Development of naphthalimide-functionalized thermochromic conjugated polydiacetylenes and their reversible green-to-red chromatic transition in the solid state

Article abstract of DOI:10.1016/j.dyepig.2019.01.058

Development of PDAs has received great attention in many fields due to their optical and chromatic properties towards external stimuli. In this article, two novel PDAs (NPI-PDA1 and NPI-PDA2) containing naphthalimide as a head group are designed and synthesized through self-assembly followed by topochemical polymerization, and these compounds show a reversible green-to-red colorimetric transition upon heating. PEO-embedded films are fabricated to facilitate the investigation of the thermochromic properties, and the surface morphologies of the films are observed by SEM analysis. The thermochromic behavior and color transitions are observed using photography and electronic absorption spectroscopy. The changes in the extended ene-yne conjugated system of the polymer during heat treatment are successfully monitored by Raman spectrometry. Moreover, PDA-embedded crayon wax-based hand-writable pens are fabricated and used efficiently on a solid substrate. NPI-PDA1 and NPI-PDA2 display excellent reversibility in the temperature ranges of 25–120 °C and 25–160 °C, respectively in the PEO polymer and paraffin wax matrices.

Full text of DOI:10.1016/j.dyepig.2019.01.058

Accepted Manuscript
Development of naphthalimide-functionalized thermochromic conjugated
polydiacetylenes and their reversible green-to-red chromatic transition in the solid
state
Naveen Mergu, Hyorim Kim, Gisu Heo, Young-A. Son
PII:
S0143-7208(19)30033-6
https://doi.org/10.1016/j.dyepig.2019.01.058
DYPI 7331
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 January 2019
Revised Date: 28 January 2019
Accepted Date: 30 January 2019
Please cite this article as: Mergu N, Kim H, Heo G, Son Y-A, Development of naphthalimide-
functionalized thermochromic conjugated polydiacetylenes and their reversible green-to-red chromatic
transition in the solid state, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.01.058.
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Graphical abstract

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Development of naphthalimide-functionalized thermochromic
conjugated polydiacetylenes and their reversible green-to-red
chromatic transition in the solid state
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Naveen Mergu, Hyorim Kim, Gisu Heo, Young-A Son*
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Department of Advanced Organic Materials Engineering, Chungnam National University, 220
Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea
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*Corresponding author. Tel.: +82 42 821 6620; Fax: +82 42 821 8870.
E-mail addresses: yason@cnu.ac.kr (Y.-A. Son).
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Abstract
Development of PDAs has received great attention in many fields due to their optical and
chromatic properties towards external stimuli. In this article, two novel PDAs (NPI-PDA1 and
NPI-PDA2) containing naphthalimide as a head group are designed and synthesized through
self-assembly followed by topochemical polymerization, and these compounds show a reversible
green-to-red colorimetric transition upon heating. PEO-embedded films are fabricated to
facilitate the investigation of the thermochromic properties, and the surface morphologies of the
films are observed by SEM analysis. The thermochromic behavior and color transitions are
observed using photography and electronic absorption spectroscopy. The changes in the
extended ene-yne conjugated system of the polymer during heat treatment are successfully
monitored by Raman spectrometry. Moreover, PDA-embedded crayon wax-based hand-writable
pens are fabricated and used efficiently on a solid substrate. NPI-PDA1 and NPI-PDA2 display
excellent reversibility in the temperature ranges of 25–120 °C and 25–160 °C, respectively in the
PEO polymer and paraffin wax matrices.
