Construction and molecular understanding of an unprecedented, reversibly thermochromic bis-polydiacetylene

Article abstract of DOI:10.1002/adfm.201304147

A new type of bis-PDA, in which two PDAs are linked via an intervening p-phenylene group, is developed. The Bis-PDA-Ph displays exceptional thermochromic reversibility owing to the presence of unique hydrophobic interactions between alkyl chains as well as aryl moieties. The Bis-PDA-Ph has a well-packed structure that causes it to display a clear blue to red colorimetric transition at elevated temperatures. To elucidate the molecular origin of the thermochromic response, a theoretical simulation of the new PDA, the results of which successfully explain the thermochromic reversibility phenomenon, is conducted. Furthermore, Bis-PDA-Ph-embedded polymer fibers are used as a thermochromic sensor material. It displays excellent reversibility between 20-120 °C, which is the largest temperature range reported thus far for PDA based sensors. Finally, transient absorption spectroscopy is employed for the first time to analyze the temperature-dependent fluorescence change of the new PDA. The Bis-PDA-Ph (two PDAs are linked via an intervening p-phenylene group) displays exceptional thermochromic reversibility with a blue to red colorimetric transition at elevated temperatures. To elucidate the molecular origin of the thermochromic response, a theoretical simulation of the new PDA which explains the reversibility phenomenon, is conducted. Furthermore, Bis-PDA-Ph-embedded fibers display excellent reversibility between 20-120 °C.

Full text of DOI:10.1002/adfm.201304147

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Construction and Molecular Understanding
of an Unprecedented, Reversibly Thermochromic
Bis-Polydiacetylene
Songyi Lee, Joonseong Lee, Minji Lee, Yu Kyung Cho, Junwoo Baek, Jinwook Kim,
Sungnam Park, Myung Hwa Kim, Rakwoo Chang,* and Juyoung Yoon*
≈550 nm, when they are subjected to
A new type of bis-PDA, in which two PDAs are linked via an intervening
p-phenylene group, is developed. The Bis-PDA-Ph displays exceptional ther-
mochromic reversibility owing to the presence of unique hydrophobic interac-
tions between alkyl chains as well as aryl moieties. The Bis-PDA-Ph has a
well-packed structure that causes it to display a clear blue to red colorimetric
transition at elevated temperatures. To elucidate the molecular origin of the
thermochromic response, a theoretical simulation of the new PDA, the results
of which successfully explain the thermochromic reversibility phenomenon,
is conducted. Furthermore, Bis-PDA-Ph-embedded polymer fibers are used as
a thermochromic sensor material. It displays excellent reversibility between
20–120 °C, which is the largest temperature range reported thus far for PDA
based sensors. Finally, transient absorption spectroscopy is employed for the
first time to analyze the temperature-dependent fluorescence change of the
new PDA.
various types of environmental stimula-
tion. The results of independent studies by
Kim[6] and Jelinek[7] and their respective co-
workers in 2005, which demonstrated that
PDAs can serve as novel sensor systems,
have stimulated the large effort that has
been recently given to the development of
new chemosensors based on PDAs.[8–27]
Temperature is one of the common
environmental stimulations that cause col-
orimetric transitions of PDAs. A number
of studies have focused on thermochromic
transitions of these materials, which in
some cases are reversible.[28–35] However,
in most of the cases reported earlier, the
temperature ranges for the reversible ther-
mochromic PDA transitions were rather
narrow and the production of clear blue to
red transitions was rare and in some cases
not fully reversible (i.e., initial blue color cannot be fully recov-
ered). For example, interesting bis-PDAs were recently found to
be temperature sensors, but the temperatures over which they
operate are in the 25–55 °C range.[33] Moreover, even though the
sensory properties of PDA systems have been actively studied,
no theoretical studies or simulations of the transitions taking
place in these materials have been described.
