the color transition temperature of the
polydiacetylene to be tuned in the range of 20 °C to over 90 °C. The results provide a fundamental idea
about the factors affecting the thermochromic properties of polydiacetylenes toward the development of
materials for universal thermal indicators.
Introduction
and biological5 sensors. One of the most commonly used amphi-
philic monomers for preparation of vesicles is 10,12-pentacosa-
Although temperature is one of the most important physical
parameters affecting quality of various products such as foods,
beverages, medicines, and chemicals, temperature indicators for
these products are rarely spotted due largely to the lack of
economical and biologically safe materials suitable for sensing
and displaying the temperature of different types of products.
Mercury, the earliest and most widely used temperature sensing
material, is too toxic to be safely fabricated on any consumer
products. The more recently developed temperature sensing
materials such as small molecule liquid crystals and leuco dyes
are so precarious that they generally require additional encapsu-
lation for improved stability and safety. Because of their poly-
meric nature and distinct thermochromic color transition,
polydiacetylenes (PDAs) stand a good chance to be developed
into economical and safe universal temperature indicators.
The preparation of PDAs was initially achievable via solid-
state polymerization of certain diacetylene compounds in their
crystalline forms.1 Later, amphiphilic diacetylenes were found to
be polymerized in forms of lipid monolayer and bilayer assem-
blies.2 This type of amphiphilic diacetylene has recently emerged
as one of the most studied class of diacetylene monomers as they
can also form various nanostructures such as vesicles, tubes, and
ribbons.3 The formation of nanovesicles allows the diacetylenes
to be dispersed in aqueous media which can be polymerized
efficiently by UV-irradiation to produce homogeneous sol of
PDA vesicles that spur immense interests in the field of chemical4
diynoic acid (PCDA) lipid which gives an intense blue sol of
poly(PCDA) upon photopolymerization. Poly(PCDA) vesicle
sol shows irreversible color transition from blue to red upon
increasing temperature, pH, or lipophilicity.6
Thermochromism is one of the earliest reported chromic
properties of polydiacetylene7 that its mechanism and applica-
tions have become subjects of interest in the field of organic
materials.8 The availability of methods for preparation of homo-
geneous sol of PDA vesicles and new structural relevance of PDA
thermochromism have recently been reported. Polydiacetylene
vesicles which composed of different position of diacetylene
functional group and chain length gave variable colorimetric
responses to temperature.9 Hydrogen bonding between the
carboxylic acid headgroups and the interactions between aro-
matic moiety in lipid assemblies have been proven to be essential
for the reversibility of thermochromism of PDAs.10 These in-
vestigations have provided important information for systematic
development of PDAswithtunable thermochromic properties. In
this contribution, we would like to report synthesis and study of
another class of diacetylene lipids which contain one or two
hydrogen bond forming amide headgroups. The amide func-
tional group not only can form strong hydrogen bond compar-
able to the carboxylic group but also can be attached to a
substituent without losing all its hydrogen bond forming ability.
We synthesized diacetylene lipids containing either one or two
amide moieties from the condensation of PCDA with various
diamine compounds. The studies on thermochromism of PDAs
synthesized in this work allow us to grade the thermochromic
*Corresponding author. E-mail smongkol@chula.ac.th.
pubs.acs.org/Macromolecules
Published on Web 12/29/2009
r 2009 American Chemical Society

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Macromolecules, Vol. 43, No. 2, 2010 717
Scheme 1. Synthesis of Diacetylene Lipid Monomers
reversibility and tune the color transition temperature to provide
structural design and increase the assortment of PDA-based
universal temperature sensing materials.