Keywords: Polydiacetylene, Thermochromism, Green-to-red color transition, PEO films,
Crayon pen
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1. Introduction
Polydiacetylenes (PDAs) are important conjugated polymers that have alternating double
and triple bonds in the main polymer chain. PDAs are generally obtained by irradiation of self-
assembled diacetylene monomers with UV (254 nm) or γ-rays, and these reactions do not require
chemical initiators or catalysts nor are purification steps necessary to obtain pure PDAs. The
polymerization takes place because the spacing between the monomer alkyl chains is small and
similar to the spacing required for the diacetylene backbone to form and because of the
intramolecular interactions among the monomer’s sidechains [1–6]. However, for polymerization
in the solid state, the diacetylene units must be properly oriented in relation to each other. A
distance of 4.9 Å between the two reactive carbon atoms and an angle of 45° relative to the
acetylene axis are required for this polymerization [7,8].
PDAs have received a substantial amount of attention since their discovery due to their
facile self-polymerization and their optical and chromic properties including the unique blue-to-
red and non-fluorescence to red-fluorescence transitions upon exposure to external stimuli such
as heat, solvent, pH changes, mechanical stress, electrical current and ligand-receptor interaction
[9–20]. The color change can easily be observed by the naked-eye. The color transformations of
PDAs are due to the π-π* transition in the linear π-conjugated polymer backbone. The monomers
alone do not show an absorption band, but an absorption band appears at approximately 650 nm
upon polymerization. If the effective conjugation length is reduced due to strain or torsion
applied to the backbone, the absorption maximum shifts to approximately 550 nm, which
corresponds to a red color. These smart materials have been effectively used as temperature
sensors and to monitor various biologically, chemically and environmentally important analytes
[21–38]. To date, the majority of reported thermochromic PDAs show blue-to-red color changes
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above room temperature (typically >40 °C). In the past few years, PDA-like conjugate polymers
have been important materials for developing advanced systems and devices in various fields due
to their intrinsic electronic, magnetic, photonic and spectroscopic properties [39–44].
To date, numerous PDAs were reported, which are mainly showing a reversible
thermochromic behavior with a one-way blue-to-red color transitions. Also, the PCDA
consisting of ethylene diamine was also reported, which showed blue-to-red color transition at
room temperature [45]. In previous reports, aza-substituted PCDAs were used to achieve green-
colored PDAs; however, due to steric hindrance, some of these compounds could not be
polymerized, and some of those that did polymerize, produced regular blue-colored PDAs [46–
49]. A green-to-red color transition was achieved by a subtractive color mixing method (external
mixing) and reported recently by our research group [50]. From the knowledge of the color
theory, we have extended the strategy to obtain PDAs that shows direct green-to-red color
transition without mixing additional color. Recently, the development of directly writable
techniques for depositing functional materials on solid substrates has attracted great attention
[51–57]. In particular, pen-on-paper type approaches are very interesting since they enable the
preparation of diverse patterned images on solid substrates with the flexibility afforded by
drawing in an inexpensive manner [58].
In the present work, we have developed new PDAs (NPI-PDA) containing naphthalimide
as a head group by a condensation reaction using 10,12-pentacosadiynoic acid (PCDA) (see
Figure S1 in the SI). This is the first example of a PDA with a reversible green-to-red color
transition at 45 °C instead of a blue-to-red transition, and the compound can return to its initial
color (green) when temperature is decreased. Furthermore, thermochromic color transitions
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displayed by NPI-PDA-embedded PEO films are highly reversible and occur over a short
temperature range (25 to 45 °C).
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2. Experimental Section
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2.1. Materials and Instruments
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4-Bromo-1,8-naphthalic anhydride, n-butylamine, anhydrous ethylenediamine, anhydrous
piperazine, N,N′-dicyclohexylcarbodiimide and 4-dimethylaminopyridine were purchased from
TCI. 10,12-Pentacosadiynoic acid (PCDA) was purchased from Sigma-Aldrich. All other
solvents were purchased from commercial sources and used without further purification. The 1H
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and C nuclear magnetic resonance (NMR) spectra were collected on a Bruker AVANCE III
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spectrometer at 300 and 600 MHz, and CDCl3 was used as the solvent. Mass spectra were
recorded on AB Sciex 4000 QTRAP and Bruker micrOTOF-Q mass spectrometers. Solid-state
absorption spectra were acquired on a SolidSpec-3700 diffuse reflectance UV-vis-NIR
spectrophotometer. UV-vis spectra of the PEO films were collected using a Shimadzu UV-2600
spectrophotometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) were conducted on a Mettler-Toledo TGA/DSC1 thermal analyzer at a heating/cooling
rate of 10 °C/min in a nitrogen stream. SEM analysis for the polymer thin films was conducted
using a Zeiss MERLIN field emission scanning electron microscope. Raman spectra were
obtained on a Horiba Jobin Yovn LabRAM HR-800 Raman spectrometer with excitation at 785
nm laser under ambient conditions.