1. Introduction
Since the time of the first report by Wegner in 1969,[1] polydia-
cetylenes have become an extensively studied family of conju-
gated polymers.[2–5] PDAs are generated by UV light irradiation
of self-assembled diacetylene monomers, which promotes rapid
polymerization to form ene-yne conjugated, blue colored poly-
mers. The blue PDAs, which typically have absorption maxima
at ≈640 nm, undergo a dramatic, naked-eye observable colori-
metric change to red corresponding to absorption maxima at
In the investigation described below, we prepared a new
type of PDA, (Bis-PDA-Ph), in which two PDA moieties are
connected via p-phenylene group. Bis-PDA-Ph displays revers-
ible, blue to red and red to blue, temperature induced color
changes even though it does not possess the typical types of
groups (hydrogen bonding), which promote this phenom-
enon. Furthermore, thermochromic changes displayed by
Bis-PDA-Ph-embedded electrospun fibers are highly revers-
ible and occur over the rather wide temperature range of
20 to 120 °C, the largest recorded thus far. In addition, we car-
ried out the first theoretical simulation of a PDA system and
used the results to explain the thermochromic reversibility of
the new PDA. We believe simulations, like those performed
in this effort, will provide detailed information about the
molecular basis for the reversible thermochromic behavior of
PDAs. Finally, the results of transient absorption studies pro-
vided important information about the nature of fluorescence
changes associated with the thermochromic transitions of the
new PDA.
S. Lee, M. Lee, Y. K. Cho, Prof. M. H. Kim, Prof. J. Yoon
Department of Chemistry and Nano Science
Global Top 5 Research Program
EwhaWomans University
Seoul 120–750, Korea
E-mail: jyoon@ewha.ac.kr
J. Lee, Prof. R. Chang
Department of Chemistry
Kwangwoon University
Seoul 139–701, Korea
E-mail: rchang@kw.ac.kr
J. Baek, J. Kim, Prof. S. Park
Department of Chemistry
Korea University
Seoul 136–701, Korea
DOI: 10.1002/adfm.201304147
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Scheme 1. Synthesis of Bis-PCDA-Ph.
2. Results and Discussion
2.1. DA monomer and PDA preparation, Dynamic Light
Scattering (DLS)
The DA monomer, Bis-PCDA-Ph (1), used in this study was
prepared using the route shown in Scheme 1. 10, 12-Pentaco-
sadiynoic acid (PCDA) was first activated by reaction with oxalyl
chloride. The formed acid chloride was then reacted with p-hyd-
roquinone to generate 1 in 90% yield. This substance was fully
characterized by using 1H NMR and 13C NMR spectroscopy, and
FAB high resolution mass spectrometry. UV irradiation of self-
assembled 1 in water for 30 s induced formation of the blue-
colored Bis-PDA-Ph (Scheme 2). Furthermore, the particle size
of Bis-PDA-Ph has been characterized with dynamic light scat-
tering (DLS). This experiment was repeated 3 times in order to
acquire an average data in Table S1 (Supporting Information).
2.2. Thermochromism of Bis-PDA-Ph in solution
Figure 1a shows images of a vial containing an aqueous solution
of Bis-PDA-Ph at various temperatures. The blue color of the Bis-
PDA-Ph polymer at room temperature changes to red as the tem-
perature is increased to 100 °C. As explained in the introduction,
previously reported PDAs display thermochromic behavior over
relatively narrow temperature ranges or initially they undergo
color changes that deteriorate to violet (or purple) to red rather
than blue to red after the first cycle. The observations demon-
strate that, unlike previously reported PDAs, Bis-PDA-Ph dis-
plays a highly distinct blue color at 20 °C and red color at 100 °C.
In addition, the temperature induced color change is highly
reversible, surviving more than 20 heating-cooling cycles without
significant color bleaching (Figure 1b). The UV absorption spec-
trum of an aqueous solution of Bis-PDA-Ph at 20 °C and 100 °C,
displayed in Figure 1c, shows that the blue color at room temper-
ature is associated with an absorption maximum at ≈630 nm and
that of the red form of this PDA at 100 °C absorbs at 540 nm.
Scheme 2. Self-assembly and polymerization of Bis-PCDA-Ph.
used in the simulation is described fully in the Experimental
Section and summarized in Figure 2 The model involves the
use of the monomer stage to construct an ene-yne conjugated
Bis-PDA-Ph polymer composed of 8 monomeric units, all con-
nected in the same manner so that they represent infinitely
long polymer chains. Sheets composed of polymeric PDA
are stacked side by side with a separation of 0.5 nm to form
a membrane. Finally, the PDA membrane is encased in water
with water molecules within 0.28 nm of the membrane being
absent. The resulting initial configuration, consisting of 14939
atoms and dimensions of 4.0 nm × 3.9 nm × 10.4 nm, is then
subjected to molecular dynamics (MD) simulations using the
CHARMM package (v. 38a1)[35] with the help of the CHARMM-
GUI website.[36] The force fields for the model PDA system
were obtained from CHARMM general force fields[37] and the
TIP3P water model was used.