Table 1. Hydration Property of the DA Lipids upon Sonication in
Water, Color Appearance, and the Maximum Absorption Wavelength
(λmax) of the DA Sols after UV Irradiation
Results and Discussion
Preparation of Diacetylene Monomers. The diacetylene
lipid monomers were synthesized via condensation of com-
mercially available PCDA with an appropriate diamine
shown in Scheme 1. In condition A, PCDA was reacted
with 1.5 equiv of the diamine, i.e., 1,2-phenylenediamine, 1,3-
phenylenediamine, and 1,4-phenylenediamine, in the presence
of 1-ethyl-3-(30-dimethylamino)carbodiimide HCl salt (EDCI)
as a coupling reagent to afford monoamidodiacetylene. N-(2-
Aminophenyl)pentacosa-10,12-diynamide (o-PA-1DA), N-(3-
aminophenyl)pentacosa-10,12-diynamide (m-PA-1DA), and
N-(4-aminophenyl)pentacosa-10,12-diynamide (p-PA-1DA)
were isolated as a white solid in low to moderate yield
(29-61%). Low isolated yield was partially caused by the
formation of the diamido compounds (ca. 10%) which had to
be separated from the monoamido products by column chro-
matography. In the synthesis of diamidodiacetylenes (condi-
tion B), 2 equiv of PCDA was reacted with the diamines to
afford the desired product: N,N0-(1,2-phenylene)dipentacosa-
10,12-diynamide (o-PA-2DA), N,N0-(1,3-phenylene)dipen-
tacosa-10,12-diynamide (m-PA-2DA), and N,N0-(1,4-pheny-
lene)dipentacosa-10,12-diynamide (p-PA-2DA) in moderate
yield (52-60%). The products were conveniently purified
by recrystallization in MeOH. Besides phenylenediamine,
1,2-ethylenediamine, 1,4-butylenediamine, 1,6-hexylenedi-
amine, 1,2-cis-diaminocyclohexane, rac-1,2-trans-diamino-
cyclohexane, and bis((3-aminopropyloxy)ethylene)oxide were
also used to react with 2 equiv of PCDA to generate EA-2DA,
BA-2DA, HA-2DA, c-CA-2DA, t-CA-2DA, and APOEO-
2DA in moderate to good yields (52-75%).
* Good = translucent sol obtained, fair = semitranslucent sol obtained,
poor = no dispersion.
other hand, diamidodiacetylene such as o-PA-2DA, m-PA-
2DA, p-PA-2DA, t-CA-2DA, c-CA-2DA, BA-2DA, HA-
2DA, and APOEO-2DA gave poor dispersion after normal
sonication condition due to their high melting temperature
(90-128 °C). The dispersion can be improved by repeating
the heating and the sonicating process until the translucent
sols are obtained. Upon subsequent irradiation of the
dispersed monomers with UV light, the colorless sols of
o-PA-1DA, m-PA-1DA, and p-PA-1DA readily turned to
distinct blue sols, signifying the polymerization of diacetylene
Preparation of Polydiacetylene Sols. The synthesized
monomers were transformed into aqueous polydiacetylene
(PDA) sols by sonication of lipid monomer in Milli-Q
water followed by irradiation with UV light. The ability
to be hydrated and the color of polymerized diacetylenes
are presented in Table 1. All monoamide monomers,
o-PA-1DA, m-PA-1DA, and p-PA-1DA, could be hydrated
to give colorless sol upon sonication at 70-80 °C. On the

718 Macromolecules, Vol. 43, No. 2, 2010
Wacharasindhu et al.
Figure 1. Electronic absorption spectra of PDA sols recorded during stepwise heating.
monomers to form ene-yne conjugated PDAs. The
results also support the formation of ordered diacetylene
monomer assembly before the polymerization step. The sols
of poly(o-PA-1DA), poly(m-PA-1DA), and poly(p-PA-
dihedral angle between the 1,2-cyclohexylene bonds, con-
necting to the amide groups, in the diequatorial trans-isomer
and the axial-equatorial cis-isomer do not allow proper
packing between the diactylene chains. In an attempt to
generate the blue sols of poly(t-CA-2DA) and poly(c-CA-
2DA), the irradiation was conducted in an ice bath (∼5 °C) to
reduce the bond vibration. A clear blue sol was produced in
the case of t-CA-2DA while a purple sol was obtained from c-
CA-2DA, implying a slightly poorer chain packing of c-CA-
2DA compared to t-CA-2DA. In addition, the colorless sol
of EA-2DA having ethylene chain as a linker rapidly turned
blue (λmax ∼ 636 nm) upon exposure to UV irradiation at
room temperature. Interestingly, with longer linkers like
butylene and hexylene, PDA sols prepared from BA-2DA
and HA-2DA showed a new absorption maximum at cur-
iously longer wavelength of over 675 nm, along with the
1DA) showed the maximum absorption wavelength (λmax
)
near 630 nm along with the phonon sideband around 580
nm. The similar blue sols were also obtained from the
irradiation of m-PA-2DA and p-PA-2DA sols. Nevertheless,
o-PA-2DA rapidly formed blue aggregates upon irradiation.
This observation is probably associated with the intramole-
cular hydrogen bonding between N-H and CdO moieties at
the ortho-position, reducing the stability of the colloidal
particles.
For t-CA-2DA and c-CA-2DA, reddish-purple sols with
max below 600 nm were obtained upon exposure to UV at
λ
room temperature. The results suggest that the nonplanar

Article
Macromolecules, Vol. 43, No. 2, 2010 719
usual absorption bands around 620 and 575 nm. The three
absorption peaks including the one at 675 nm observed here
is quite different from the recently reported observation
of long wavelength absorption caused by the crystal size
effect.11 It is also important to note here that, with the same
starting concentration of the monomeric lipids, the polymers
obtained from BA-2DA and HA-2DA show the deepest blue
color. The low-energy absorption and the highly intense blue
color suggest either unusually high backbone strain or long
conjugation length associated with superior packing of the
long aliphatic linkers between the amide groups. The low-
energy absorption band was not observed when the aliphatic
alkyl chain linker was replaced with a glycolic chain as in
poly(APOEO-2DA) of which only the typical λmax of 633
and 582 nm were observed.