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2.2. Preparation of diacetylene
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Synthesis of naphthalimide (A): A was prepared according to the literature method [59]. A
mixture of 4-bromo-1,8-naphthalic anhydride (2.77 g, 10 mmol) and butylamine (0.88 g, 12
mmol) in ethanol (50 mL) was refluxed for 8 hours. Then, the reaction mixture was concentrated
under reduced pressure and purified by column chromatography using a mixture of hexane/DCM
(7:3) as the eluent. After drying under reduced pressure, the reaction yielded A as a white solid
(3.0 g, 90%). 1H NMR (300 MHz, CDCl3, δ): 8.55 (dd, J = 1.2, 7.2 Hz, 1H; Ar H), 8.45 (dd, J =
1.2, 8.4 Hz, 1H; Ar H), 8.31 (d, J = 8.1 Hz, 1H; Ar H), 7.94 (d, J = 7.8 Hz, 1H; Ar H), 7.75 (t, J
= 7.2 Hz, 1H; Ar H), 4.09 (t, J = 7.5 Hz, 2H; CH2), 1.69–1.59 (m, 2H, CH2), 1.37 (sext, J = 7.5
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Hz, 2H; CH2), 0.90 (t, J = 7.5 Hz, 3H; CH3); C NMR (150 MHz, CDCl3, δ): 133.1, 131.9,
131.1, 131.0, 130.5, 130.1, 128.9, 128.0, 123.1, 122.2, 40.3, 30.1, 20.3,13.8.
Synthesis of naphthalimide derivative (B): A (1.0 g, 3 mmol), ethylenediamine (1.8 g, 30
mmol), and piperazine (1.3 g, 15 mmol) were refluxed in ethanol (30 mL) for 72 hours. The
mixture was then concentrated and purified by column chromatography using DCM/MeOH
(90:10 for B1 and 95:5 for B2) as the eluent, and B was isolated as a yellow solid (0.79 g, 84%
of B1 and 0.89 g, 87% of B2).
B1: 1H NMR (600 MHz, CDCl3, δ): 8.53 (d, J = 7.2 Hz, 1H; Ar H), 8.41 (d, J = 8.4 Hz, 1H; Ar
H), 8.15 (d, J = 8.4 Hz, 1H; Ar H), 7.57 (t, J = 7.2 Hz, 1H; Ar H), 6.65 (d, J = 8.4 Hz, 1H; Ar H),
6.19 (s, 1H, NH), 4.14 (t, J = 7.8 Hz, 2H; CH2), 3.39 (q, J = 6.0 Hz, 2H; CH2), 3.16 (t, J = 6.0 Hz
2H; CH2), 1.72–1.67 (m, 2H, CH2), 1.56 (s, 2H, NH2), 1.42 (sext, J = 7.8 Hz, 2H; CH2), 0.96 (t, J
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= 7.8 Hz, 3H; CH3); C NMR (150 MHz, CDCl3, δ): 164.7, 164.1, 149.6, 134.4, 131.0, 129.7,
126.2, 124.6, 123.1, 120.4, 110.3, 104.4, 44.9, 40.2, 39.9, 30.3, 20.4, 13.8. ESI-MS m/z: [M +
H]+ calcd for C18H21N3O2, 312.2; found, 312.3.