2.3. Theoretical Simulations of PDAs Systems
The results of calculations show that the initial Bis-PDA-
Ph membrane system maintains its general structure upon
heating from 300 K to 400 K, presumably the consequence of
We carried out what we believe to be the first theoretical sim-
ulation of PDA systems. The setup of the Bis-PDA-Ph system
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(a)
cooling
heating
heating
heating
20
°C
70
°C
100°
C
20°C
(b)
(c)
70
0.30
0.25
0.20
0.15
0.10
0.05
0.00
60
50
40
30
20
10
0
100°C
100°C
20°C
20°C
0
1
2
3
4
5
6
7
8
9
10 11
350
400
450
500
550
600
650
700
Thermal Cycles
Wavelength (nm)
Figure 1. a) Photographs of as aqueous solution of Bis-PDA-Ph at 20 °C, 70 °C, and 100 °C. b) Colorimetric response value (see Experimental Sec-
tion) of the Bis-PDA-Ph solutions to cycles of temperature changes in the ≈20–100 °C range. c) UV-Vis spectra of an aqueous solution of Bis-PDA-Ph
at 20 °C and 100 °C.
the presence of the rigid π-conjugation that exists along the
backbone of the polymer.
Representative snapshots of the equilibrated Bis-PDA-Ph sys-
tems at these two temperatures are shown in Figure 3 and the
equilibrium structural properties are summarized in Table S2
(Supporting Information). The first noticeable structural fea-
ture of the membrane is that the Bis-PCDA-Ph monomer tails
are significantly tilted from the membrane normal direction
(θ = 23.2° at T = 300 K). Because of this
arrangement, the membrane thickness
decreases significantly from its initial value
of 6.23 nm to 5.76 nm and at the same time
the area per monomer decrease slightly from
0.24 nm2 to 0.22 nm2. Interestingly, the area
per monomer values arising from these cal-
culations are consistent with those coming
from previous studies,[38] which are 0.24,
0.24, and 0.19 nm2 for monomeric, blue, and
red phases of mono-PDA Langmuir films,
respectively.
As temperature increases to 400 K, the
hydrocarbon tails of Bis-PCDA-Ph mono-
meric units move back in the membrane
normal direction with the tilt angle θ = 14.9°
but, interestingly, the membrane thickness
does not change. On the other hand, the
area per monomer increases slightly from
0.22 nm2 to 0.23 nm2, a change that is mostly
caused by the increased distance between the
PDA polymer sheets from 0.449 nm at T =
300 K to 0.464 nm at T = 400 K.
The structural response of the Bis-PDA-Ph
membrane system to changes in temperature
was examined by measuring the orientation
Figure 2. Simulation model step: a) Bis-PCDA-Ph (1) monomer, b) Bis-PDA-Ph sheet, and
correlation function, C1, which is defined
c) side (upper) and top (lower) views of Bis-PDA-Ph membrane system in the presence of water.
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(a)
0.980
0.975
0.970
0.965
0.960
0.955
0.950
0.945
0.0
0.2
0.2
0.2
0.4
0.4
0.4
0.6
Time (ns)
0.8
0.8
0.8
1.0
1.0
1.0
(b)
26
24
22
20
18
16
14
0.0
0.6
Time (ns)
(c)
400
380
360
340
320
300
Figure 3. Representative simulation snapshots at a) T = 300 K and b) T =
400 K for the Bis-PDA-Ph membrane system. Upper figures are views
from the direction normal to the Bis-PDA-Ph polymer sheets and lower
figures from the top with the omission of water molecules. Snapshots and
animations were generated by using VMD.[34]
as the inner product between two unit normal vectors of the
neighboring conjugated ene-yne planes along the Bis-PDA-Ph
polymer backbone. The correlation function is a measure of
the conjugation length along the Bis-PDA-Ph polymer back-
bone. In Figure 4 are shown C1 as well as tilt angle θ responses
to linear temperature changes between 300 K and 400 K at a
rate of 1.0 K ps-1 (an animation of changes in the Bis-PDA-Ph
system upon changing temperatures is provided in Supporting
Information). Because the ene-yne conjugated backbone of the
Bis-PDA-Ph is rigid, C1 changes only slightly. However, the
finite skeletal changes reflected in changes in C1 are clearly dis-
cernable and, except at the initial stage, they are well correlated
with the temperature change even if it takes place rapidly. This
observation indicates that π–π interactions between the benzene
rings along with the rigid ene-yne network are mainly respon-
sible for promoting the reversibility of the thermochromic tran-
sition of Bis-PDA-Ph.