The dynamic light scattering (DLS) was performed to
determine the particle size distribution of selected PDA sols.
The average particle sizes of poly(m-PA-1DA), poly(m-PA-
2DA), and poly(BA-1DA) were 139, 169, and 157 nm,
respectively (see Supporting Information, Figures
S12-S14). The morphologies of dry samples of the PDA
sols on mica were examined by AFM. The AFM images
revealed spherical-shaped particles for poly(m-PA-1DA)
while significant aggregation were also observed in case of
poly(m-PA-2DA) and poly(BA-1DA). These observation
supports the a relatively smaller particle size and unimodal
distribution of poly(m-PA-1DA) from DLS measurement
(see Supporting Information, Figure S15).
Figure 2. Color of PDA sols recorded by photography during the
heating process displaying the variation of color transition temperature.
attractive interactions, viz. hydrogen bonding and hydro-
phobic interaction, among the side chains in these polymers.
It is also interesting to note that the trans-geometry exhibited
higher color transition temperature compared to the cis-
geometry. The observation suggested that the diequatorial
arrangement of the diamide side chains allow better
packing than the axial-equatorial orientation. According
to results described above, the color transition temperatures
observed by eyes were in the following order: poly(HA-2DA)
> poly(BA-2DA) > poly(m-PA-2DA) > poly(p-PA-2DA) >
poly(o-PA-1DA) > poly(m-PA-1DA) > poly(p-PA-1DA) >
poly(BA-2DA) > poly(APOEO-2DA) > poly(t-CA-2DA) >
poly(c-CA-2DA). This order indicates the importance of the
number of amide groups, the geometry of the linkers, and the lone
pair electron presence in the linkers. A greater number of amide
groups can lead to higher color transition temperature (poly(m-
PA-2DA), poly(p-PA-2DA) > poly(o-PA-1DA), poly(m-PA-
1DA), poly(p-PA-1DA)). The planar geometry of the linker
connecting the two amides groups (poly(HA-2DA), poly(BA-
2DA), poly(m-PA-2DA), and poly(p-PA-2DA)) give higher
color transition temperature than those of nonplanar geometry
(poly(t-CA-2DA) and poly(c-CA-2DA)). The lone electron pairs
within the linker can reduce the color transition temperature
(poly(APOEO-2DA) < poly(EA-2DA), poly(HA-2DA), and
poly(BA-2DA)).
To quantitatively evaluate the color transition of the PDA
sols, the absorbance from the electronic absorption spectra
was translated into colorimetric response (%CR). The %CR
is defined as percentage change in the maximum absorption
of the blue phase with respect to the total absorption of both
red and blue phases.12 The plot of %CR against the tem-
perature of all PDA studied yielded sigmoidal curves as a
result of blue to red transition upon raising temperature
(Figure 3). The colorimetric response plot displays a similar
tendency with the photographic observation. The order of
color transition temperature assessed from 40% CR is found
in the following order: poly(HA-2DA) ∼ poly(BA-2DA) >
poly(m-PA-2DA) > poly(p-PA-2DA) > poly(o-PA-1DA) >
poly(m-PA-1DA) > poly(p-PA-1DA) > poly(BA-2DA >
poly(APOEO-2DA) > poly(t-CA-2DA) > poly(c-CA-2DA).
The %CR plots also provided another piece of valuable
information. Evidently, PDAs derived from c-CA-2DA, t-CA-
2DA, and APOEO-2DA exhibited a rather steep sigmoid,
meaning that they undergo sharp color transition within the
Thermochromic Properties of Polydiactylene Sols. The sols
were filtered and subjected to the stepwise heating from 25 to
90 °C within a Peltier heating cell of a variable temperature
UV-vis spectrometer. Figure 1 illustrates the annealing
temperature dependence of the electronic absorption spec-
tra. Upon heating, the absorption λmax near 630 nm and the
phonon sideband of poly(o-PA-1DA), poly(m-PA-1DA),
and poly(p-PA-1DA) gradually shifted toward shorter
wavelength. At 95 °C, the blue sols of poly(o-PA-1DA), poly-
(m-PA-1DA), and poly(p-PA-1DA) turned completely red
with the absorption maximum near 540 nm, indicative of the
wider band gap between the ground and excited states of the
conjugated backbone. Similarly, the PDA derived from the
diamidodiacetylene monomer demonstrated the same phe-
nomena. For example, the absorption bands of poly(m-PA-
2DA) and poly(p-PA-2DA) were hypsochromically shifted
from λmax ∼640 to ∼550 nm (Figure 1). The color transition
temperature of the investigated PDAs, however, differed
among them. The photographs of PDAs sols were recorded
upon stepwise heating from 25 to 90 °C, and the results are
presented in Figure 2. PDAs derived from monoamidodi-
acetylene, for example, poly(o-PA-1DA), poly(m-PA-1DA),
and poly(p-PA-1DA), exhibited the blue to red color transi-
tion around 65-75 °C. The higher color transition tempera-
tures were observed in the case of PDAs prepared from
diamidodiacetylene carrying a phenylenediamine linkage
such that the blue sols of poly(m-PA-2DA) and poly(p-PA-
2DA) turned red around 80-90 and 70-80 °C, respectively.