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B2: 1H NMR (600 MHz, CDCl3, δ): 8.57 (d, J = 7.2 Hz, 1H; Ar H), 8.51 (d, J = 7.8 Hz, 1H; Ar
H), 8.41 (d, J = 8.4 Hz, 1H; Ar H), 7.68 (t, J = 7.8 Hz, 1H; Ar H), 7.20 (d, J = 7.8 Hz, 1H; Ar H),
4.16 (t, J = 7.2 Hz, 2H; CH2), 3.24–3.19 (m, 8H, CH2), 1.70 (qn, J = 7.8 Hz, 2H; CH2), 1.44
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(sext, J = 7.2 Hz, 2H; CH2), 1.24 (s, 1H, NH), 0.96 (t, J = 7.2 Hz, 3H; CH3); C NMR (150
MHz, CDCl3, δ):164.5, 164.0, 156.3, 132.5, 131.20, 130.2, 129.9, 126.2, 125.6, 123.3, 116.8,
114.9, 54.4, 46.2, 40.1, 30.3, 20.4, 13.8. ESI-MS m/z: [M + H]+ calcd for C20H23N3O2, 338.2;
found, 337.8.
Synthesis of diacetylene monomers (NPI-DA): As shown in Scheme 1, a dichloromethane
(DCM) solution containing B (1.5 mmol), 10,12-pentacosadiynoic acid (PCDA) (0.56 g, 1.5
mmol), N,N′-dicyclohexylcarbodiimide (DCC) (0.5 g, 2.5 mmol) and 4-(dimethylamino)pyridine
(DMAP) (30 mg) was stirred overnight at room temperature. The mixture was concentrated
under reduced pressure and purified by column chromatography (DCM/MeOH; 99:1) to afford
NPI-DA as green-yellow solid (750 mg, 75% of NPI-DA1 and 821 mg, 79% of NPI-DA2).
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NPI-DA1: H NMR (600 MHz, CDCl3, δ): 8.56 (d, J = 7.2 Hz, 1H; Ar H), 8.38 (d, J = 7.8 Hz,
1H; Ar H), 8.23 (d, J = 8.4 Hz, 1H; Ar H), 7.63 (t, J = 7.2 Hz, 1H; Ar H), 7.20 (s, 1H, NH), 6.48
(d, J = 8.4 Hz, 1H; Ar H), 6.37 (t, J = 6.6 Hz, 1H; NH), 4.15 (t, J = 7.2 Hz, 2H; CH2), 3.76 (q, J
= 6.0 Hz, 2H; CH2), 3.42 (q, J = 4.2 Hz, 2H, CH2), 2.28 (t, J = 7.2 Hz, 2H; CH2), 2.23 (t, J = 7.2
Hz, 2H; CH2), 2.16 (t, J = 7.2 Hz, 2H; CH2), 1.73–1.69 (m, 2H, CH2), 1.67–1.63 (m, 2H, CH2),
1.50 (qn, J = 7.2 Hz, 2H; CH2), 1.46–1.38 (m, 4H, CH2), 1.37–1.13 (m, 23H, CH2), 1.15–1.08
(m, 3H, CH2), 0.96 (t, J = 7.2 Hz, 3H; CH3), 0.87 (t, J = 7.2 Hz, 3H; CH3); 13C NMR (150 MHz,
CDCl3, δ): 176.6, 164.8, 164.3, 150.1, 134.4, 131.1, 129.8, 127.2, 124.9, 122.8, 120.4, 109.9,
103.1, 77.6, 77.3, 65.4, 65.2, 46.7, 39.9, 38.9, 36.6, 33.9, 31.9, 30.3, 29.6, 29.6, 29.5, 29.4, 29.3,
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29.1, 29.0, 28.9, 28.8, 28.8, 28.6, 28.4, 28.2, 25.7, 25.6, 24.9, 22.7, 20.4, 19.1, 14.1, 13.9. HRMS
m/z: [M + Na]+ calcd for C43H61N3O3, 690.4611; found, 690.4600.