0.0
0.6
Time (ns)
Figure 4. Responses of a) the orientation correlation function, C1, and
b) the monomer tilt angle, θ, to temperature changes for the Bis-PDA-Ph
membrane system. The rate of the linear temperature change is 1.0 K/ps
and the system temperature is also shown in (c).
absorption (TA) experiments were carried out to assess the
electronic relaxation dynamics of Bis-PDA-Ph and PCDA-PDA.
Bis-PDA-Ph and PCDA-PDA were electronically excited by irra-
diation with 630 nm light pulses at 22 °C and 525 nm light
pulses at 80 °C, and the rates of relaxation of the corresponding
electronic excited states were measured by using time-delayed
probe pulses at different wavelengths (Figure 5, see Supporting
Information). Decay profiles for Bis-PDA-Ph and PCDA-PDA
at 22 °C are characterized by an initial fast relaxation process
with a large amplitude at short times followed by a slow decay
component with a very small amplitude at long times. Approxi-
mately 70% of TA signals decay within the first few ps. The slow
relaxation behaviors of excited Bis-PDA-Ph and PCDA-PDA
2.4. Transient Absorption (TA) Spectroscopy Studies
PCDA-PDA, derived by polymerization of the PCDA monomer,
was prepared as a control material for the TA studies. Transient
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1H NMR and 13C NMR spectra were recorded using a 300 MHz NMR
spectrometer. Chemical shifts were expressed in ppm and coupling
constants (J) in Hz.
1.0
Bis-PDA-Ph, 22 oC
Bis-PDA-Ph, 80 oC
PCDA-PDA, 22 oC
PCDA-PDA, 80 oC
0.06
0.04
0.02
0.00
Synthesis of Bis-PCDA-Ph: Oxalyl chloride (0.63 mL, 4.96 mmol) was
added dropwise to a methylene chloride solution containing 0.502 g
(1.34 mmol) of 10, 12-pentacosadiynoic acid. After 1 h of stirring, one
drop of dimethylformamide (DMF) was added to the solution and
the resulting mixture was stirred for 4 h. To the residue obtained by
concentration of the solution was added a solution containing 0.07 g
(0.67 mmol) of hydroquinone and 0.4 mL (4 mmol) of triethylamine
in 20 mL of THF. The resulting solution was stirred overnight at room
temperature under N2. The residue obtained by concentration was
subjected to silica gel column chromatography (CH2Cl2 100%) to give
Bis-PCDA-Ph (0.50 g, 90%) as a white solid. 1H NMR (300 MHz, CDCl3):
δ (ppm): 0.83–0.87 (t, J = 6.9 Hz, 6H), 1.23–1.53 (m, 52H), 1.68–1.73
(m, 9H), 1.69–1.76 (m, 4H), 2.20–2.24 (m, 8H), 2.50–2.55 (t, J = 7.5 Hz,
4H), 7.06 (s, 4H). 13C NMR (300 MHz, CDCl3) δ (ppm) 172.15, 148.05,
122.38, 77.44, 77.01, 76.59, 65.31, 34.31, 31.92, 29.65, 29.63, 29.61,
29.48, 29.35, 29.10, 29.08, 29.03, 28.90, 28.86, 28.75, 28.35, 28.29, 24.86,
22.63, 19.21, 19.19. FAB HRMS m/z = 823.6606 [M]+, calc. for C56H87O4
= 823.6610.
Preparation and Polymerization of Polymers: Preparation of PDA
vesicles in aqueous solution was achieved by using the following
method. A solution of Bis-PCDA-Ph monomer (16.5 mg, 0.02 mmol)
in THF (2 mL) was injected into 18 mL of deionized water while
shaking to yield a solution containing a total monomer concentration
of 1 mM. The sample was then sonicated at 80 °C for 30 min. The
resulting solution was filtered through a 0.8 µm filter, giving a filtrate
that was cooled to 4 °C for 12 h. Polymerization was carried out at
room temperature by irradiating the solution with a 254 nm UV light
(1 mW cm−2) for 30 s.