Substituting phenylenediamine with butanediamine or
hexanediamine, the color transition temperatures were even
higher. The deep blue sols of poly(BA-2DA) turned to red
color around 85-95 °C while the color of poly(HA-2DA) did
not appear red even at 95 °C. The other groups of PDAs,
poly(c-CA-2DA), poly(t-CA-2DA), and poly(APOEO-
2DA), showed relatively lower color transition tem-
peratures around 20-25, 45-55, and 50-60 °C, respectively
(Figure 2). The lower color transition temperature of poly-
(t-CA-2DA), poly(c-CA-2DA), and poly(APOEO-2DA) in
comparison to the rest of the series confirmed the weaker

720 Macromolecules, Vol. 43, No. 2, 2010
Wacharasindhu et al.
Figure 3. Colorimetric responses (%CR) of the PDA sols to the temperature upon heating.
Table 2. Thermochromic Reversibility of the PDA Sols Illustrated by Color Photographs
narrow ranges of temperature. The %CR of these PDAs
increased from 20% to 60% within the temperature range of
no more than 10 °C. On the other hand, poly(BA-2DA), poly(o-
PA-1DA), poly(m-PA-1DA), and poly(p-PA-1DA) showed
more gradual change in %CR upon heating that generally
required the temperature increase of 20-25 °C to achieve the
%CR transition from 20% to 60%. The PDAs derived from the
monomers such as m-PA-2DA, p-PA-2DA, BA-2DA, and HA-
2DA showed even slower blue to red color transition that
required the increase of more than 25 °C in the temperature
to attain the level of 20% to 60% CR change. The most notable
trend described above is that the lower the color transition
temperature, the sharper the temperature-induced colorimetric
response. The intramolecular side chain interaction is again
likely to be the main contribution governing this trend. The
blue-to-red transition is associated with the movement of the
side chains that result in the release of backbone strain or disturb
the ene-yne planarity.8 The side chain movement will be
sluggish and will require more energy if the hydrophobic
interaction between the side chains and the hydrogen bonding
between the headgroups are strong. Since all diacetylene mono-
mers investigated contain the same C25 aliphatic structure, the
two hydrogen bond forming points between the headgroups are
thus the main contributor to both broad temperature-induced
colorimetric response and high color transition temperature.
Although c-CA-2DA, t-CA-2DA, and APOEO-2DA also have
two hydrogen forming points, they give PDAs with lower color
transition temperature and sharper colorimetric response even
when compared with the monoamide lipids like o-PA-1DA,
m-PA-1DA, and p-PA-1DA. The results reiterate the effect of
the geometry and the electron lone pair repulsion on the O
atoms of the linkers between the diamide groups.
Thermochromic Reversibility of PDA Sols. During the
course of the thermochromism studies, the reversibility of
the color transition has come to our attention. The color of
the PDA sols observed at 25 °C before heating, after heating
to 95 °C, and after cooling back to 25 °C provided an
approximate idea about the reversibility of these PDAs
(Table 2). The PDAs possessing one amide headgroup such
as poly(o-PA-1DA), poly(m-PA-1DA), and poly(p-PA-
1DA) exhibited none to partial reversibility of the color
transition while the PDAs containing diamide headgroup
such as poly(m-PA-2DA), poly(p-PA-2DA), poly(EA-
2DA), poly(BA-2DA), and poly(HA-2DA) displayed com-
plete reversibility of the color transition, suggesting that
the two amide headgroups are necessary for complete

Article
Macromolecules, Vol. 43, No. 2, 2010 721
Figure 4. Absorbance at the λmax of the blue sols of the PDA recoded at 25 and 95 °C in the heating-cooling cycles.
reversibility. Poly(c-CA-2DA), poly(t-CA-2DA), and poly-
(APOEO-2DA), however, displayed very little reversibility of
the color transition despite having two amide headgroups. The
results again confirm that double hydrogen bond forming
among the amide headgroups in poly(c-CA-2DA), poly(t-CA-
2DA), and poly(APOEO-2DA) is not geometrically feasible.

