Tuning of the Topochemical Polymerization of Diacetylenes Based on

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Tuning of the topochemical polymerization of diacetylenes based on an odd/even effect of the peripheral alkyl chain: Thermochromic reversibility in a thin film and a single component ink for a fountain pen Myeongjin Kim, Satheshkumar Angupillai, Kyeongsu Min, Manivannan Ramalingam, and Young-A Son ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05896 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Materials & Interfaces

Tuning of the Topochemical Polymerization of Diacetylenes Based on an Odd/Even Effect of the Peripheral Alkyl Chain: Thermochromic Reversibility in a Thin Film and a Single Component Ink for a Fountain Pen Myeong Jin Kim‡, Satheshkumar Angupillai‡, Kyeongsu Min, Manivannan Ramalingam, Young-A Son*

Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea

*Corresponding author. Tel: +82 42 821 6620; Fax: +82 42 821 8870 E-mail address: [email protected] ‡

These authors contributed equally to this work.

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Abstract: The topochemical polymerization of diacetylenes (DAs) is closely related to the length of their alkyl chain. Diacetylene monomers have two types of alkyl chain side groups: the inner alkyl chain and the outer alkyl chain, i.e., the peripheral alkyl chain. Herein, we designed and synthesized a series of diacetylene monomers that possess bis-amide linkages with different peripheral alkyl chains (n=6-9; DA1-DA4)). The peripheral alkyl chain length of these diacetylene monomers exhibits an odd/even effect on topochemical polymerization. The polymerization of DAs was achieved only when n is an odd number, while no polymerization occurred when n is an even number. The odd/even effect on the topochemical polymerization was also investigated using ab initio DFT calculations. The thermochromic reversibility of PDAs was investigated using UV-Vis absorption spectroscopy at temperatures ranging from 20 to 60° C. Monomer DA2 was used as a single component ink solution in a fountain pen that can be transformed into thermochromic letters on conventional paper. Furthermore, a PDA-embedded PEO film was included to monitor the thermochromic reversibility and was found to exhibit excellent thermochromic reversibility between 20 and 100° C and stability, enabling storage for a few months without deformation. Finally, a green-colored patterned polymer film is readily obtained by a subtractive color (blue and yellow) mixing method and exhibits high thermochromic reversibility at temperatures between 20 and 100° C.

Keywords: Odd/even effect, Polydiacetylene, Thermochromic, Subtractive color mixing, Single component ink, Fountain pen.


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1. Introduction Polydiacetylenes (PDAs) are conjugated polymers that have received great attention due to their unique optical properties. Upon irradiation with short UV light, 1,3-diynes undergo topochemical polymerization to blue color PDAs due to the existence of extended π-conjugation. Since 1969, PDAs have been used by various research groups due to their property of drastic color changes in response to external stimuli, such as temperature, pressure, light, ions, and biochemicals.1-9 These ene-yne-type PDAs are promising thermochromic materials due to the twisting of the polymer backbone and changes in the degree of conjugation in the system.10 Despite several decades of research, there have been very few reported investigations on the elucidation of the roles of polymer length and peripheral groups in the chromatic transformation of PDA.11 Very recently, Tachibana et al. have studied the polymerization and thermochromic effect of urethane-substituted diacetylenes with varying alkyl chain lengths. The results reveal that the angle between the diacetylene backbone and the stacking axis and the distance between the neighboring reacting carbon atoms play a vital role in the diacetylene polymerization reaction.12 Sasikarn and coworkers have demonstrated the roles of the number of methylene units within the diacetylene monomers and the thermochromic reversibility of their corresponding PDAs.13 Likewise, a few research groups have proved that the odd/even number of the inner alkyl chain of diacetylene molecules has a pronounced effect on the optical properties of PDAs.14-17 However, the odd/even effect of the peripheral alkyl chain on PDAs is still rare or does not exist in the literature. Another important aspect of the PDA structure that has been almost entirely ignored until recently is the difunctionalization of PDAs. Very recently, Jong-Man Kim et al. have investigated three bis-urea substituted inkjet-printable PDA, showing that bis-peripheral groups



such as bis-urea facilitate the hydrogen bonding network and self-assembly of monomers.18 The literature suggests that the polymerization ability of a diacetylene monomer is based on the ease with which the units self-assemble and the peripheral alkyl chain length.15,18 To date, while various approaches have been developed to achieve DA-based ink and successfully apply it to an inkjet printable system, DAs have not been used as ink in a pen.19,20 However, the main disadvantage of the reported DA ink is the formation of a vesicle solution in which the DA monomers are embedded in organic microdroplets with an aqueous surfactant solution in the presence of a co-surfactant. In general vesicle solutions consist of three components, i.e., the diacetylene, surfactant and co-surfactants, making ink solution preparation a rather tedious process.19 The achievement of writable ink is essential for the development of anti-counterfeiting materials. Accordingly, in the present research we developed a single component ink system that is highly suitable for use as writable ink consisting of a diacetylene (for self-assembly), an amide (for enhanced self-assembly) and a peripheral alkyl chain (for improved solubility). From the standpoint of practical applications, the preparation of a flexible batch-type thin film is more convenient than that of powder samples. Nevertheless, few studies have succeeded in the use of a flexible batch-type PDA film for thermochromic and hydrochromic applications.21,22 Moreover, with regard to color, only blue is currently available in PDAs motifs. According to subtractive color mixing, the mixing of the primary colors yellow and blue yield green. Even though a few studies have been reported in the literature for the attachment of an azo dye to PDAs, the color of PDA remains blue.23,24 Although even the use of this yellow azo chromophore fails to induce green color in PDAs, this may be due to the weaker intensity of the yellow color. A survey of the literature shows that achieving different colors other than blue in PDA is still a challenging task.