NPI-DA2: 1H NMR (600 MHz, CDCl3, δ): 8.60 (dd, J = 1.2, 7.2 Hz, 1H; Ar H), 8.52 (d, J = 7.8
Hz, 1H; Ar H), 8.42 (dd, J = 1.2, 8.4 Hz, 1H; Ar H), 7.72 (t, J = 7.2 Hz, 1H; Ar H), 7.22 (d, J =
8.4 Hz, 1H; Ar H), 4.16 (t, J = 7.2 Hz, 2H; CH2), 3.94 (s, 2H, CH2), 3.79 (s, 2H, CH2), 3.25–3.21
(m, 4H, CH2), 2.24 (t, J = 7.8 Hz, 2H; CH2), 2.23 (q, J = 7.8 Hz, 4H; CH2), 1.73–1.65 (m, 4H,
CH2), 1.50 (sext, J = 7.2 Hz, 4H; CH2), 1.44 (q, J = 7.2 Hz, 2H; CH2), 1.41–1.25 (m, 26H, CH2),
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0.97 (t, J = 7.2 Hz, 3H; CH3), 0.87 (t, J = 7.2 Hz, 3H; CH3); C NMR (150 MHz, CDCl3, δ):
171.9, 164.3, 163.9, 155.1, 132.3, 131.2, 129.8, 129.7, 126.3, 126.1, 123.5, 117.7, 115.4, 77.6,
77.4, 65.3, 65.2, 53.1, 53.1, 45.8, 41.7, 40.1, 33.3, 31.9, 30.2, 29.6, 29.6, 29.6, 29.4, 29.4, 29.3,
29.2, 29.1, 28.9, 28.8, 28.7, 28.3, 28.3, 25.3, 22.7, 20.4, 19.2, 14.1, 13.8. HRMS m/z: [M + Na]+
calcd for C45H63N3O3, 716.4767; found, 716.4767.
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Scheme 1. Synthesis route for the preparation of PCDA derivatives containing a naphthalimide
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unit.
2.3. Preparation of polymerized NPI-DAs
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The NPI-DA monomer was ground into a fine powder using a mortar and pestle and then
polymerized with UV irradiation using a 254 nm lamp for 1 h at room temperature. Finally, a
green-colored NPI-PDA was collected, and it was used without further purification.
2.4. Preparation of the NPI-DA-embedded PEO film
Poly(ethylene oxide) was used as the polymer matrix to prepare the polymeric films. As
depicted in the schematic representation (Figure 1), to the stirred solution of CHCl3 (3 mL)
containing 100 mg of PEO was added 20 mg of NPI-DA monomer. After the PEO had
completely dissolved, the transparent solution was poured into a petri dish. The solvent was
allowed to slowly evaporate under a vacuum dryer for 1 h. The thin PEO films were cut into 2×2
cm square pieces and then removed from the petri dish.
2.5. Fabrication of an NPI-DA-embedded hand-writable pen
For drawing purposes, a paraffin wax-based hand-writable pen containing NPI-DA
monomer was constructed. As demonstrated in Figure 1, a hot solution of paraffin wax (90%)
containing DA monomer (10%) was poured into a mold. After cooling to room temperature, the
yellow-colored stick was removed from the mold and could then be used as a thermochromic
drawing tool. As shown in the figure, the color transition takes place upon heating a solid surface
that has already been drawn with the PDA-wax pen and polymerized.
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Figure 1. (a) Schematic representation of the steps involved in the preparation of polymer thin
film. (b1) Fabrication of a diacetylene monomer-embedded paraffin pen. (b2) Color transition of
the paraffin wax drawing upon polymerization and heating.