722 Macromolecules, Vol. 43, No. 2, 2010
Wacharasindhu et al.
To assess the degree of thermochromic reversibility of the
PDAs, the blue absorbance (Ablue), defined as the absor-
bance at the λmax of the “blue phase”, was monitored for nine
cycles of heating and cooling between 95 and 25 °C. The plots
between the absorbance and the cycle numbers of poly-
(m-PA-2DA), poly(p-PA-2DA), poly(EA-2DA), poly(BA-
2DA), and poly(HA-2DA) displayed a zigzag pattern with
virtually full recovery of the initial absorbance in every cycles
(Figure 4), indicating the complete thermochromic reversi-
bility of these PDAs. Again, two points of hydrogen bond
forming between the diamide headgroups probably play the
crucial roles in this reversibility. The rest of the PDAs did not
show full reversibility as the blue absorbance dropped dra-
matically after the first heating-cooling cycle. Interestingly,
all of these PDAs, except poly(c-CA-2DA), however,
showed certain degrees of consistent reversibility from the
second heating-cooling cycle. The abilities to regain the
intensity of the initial blue absorbance are diverse among
them. For comparison, thermochromic reversibility of poly-
(PCDA) was studied in the same manner. Poly(PCDA)
showed a low degree but consistent recovery of the blue
phase absorbance after the second heating-cooling cycle.
The recoveries of the blue absorbance of the monoamide
PDAs whose side chains contain aromatic rings, such as
poly(o-PA-1DA) and poly(p-PA-1DA), were notably greater
than that of poly(PCDA). The results signify the contribu-
tion of the π-π interaction between the aromatic rings to the
reversibility of the color transition.
Table 3. Degree of Reversibility and the Classification of the PDA
Sols Determined from the Absorption Spectra in Figure 4
PDA
degree of reversibility (%) classification of PDA
poly(PCDA)
2.2
1.7
1.2
1.2
4.3
irreversible
irreversible
irreversible
irreversible
irreversible
partially reversible
partially reversible
fully reversible
fully reversible
fully reversible
fully reversible
fully reversible
poly(m-PA-1DA)
poly(c-CA-2DA)
poly(t-CA-2DA)
poly(APOEO-2DA)
poly(o-PA-1DA)
poly(p-PA-1DA)
poly(m-PA-2DA)
poly(p-PA-2DA)
poly(EA-2DA)
poly(BA-2DA)
poly(HA-2DA)
55
13
101
105
100
100
105
irreversible process, the positive enthalpy changes (ΔHhb
,
ΔHhi, and ΔHde) are either less than or comparable to the
negative enthalpy changes (ΔHcs and ΔHbs). In the other
words, the thermodynamic stability gained from the hydro-
gen bonding, hydrophobic interaction, and delocalization
energy in the blue form are not enough to restore the original
conformation and backbone strains. The partially reversible
thermochromism observed for poly(o-PA-1DA) and poly(p-
PA-1DA) indicates that the red form is a metastable form
present only at high temperature, where the entropic term
(TΔS) becomes significant, while the purple color is the
stable phase that exists after the PDAs are cooled back to
room temperature (see Table 2 for the appearing color). This
stable purple color obtained is different from the purple form
observed at intermediate temperature (∼50-60 °C) during
the first heating which is only metastable akin to the meta-
stable purple form of poly(PCDA).10c In these polymers, the
extra benzene rings add hydrophobic and π-π interaction to
help in restoring some of the original conformation and
backbone strains. The complete reversibility observed for
poly(m-PA-2DA), poly(p-PA-2DA), poly(BA-2DA), and
poly(HA-2DA) is for the most part attributed to the dual
hydrogen bond forming between the diamide headgroups. It
is likely that at least one of the hydrogen bonds per head-
group remained strongly present up to 95 °C. The red form of
theses PDAs is a metastable form which is present only at
high temperature where the entropic term becomes more
influential.
To obtain a quantitative value of the degree of reversibility
(%DR), the average value of the absorbance change (ΔAavg
)
from the second to ninth heating is compared against the
absorbance change in the first heating (ΔA1) according to the
following equation: %DR = 100 ꢀ ΔAavg/ΔA1 where ΔA =
A25 °C - A95 °C. On the basis of %DR presented in Table 3,
three reversibility classes of the PDAs, i.e., fully reversible
PDA (%DR g 90%), partially reversible PDA (10 < %DR
< 90%), and irreversible PDA (%DR e 10%), can be
perceived. Using the classification mentioned above, poly-
(m-PA-1DA), poly(t-CA-2DA), poly(c-CA-2DA), poly-
(APOEO-2DA), and poly(PCDA) possess irreversible
thermochromic property. After the second heating-cooling
cycle, these polymers appeared as red color at both 95 and 25
°C (Table 3). The small recovery (1-5%) observed in these
PDAs is conceivably the result of either the difference in the
side chain packing or the altering of the vibronic state
population at different temperatures.