0.8
0.6
0.4
0.2
0.0
200
400
600
t (ps)
800
1000
0
200
400
600
800
1000
t (ps)
Figure 5. Transient absorption (TA) signals of Bis-PDA-Ph and PCDA-
PDA at 22 and 80 °C. The inset shows the TA signals at long relaxation
times (t > 90 ps). TA signals of Bis-PDA-Ph at 22 and 80 °C and PCDA-
PDA at 22 °C decay almost completely at 1000 ps. In contrast, TA signal
of PCDA-PDA at 80 °C decays on a much longer timescale.
at long times are shown in the inset of Figure 5. In addition,
intermediate electronic relaxation processes connect the initial
fast and slow decay components. The multi-timescale relaxa-
tion processes reflect the complicated electronic structures of
chromophores present in Bis-PDA-Ph and PCDA-PDA.
Determination of CR Values: To determine the thermochromic
properties of the PDA, UV spectroscopy measurements were carried
out using a 1 cm optical path length cell. The blue polymer was first
transformed to its red phase by using a heating-cooling cycle. UV-Vis
spectra were recorded during the first cycle. To quantify the extent of the
blue-to-red color transitions, the CR value (%) was calculated using the
following equation,
3. Conclusion
In the study described above, we prepared the new bis-PDAs,
Bis-PDA-Ph, in which two PDA moieties are linked via a p-phe-
nylene group. Bis-PDA-Ph displayed excellent thermochromic
reversibility as a consequence of the existence of unique hydro-
phobic interactions between alkyl chains as well as π-π interac-
tions between aryl groups. The tightly packed structure of this
PDA is responsible for the deep blue to red colorimetric transi-
tions that take place in response to temperature changes over a
range that is larger than those displayed by other types of PDAs
developed to date. For the first time, a theoretical simulation
of the PDA system was carried out. The results of this effort
provide information about the molecular nature of reversible
thermochromic process. In addition, Bis-PDA-Ph-embedded
polymer fibers were prepared and utilized as a thermochromic
sensor material. The fibers were observed to undergo a revers-
ible color transition between 20 and 120 °C, which is the largest
temperature range reported thus far for PDAs. Finally, tran-
sient absorption spectroscopy studies were conducted to gain
information about the origin of fluorescence changes that some
PDAs undergo in response to temperature.
CR = (PB −PB1)/PB0 × 100
(1)
[
]
0
where PB = Ablue/(Ablue + Ared), Ablue and Ared are the respective
absorbances at 630 nm and 540 nm in the UV-Vis spectrum. PB0 and
PB1 are the respective pre- and post-thermal perturbations values. The
temperature of the aqueous PDA solution was controlled by using a
water-bath system.
Molecular Model and Simulation Methods: A schematic for the set up
of the model system is given in Figure 3. The Bis-PDA-Ph monomers,
consisting of two PDA molecules linked by an aryl group, were
transformed to the ene-yne conjugated polymer in the form of a sheet.
The first and last monomers were connected by the same conjugation,
a configuration which when combined with the periodic boundary
condition imposed by the computer simulation represents an infinitely
long polymer. The number of monomers per polymer was set at 8 in
this model system. Eight polymeric PDA sheets were then stacked
side by side to form a membrane. The initial distance between the
neighboring sheets was set to 0.5 nm. Finally, the PDA membrane was
soaked in water solution and water molecules within 0.28 nm from the
membrane were removed. The resulting initial configuration consists of
14 939 atoms with 4.0 nm × 3.9 nm × 10.4 nm in dimension.
The molecular dynamics (MD) simulations were performed using
the CHARMM package (v. 38a1)[31] with the help of the CHARMM-GUI
website.[32] The force fields for the model PDA system were obtained
from CHARMM general force fields[33] and the TIP3P water model was
used in this study. The long-range electrostatic interaction between
charged atoms was treated by the PME method with the real space
cutoff of 1.6 nm and the cutoff distance for the Lennard-Jones potential
between non-bonded atom pairs was also set to 1.6 nm.
4. Experimental Section
Methods: Unless otherwise noted, materials were obtained from
commercial suppliers and were used without further purification.
Flash chromatography was carried out on silica gel (230–400 mesh).
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20 °C
40 °C
60 °C
80 °C
100 °C
120 °C
heating
cooling
heating
20 °C
PDA-embedded
20 °C
120 °C
Electronspun Fiber
Figure 6. Temperature promoted color transitions of the Bis-PDA-Ph based electrospun fibers.