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3. Results and Discussion
3.1. Thermochromism of green PDAs
Green PDAs were obtained upon polymerization of the diacetylene monomers NPI-DA1
and NPI-DA2 under UV light irradiation. To date, the majority of PDAs and their derivatives are
blue in color, and this is the first report of using a yellow-colored naphthalimide derivative as a
head group to prepare green PDAs. The topochemical polymerization initiated and accelerated
by UV irradiation generates the conjugated polymer network [7,8]. To gain a better
understanding of the development of green PDA, an illustration showing the self-assembly of the
NPI-DA monomer, the polymerization and the color transition upon heat treatment is shown in
Figure 2. As seen in the figure, the monomers approach each other in a single plane upon self-
assembly. Under UV irradiation, the adjacent highly ordered monomers join together to produce
polymer of NPI-DA. The developed green-colored PDAs turns red under thermal stress in the
solid state.
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Figure 2. A detailed schematic illustration of the steps involved in producing a green PDA from
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the DA monomer and its thermochromic behavior.
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Initially, the monomers are yellow, and they turn green upon polymerization (see Figure
S1 in the SI). UV-vis absorption spectra corresponding to the green and red phases of the PDAs
in the powder state were recorded and are shown in (see Figure S2 in the SI). For the green
phase, the most prominent spectral features are two broad peaks centered at approximately 450
and 620 nm for NPI-PDA1 and 400 and 600 nm for NPI-PDA2. The PDA undergoes a color
transition from green to red when the temperature is increased to 45 °C, and the PDA can be
returned to its original green color by decreasing the temperature. The NPI-PDAs displayed
excellent thermochromic reversibility in the temperature range of 25–160 °C. In the red phase,
unexpected absorption shifts were observed due to inadequate polymerization in the powder
phase. To avoid this, we then prepared and characterized PEO polymer thin films doped with
NPI-DA monomer. Additionally, the thin polymer films are clear to the naked-eye and show
good spectral properties. Notably, higher temperatures of 180 and 160 °C, which are equal to the
melting points of NPI-PDA1 and NPI-PDA2, respectively, will cause irreversible
thermochromic behavior in the solid state.
To determine the thermal stability of the synthesized PDAs, thermogravimetric analysis
(TGA) was conducted over the range of 25–800 °C. As shown in Figure 3a, NPI-PDA1 and
NPI-PDA2 have similar thermal decomposition behavior, and both compounds started
decomposing at approximately 350 °C. In the DSC curve of NPI-PDA1, two endothermic peaks
were observed at 98.39 and 131.26 °C, while NPI-PDA2 displayed a single endothermic peak at
84.24 °C (Figure 3b). According to the obtained TGA and DSC data, the prepared PDAs can be
analyzed and display their thermochromic properties at reasonably high temperatures.
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Figure 3. TGA (a) and DSC (b) curves of NPI-PDA1 and NPI-PDA2. The heating rate was 10
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3.2. Thermochromism of NPI-DA-embedded PEO films
Due to its uniformity and the availability of higher surface area to polymerization, we
prepared NPI-DA-embedded PEO films. These thin films displayed an extremely quick response
to UV irradiation to yield a green color. The fabricated yellow-colored PEO-based films were
irradiated with a UV lamp (254 nm) at room temperature for 5 min to achieve the polymerized
green-colored films, and the corresponding UV spectra are shown in Figure 4. The
naphthalimide moiety connected to ethylenediamine (primary amine) produced brighter green
compared to piperazine (secondary amine) due to the difference in their original yellow colors.