Conclusion
The color transition temperature and thermochromic reversi-
bility of polydiacetylenes can be tuned by the variation of the
number of internal amide functional groups in the monomers and
the structures of the linkers between the amide groups. Both color
transition temperature and reversibility of polydiacteylenes were
found to be mainly associated with the number and orientation of
the hydrogen bond forming groups. The sensibility and degree of
colorimetric reversibility, which are extremely important factors
for development of materials for temperature indicators, can
be assessed via the determination of the colorimetric response
(%CR) and the degree of reversibility (%DR), respectively. The
capability of tuning the color transition temperature as well as the
reversibility of the color transition will surely expand the scope
where polydiacetylenes can be applied.
In consideration of thermodynamic reversibility of the
color transition of PDAs, the enthalpy change (ΔH) is likely
to be the major contributor to the free energy change (ΔG =
ΔH - TΔS) as the entropic term (TΔS) is insignificant in
condensed matter such as vesicles. Providing that the thermo-
chromic process is a unimolecular process excluding solvent
interference, the enthalpy term (ΔH) can be thought as a sum
of several bonding and steric energy terms, i.e., hydrogen
bonding between the headgroups (ΔHhb), hydrophobic in-
teraction among the side chains (ΔHhi), conformational
strain of individual side chain (ΔHcs), backbone strain
(ΔHbs), and delocalization energy within the conjugated
system (ΔHde). Thus, ΔH = ΔHhb þ ΔHhi þ ΔHcs þΔHbs
þ ΔHde. Upon heating, ΔHhb, ΔHhi, and ΔHde are positive
while ΔHbs is negative because the hydrogen bonding,
hydrophobic interaction, delocalization energy, and back-
bone strain are present in the original blue form to a greater
extent compared to the red form. Earlier literature works
have reported that the side chains in the blue form
contain more gauche conformation than those in the red
form,13 suggesting that ΔHcs is negative upon heating. In the
Experimental Section
Chemicals and Instrumentation. 10,12-Pentacosanoic acid
(PCDA) was purchased from GFS Chemical, and other reagents
were purchased from Sigma-Aldrich and Fluka. Analytical
grade solvents such as chloroform and methylene chloride were
used without further purification. Flash chromatography was

Article
Macromolecules, Vol. 43, No. 2, 2010 723
carried out on silica gel 60 (230-400 mesh; Merck). Thin layer
chromatography (TLC) was carried out using Merck 60 F254
plates with a thickness of 0.25 mm. The diacetylene monomers
were purified by filtration to remove the polymerized lipid before
used. The 1H and 13C NMR spectra were collected on a 400 MHz
NMR spectrometer (Murcury 400, Varian). The molecular
weights were obtained from a low-resolution quadupole mass
analyzer (Quattro Micro API 2000, Micromass) using the electro-
spraying ionization (ESI) technique. The electronic absorp-
tion spectra were recorded on a temperature variable UV-vis
spectrophotometer (Carry 100Bio, Varian).The AFM images
were acquired with Nano Scope IV (Vecco Metrology Group)
operating by noncontact mode. The particle size measurements
were made by a dynamic light scattering using a nanosizer
(Malvern Instruments).
1H NMR (400 MHz, CDCl3): δ = 7.47 (4 H, s, 4ArH), 7.07
(2H, br, NHdCO), 2.33 (4H, t, J = 7.0 Hz, 2CH2), 2.24 (8H, t,
J = 7.0 Hz, 2CH2), 1.78-1.18 (64H, m, 32CH2), 0.88 (6H, t,
J = 7.0 Hz, 2CH3). LMRS calcd for C56H87N2O2 [M - H]-:
819.68; found: 819.83. c-CA-2DA (323 mg, 52%); mp 62-64 °C.