The initial PDA membrane system was first energy-minimized
S(t) = A exp −t /τ
i
(2)
(
)
∑
i
by the steepest descent and the Newton-Raphson methods with
500 steps before being heated gradually up to 300 K with the rate of
10 K ps−1 at constant volume. The heated PDA membrane system was
then equilibrated with equilibrium NPT MD simulations until both the
system energy and structural properties such as area per monomer and
membrane thickness converged to a plateau. The pressure of 1.0 atm
was applied in all directions independently: that is, the surface tension
was set to zero.
i=1
are summarized in Table S2 (Supporting Information). The fitted curves
are presented in Figure 6.
Preparation of Bis-PCDA-Ph Electrospun Fibers: To prepare the
electrospun fibers, 0.015 g of Bis-PCDA-Ph and 0.15 g of Polyethylene
oxide (PEO, Mw = 600 000) as a matrix polymer were added to a mixture
of 4 mL CHCl3 and 4 mL THF. The solution was vigorously stirred for 1 h
and then placed in a syringe with the needle tip at a flow rate of 30 µL h−1
via an electrospinning system (NanoNC ESR200R2). Through the needle
(gauge 27), which was connected to a high voltage (8.0 kV) power
supply, the precursor solution was ejected to form fine fibers on a silicon
(Si) wafer or filter paper on the surface of a grounded aluminum plate
located 15 cm below the tip of the needle. After being dried in air for
≈10 h, the as spun fibers were subjected to UV light irradiation at 254 nm
(4 W) for 1 min in order to induce polymerization. The structure and
morphology of the fibers were assessed using electron microscopy
(FE-SEM, JEOL JSM-6700F).
Transient Absorption (TA) Experiments: In TA experiments, visible light
pulses are used to excite electronically the PDAs and then relaxation of
the electronic excited states of the PDAs is monitored by using time-
delayed probe pulses at different wavelengths. The TA experimental
setup is shown in Figure S1 (Supporting Information). Pulses at 800 nm
with an ca. per pulse energy of 0.5 mJ and a ≈35 fs pulse duration,
generated using a femtosecond Ti:Sapphire oscillator and amplifer
system (SpectraPhysics) operating at 1 kHz, were used to pump an
optical parametric amplifier (OPA, SpectraPhysics) which then generates
the signal (1.2–1.6 µm) and idler (1.6–2.4 µm) in the near-IR range. The
pump pulses at 630 nm or 520 nm in the visible range were generated
by using second harmonic generation of the signal or fourth harmonic
generation of the idler from the OPA, respectively. White light used
as the probe pulse was generated by focusing 800 nm pulses onto a
2 mm thick sapphire disk. The instrumental response function of the
TA experimental setup was ≈200 fs. The probe beam was focused onto
the sample by using a parabolic mirror (focal length = 100 mm) and
its diameter was approximately 100 µm at the sample position. The
pump beam was focused by using a lens with a 20 cm focal length and
the diameter of the pump beam was almost 3 times larger than that of
the probe beam at the sample position. The power of the pump beams
was ca. 200 µW. After passing through the sample, the probe beam was
collimated with a parabolic mirror (focal length = 100 mm), dispersed
through a monochromator, and detected by using a Si photodiode.
TA signals were measured with a lock-in amplifier by chopping the
pump beam at 500 Hz. The sample solutions were housed in a
1 mm thick glass cell for TA experiments at room temperature (22 °C).
For TA experiments at 80 °C, a home made temperature controlled
cell with a 750 µm Teflon spacer was used and its temperature was
controlled by using a temperature controller and measured with a
K-type thermocouple. The temperature of the cell was maintained within
at 80 0.2 °C during the elevated temperature TA experiments. The
normalized TA signals associated with Bis-PDA-Ph and PCDA-PDA are
shown in Figure S2 (Supporting Information). For quantitative analyses,
the normalized TA signals of Bis-PDA-Ph and PCDA-PDA, fitted by using
the multi-exponential function,
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
S.L., J.L., and M.L. contributed equally to this work. This study
was supported by a grant from the National Research Foundation
of Korea (NRF) funded by the Korean government (MSIP)
(No. 2012R1A3A2048814 for J.Y.; No. 2013R1A1A1A05009866 for R.C.;
No.20100020209 for S.P.; No. 2010–0022028 for M.H.K.).
Received: December 12, 2013
Revised: January 14, 2014
Published online: February 26, 2014
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