The color of PDA is also depends on the intermolecular hydrogen bonds between two adjacent
polymerizable monomers [60]. Thus, a strong color observed for NPI-PDA1 could be due to the
formation of strong intermolecular hydrogen bonds that induces the polymerization of
monomers. The NPI-DA monomer showed a very broad and more intense peak in the region of
400–500 nm that can be attributed to the naphthalimide chromophore, no peak was observed in
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the range of 550–700 nm, and a peak centered at 640 nm with a shoulder at approximately 590
nm was observed during the polymerization process. In general, a very strong absorption appears
at around 650 nm for PDA macromolecules. Instead, a broad absorption peak in the 550–700 nm
region was observed, which mainly due to the overlap with the broad and strong absorption band
of naphthalimide chromophore. As shown in the spectra, the λmax at approximately 640 nm
gradually increased and reached a maximum, and the yellow-colored thin films dramatically
turned green after 5 min of UV irradiation. SEM analysis was carried out to observe the
morphology of only the thin films of PEO, NPI-PDA1-embedded PEO and NPI-PDA2-
embedded PEO (Figure 5). The SEM results of PEO show that the surface is an aggregated
sheet. The morphology of the PDA/PEO mixture is completely different, and they show
aggregated needles-type morphology. The obtained green PDA-embedded PEO solid film was
further heated for the thermochromic observations. The thermochromic transition of the
successfully fabricated and polymerized thin films were carried out using photography and
electronic absorption spectroscopy.
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Figure 4. UV-vis spectral changes of (a) NPI-DA1- and (b) NPI-DA2-embedded PEO films
under UV irradiation for 5 min. Inset: The color changes of the thin films from yellow to green
upon polymerization.
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Figure 5. SEM images of PEO (a), NPI-PDA1-embedded PEO (b) and NPI-PDA2-embedded
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PEO (c) films at room temperature.
As depicted in Figure 6, the reversible significant color transition and the changes in its
absorption spectrum were observed and recorded at 2 °C increments in the temperature range of
25–45 °C. Initially, at 25 °C, the film was green and showed a broad absorption peak centered at
640 nm. The color slowly turned orange-red, and the absorbance at 640 nm gradually decreased,
and a new absorption peak at approximately 550 nm (blueshift) appeared as the temperature
increased. The peak at 640 nm almost disappeared, and the new peak at 550 nm reached its
maximum intensity at 45 °C. The band corresponding to the naphthalimide did not change as the
temperature increase, which means that the naphthalimide group was not affected during
polymerization or heat treatment. The original spectral pattern and color were completely
recovered upon cooling from 45 °C to 25 °C. It is commonly believed that the side chains that
are directly linked to the backbone of the polymer undergo twisting upon thermal treatment. Due
to the presence of twisted side chains, the conjugation within the polymer decreases, causing a
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blueshift in the absorption spectra [61–63]. On further increasing the temperature from 45–120
°C, the orange-red color slowly became orange. The thermochromic reversibility of the PDA-
embedded PEO films was successfully demonstrated up to 120 °C. Finally, upon further heat
treatment of the orange NPI-PDA1- and NPI-PDA2-embedded PEO films at 180 and 160 °C,
respectively, an irreversible yellow-orange color developed. The thermal stability for PEO films
was found to be 180 and 160 °C respectively for NPI-PDA1 and NPI-PDA2. Thus, the PDA in
the PEO polymer matrix exhibited a rapid green-to-orange response and a reversible orange-to-
green thermochromic transition. The color transition of the PDA in the polymer matrix was the
same as that observed in the powder form, and compared to the powder form, it showed
promising spectral characteristics when embedded in the polymer matrix.
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Figure 6. The color transition (a,b) and UV absorption spectral changes (c,d) of polymerized
thin films under heating; a and c correspond to NPI-PDA1, and b and d correspond to NPI-
PDA2.
Moreover, the thin films consisting of the monomer and polymer were further
characterized using Raman spectrometry. For this study, to generate irreversibly orange films,
NPI-PDA1- and NPI-PDA2-embedded films were heat treated at 180 and 160 °C, respectively.