1H NMR (400 MHz, CDCl3): δ = 6.28 (2H, br, NHdCO),
3.95-4.00 (2H, m, 2CH), 2.21 (12H, td, J = 15.0, 7.0 Hz,
6CH2), 1.89 (4H, d, J = 1.5 Hz, 2CH2), 1.70-1.13 (68 H, m,
34CH2), 0.88 (6H, t, J = 6.5 Hz, 2CH3). LMRS calcd for
C56H94N2O2Na [M þ Na]þ: 849.73; found: 850.27. t-CA-2DA
1
(379 mg, 61%); mp 62-64 °C. H NMR (400 MHz, CDCl3):
δ = 5.87 (2H, br, NHdCO), 3.74-3.56 (2H, m, 2CH), 2.49-1.97
(16H, m, 8CH2), 1.82-1.15 (68H, m, 34CH2), 0.88 (6H, t, J = 6.5
Hz, 2CH3). EA-2DA (197.9 mg, 52%); mp 125-130 °C. 1H NMR
(400 MHz, CDCl3): δ = 6.10 (2H, br, NHdCO), 3.39 (4H, br,
2CH2), 2.17 (8H, t, J = 7.0 Hz, 4CH2), 2.10 (4H, t, J = 7.5 Hz,
2CH2), 1.25-1.51 (64H, m, 32CH2), 0.88 (6H, t, J = 6.8 Hz,
2CH3). BA-2DA (451 mg, 75%); mp 117-120 °C. 1H NMR (400
MHz, CDCl3): δ = 6.34 (2H, br, NH=CO), 3.28 (4H, m, 2CH2),
2.22 (12H, t, J= 7.0 Hz, 6CH2), 1.68-1.15 (66H, m, 33CH2), 0.86
(6H, t, J = 6.5 Hz, 2CH3). LMRS calcd for C37H75N2O2 [M -
C17H27]þ: 570.51; found: 569.72. HA-2DA (404 mg, 65%); mp
General Procedure for Synthesis of Monoamidodiacetylene
Lipid Monomers. 1-Ethyl-3-(30-dimethylamino)carbodiimide
HCl salt (EDCI) (230.04 mg, 1.2 mmol) in chloroform (4 mL)
was added dropwise into a solution of 10,12-pentacosadiynoic
acid (374.61 mg, 1.0 mmol) in chloroform (5 mL). The mixture
was stirred for 1 h at 0 °C and was then added dropwise into 1.5
equiv of the selected diamine, e.g., 1,2-phenylenediamine
(162.21 mg, 1.5 mmol) in chloroform (4 mL) at 0 °C. The
reaction mixture was stirred at room temperature overnight
when the white precipitate was clearly observed. The reaction
mixture was evaporated under reduced pressure to yield the
crude product as a white solid. Purification was accomplished by
column chromatography on silica gel eluted with a mixture of
hexane and ethyl acetate (50:50) to give o-PA-1DA (386 mg,
47%) as a white solid; mp 82-84 °C. 1H NMR (400 MHz,
CDCl3): δ 9.07 (1H, s, NHCdO), 7.11 (1H, d, J = 7.5 Hz, ArH),
6.86 (1H, t, J = 7.5 Hz, ArH), 6.68 (1H, d, J = 7.5 Hz, ArH),
6.51 (1H, t, J = 7.5 Hz, ArH), 4.78 (2H, br, NH2), 2.41-2.06
(6H, m, 3CH2), 1.65-1.03 (32H, m, 16CH2), 0.83 (3H, t, J = 6.5
Hz, CH3). LRMS calcd for C31H48N2ONa [M þ Na]þ: 488.7;
1
122-125 °C. H NMR (400 MHz, CDCl3): δ = 5.56 (2H, br,
NHdCO), 3.24 (4H, q, J = 6.5 Hz, 2CH2), 2.20 (12H, td, J =
15.0, 7.5 Hz, 6CH2), 1.71-1.13 (72H, m, 36CH2), 0.88 (6H, t, J =
6.5 Hz, 2CH3). LMRS calcd for C56H96N2O2Na [M þ Na]þ:
852.75; found: 850.27. APOEO-2DA (504 mg, 72%); mp 77-80
°C. 1H NMR (400 MHz, CDCl3): δ = 6.17 (2H, br, NHdCO),
3.71-3.49 (16H, m, 8CH2), 3.35 (4H, q, J = 6.0 Hz, 2CH2), 2.18
(12H, td, J = 15.0, 7.5 Hz, 6CH2), 1.83-1.17 (64H, m, 32CH2),
0.87 (6H, t, J = 6.5 Hz, 2CH3).
Typical Method for Preparation of Polydiacetylene Vesicles. A
diacetylene monomer was dissolved in chloroform (0.5 mL) in a
test tube, and the solvent was removed under reduced pressure.