As can be seen in Figure 7, peaks corresponding to the alkene and alkyne groups in the NPI-
DA1 monomer appeared at 1455 and 2092 cm⁻
1, respectively, and green NPI-PDA1 showed
Raman shifts similar to those of the monomer. The alkene and alkyne bands in the typical orange
1, respectively, and the bands typical of the initial
NPI-PDA1 appeared at 1512 and 2117 cm
⁻
green polymer were also present. Similarly, the NPI-DA2 monomer and polymer (green phase)
1
produced signals at 1454 and 2082 cm
⁻
corresponding to the alkene and alkyne groups,
respectively. The orange NPI-PDA2 exhibited the same Raman bands along with shoulder peaks
1. The spectral changes corresponding to the color transition are commonly
at 1501 and 2107 cm
⁻
due to the thermal-induced conformational changes in the alkyl side chains causing changes in
the PDA backbone [64–67].
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a
b
NPI-PDA1 (orange phase)
NPI-PDA1 (green phase)
NPI-PDA2 (orange phase)
NPI-PDA2 (green phase)
NPI-DA1 (monomer)
NPI-DA2 (monomer)
1400
1600
1800
2000
2200
2400 1400
1600
1800
2000
2200
2400
Raman shift
(
cm-1
)
Raman shift ( )
cm-1
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Figure 7. Raman spectral changes of (a) NPI-DA1- and (b) NPI-DA2-embedded PEO thin films
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before and after polymerization (green and orange phase).
3.3. Thermochromic properties of NPI-DA-embedded hand-writable pen
Due to the unreactive solid matrix, we used crayon wax to fabricate a hand-writable pen
based on the diacetylene monomer and investigated its thermochromic behavior on solid
substrates such as paper. The drawings of NPI-DA2-wax crayon on paper showed color changes
upon UV irradiation and thermal treatment, which are photographed step-by-step at various
temperatures (Figure 8). As demonstrated in Figure 9, images of a parrot and a rose were drawn
using the NPI-DA1- and NPI-DA2-wax crayons, respectively, and were initially yellow. The
drawings turned green immediately upon UV irradiation (254 nm). The obtained green images
exhibited reversible thermochromic behavior over a wide range of working temperatures from
25–160 °C and 25–180 °C for NPI-DA1- and NPI-DA2-embedded wax crayons, respectively.
The color of each of the green drawings was monitored over various temperature ranges as
shown in the figure. The images turned greenish-red and orange-red at 35 and 45 °C, and these
color changes are completely reversible. On further heating, the parrot and rose turned orange at
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180 and 160 °C, respectively, and they returned to green upon cooling. The images
corresponding to NPI-DA1 and NPI-DA2 irreversibly turned yellow-orange at approximately
190 and 170 °C, respectively. The images drawn using crayon pens exhibited similar
thermochromic properties as the monomers showed in the powder state. Thus, due to their high
thermal stability and immediate thermochromic reversibility, it is possible to utilize in the
identification and authentication of documents.
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Figure 8. Photographs of a PDA image on paper derived from NPI-PDA2-wax upon heating and
cooling.
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Figure 9. Photographs of PDA images drawn using the hand-writable pens derived from (a)
NPI-DA1- and (b) NPI-DA2-wax crayons at various conditions.
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4. Conclusion
In the studies described above, the thermochromic behavior of newly synthesized PDAs
was investigated in the solid state as well as in a polymer matrix. PDA-embedded polymer thin
films were fabricated and characterized by SEM, UV absorption and Raman spectroscopic
studies. Obvious spectral changes corresponding to the color transition from green-to-red and
red-to-orange upon heating were observed. Thermochromic reversibility was successfully
monitored in the range of 25–120 °C in the polymer matrix. In addition, NPI-DA1- and NPI-
DA2-embedded paraffin wax-based hand-writable pens were constructed, and they exhibited
excellent reversible thermochromic performance over a wide temperature range (25–160 °C).
Due to their high thermal stability and immediate thermochromic reversibility, these materials
can be used in the identification and authentication of documents.
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Acknowledgements
This study was supported by the Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant
No. NRF-2017R1E1A1A01074266).
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