A volume of Milli-Q water was added to provide the lipid
concentration of 0.25 mM. The suspension was heated to
75-85 °C and sonicated in an ultrasonicating bath until a
translucent vesicle sol was obtained, typically requiring
30-40 min. The vesicle sol was kept at 4 °C overnight and was
then irradiated with UV lamp (254 nm, 500 μW/cm2) for 5 min at
room temperature, unless specified otherwise, and filtered
through a filter paper (No. 1) to give a clear intense blue vesicle
sol. The PDA sols were characterized by UV-vis absorption,
dynamic light scattering, and AFM imaging as described in the
Results and Discussion. Freeze-dried samples of some PDAs
were also characterized by Raman spectroscopy for the presence
of the ene-yne conjugate. Raman peaks (cm-1) for poly(m-PA-
2DA): 1510 (CdC), 2112 (CtC); poly(p-PA-2DA): 1515
(CdC), 2104 (CtC); poly(BA-2DA): 1506 (CdC), 2107
(CtC); poly(HA-2DA): 1514 (CdC), 2115 (CtC).
1
found: 487.7. m-PA-1DA (427 mg, 52%); mp 76-78 °C. H
NMR (400 MHz, CDCl3): δ 9.51 (1H, s, NHCdO), 6.88 (1H, s,
ArH), 6.64 (1H, d, J = 8.5 Hz, 2ArH), 6.21 (1H, d, J = 8.0 Hz,
ArH), 5.00 (2 H, s, NH2), 2.24 (6 H, m, 3CH2), 1.65-1.15 (32H,
m, 16CH2), 0.84 (3H, t, J = 7.0 Hz, CH3). LMRS calcd for
C31H49N2O [M þ H]þ: 465.38; found: 465.47. p-PA-1DA (452
1
mg, 55%); mp 101-103 °C. H NMR (400 MHz, CDCl3): δ
9.42 (1H, s, NHdCO), 7.19 (2H, d, J = 8.5 Hz, 2ArH), 6.48
(2H, d, J = 8.5 Hz, 2ArH), 4.98-4.77 (2H, m, NH2), 2.23 (6H,
td, J = 15.0, 7.0 Hz, 3CH2), 1.65-1.04 (32H, m, 16CH2), 0.85
(3H, t, J = 6.5 Hz, CH3). LMRS calcd for C31H48N2ONa [M þ
Na]þ: 488.73; found: 487.37.
General Procedure for Synthesis of Diamidodiacetylene Lipid
Monomers. 1-Ethyl-3-(30-dimethylamino)carbodiimide HCl salt
(EDCI) (383.4 mg, 2 mmol) in chloroform (2 mL) was added
dropwise into a solution of 10,12-pentacosadiynoic acid (637
mg, 1.7 mmol) in chloroform (2 mL). After stirring for an hour,
0.45 equiv of the selected diamine, e.g., 1,2-phenylenediamine
(81 mg, 0.75 mmol) in chloroform (10 mL), was then added
dropwise and kept stirred overnight. The reaction mixture was
evaporated under reduced pressure to yield the crude product as
a white solid. The crude product was then crystallized in
methanol to afford o-AP-2DA (370 mg, 60%); mp 112-114
°C. 1H NMR (400 MHz, CDCl3): δ = 8.05-7.98 (2H, br,
NHdCO), 7.42-7.36 (2H, m, 2ArH), 7.22-7.17 (2H, m,
2ArH), 2.36 (4H, t, J = 7.5 Hz, 2CH2), 2.24 (8H, dd, J =
9.0, 4.52 Hz, 4CH2), 1.78-1.18 (64H, m, 32CH2), 0.88 (6H, t,
J = 7.0 Hz, 2CH3). LMRS calcd for C56H88N2O2Na [M þ
Na]þ: 844.68; found: 843.74. m-PA-2DA (320 mg, 52%); mp
103-105 °C. 1H NMR (400 MHz, CDCl3): δ = 7.86-7.79 (2H,
br, NHdCO), 7.30-7.24 (2H, m, 2ArH), 7.18-7.16 (2H, m,
2ArH), 2.33 (4H, t, J = 7.5 Hz, 2CH2), 2.24 (8H, t, J = 6.5 Hz,
4CH2), 1.76-1.18 (64H, m, 32CH2), 0.88 (6H, t, J = 6.5 Hz,
2CH3). LMRS calcd for C56H88N2O2Na [M þ Na]þ: 844.68;
found: 844.16. p-PA-2DA (363 mg, 59%); mp 167-169 °C.
Acknowledgment. This study was financially supported by the
grants from National Nanotechnology Center, National Science
and Technology Development Agency (NANOTEC, NSTDA),
and the Thailand Research Fund (DIG 5180020). We also thank
Center for Petroleum, Petrochemicals and Advanced Materials,
Chulalongkorn University and the 90th Anniversary of Chulalong-
korn University Fund (Ratchadaphiseksomphot Endowment
Fund) for student scholarships.
Supporting Information Available: 1H NMR spectra of
all monomers, atomic force microscopic images, dynamic light
scattering plots, and Raman spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
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