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Taking this into account, the present study examines the factors: (i) the odd/even effect of the peripheral alkyl chain length on topochemical polymerization, for which we designed a series of diacetylenes with amide functional groups (CONH(CH2)n) on both sides of the backbone chain such as n=6, DA1; n=7, DA2; n=8, DA3; and n=9, DA4 (Scheme 1). Similar research of odd/even effects on the topochemical polymerization of diacetylene compounds have been reported by Fujita et al. and Tamaoki et al.14,15 for the inner alkyl chain, however to the best of our knowledge, the present work is the first report on the odd/even effect of the peripheral alkyl chain length on the polymerization of diacetylenes. The reversible thermochromic behavior of PDA is discussed in the solid state as well as in the film state at various temperatures. Then, the prepared DA2 is used as a single component ink in a writable pen. (ii) We have also investigated a new methodology for fabricating a new green-colored patterned PDA system in the PEO polymer, which was made by subtractive mixing of a commercially available azo dye and a blue PDA.

2. Experimental section: 2.1 Materials and methods: All of the starting materials, other reagents, polyethylene oxide (Average Mv ca. 400,000, inhibited with 200-500 ppm BHT) and analytical-grade solvents were purchased from commercial sources and used without further purification. 1H NMR and 13C-NMR spectra were recorded using a Bruker spectrometer operating at 300/600 and 75/150 MHz, respectively. Raman spectra were recorded using a Raman microscope (LabRAM HR-800 UV-Visible-NIR, HORIBA JOBIN YVON) with laser excitation at 785 nm). Solid-state UV-Vis absorption



spectra were recorded using a UV-2600 spectrophotometer (Shimadzu). Thin layer chromatography (TLC) analyses were performed on silica gel plates, and flash column chromatography was performed using silica gel (230-400 mesh).

2.2 General method for the preparation of MA1-MA4: To a solution of 6-heptynoic acid (1.0 g, 7.9 mmol) in dry DCM (30 mL) was added EDCI.HCl (1.8 g, 9.5 mmol) and alkyl amine (9.5 mmol; hexylamine for MA1, heptylamine for MA2, octylamine for MA3, nonylamine for MA4). The reaction mixture was stirred at ambient temperature under a N2 atmosphere. After 24 h, the reaction was found to be complete by TLC. The reaction mixture was diluted with 30 ml of distilled water and washed successively with 1 N aqueous NaHCO3 and 1 N HCl solution. The organic layer was dried over sodium sulfate and filtered through filter paper. The organic layer was concentrated under reduced pressure to yield a corresponding monomers MA1, MA2, and MA4 (except MA3), crude MA3 was used as a precursor for DA3, therefore here we not mention analytical data.

Synthesis of N-hexylhept-6-ynamide (MA 1): 1H NMR (CDCl3, 300 MHz), δ(ppm): 0.82 (t, 3H, J = 5.4 Hz), 1.22 (m, 6H), 1.46 (m, 4H), 1.69 (m, 2H), 1.87 (t, 1H, J=2.4 Hz), 2.14 (m, 4H), 3.15 (m, 2H), 5.43 (bs, 1H). 13C NMR (CDCl3, 75 MHz), δ(ppm): 12.98, 17.18, 21.52, 23.84, 25.56, 26.93, 28.60, 30.45, 35.18, 38.55, 67.55, 83.09, 171.53. (White solid; yield=92 %)

Synthesis of N-heptylhept-6-ynamide (MA 2): 1H NMR (CDCl3, 300 MHz), δ(ppm): 0.81 (t, 3H, J = 6.6 Hz), 1.21 (m, 8H), 1.42 (m, 2H), 1.49 (m, 2H), 1.68 (m, 2H), 1.88 (t, 1H, J=2.7 Hz), 2.15 (m, 4H), 3.18 (m, 2H), 5.47 (bs, 1H). 13C NMR (CDCl3, 75 MHz), δ(ppm): 13.03, 17.17, 21.55,


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23.82, 25.86, 26.92, 27.93, 28.62, 30.71, 35.19, 38.57, 67.55, 83.08, 171.66. (White solid; yield=96 %)

Synthesis of N-nonylhept-6-ynamide (MA 4): 1H NMR (CDCl3, 600 MHz), δ(ppm): 0.80 (t, 3H, J = 7.2 Hz), 1.22 (m, 12H), 1.41 (m, 2H), 1.49 (m, 2H), 1.68 (m, 2H), 2.10 (m, 4H), 2.20 (m, 4H), 3.17 (m, 2H), 5.38 (bs, 1H). 13C NMR (CDCl3, 150 MHz), δ(ppm): 13.18, 17.18, 21.64, 23.82, 25.90, 26.95, 28.28, 28.48, 28.66, 30.83, 35.21, 38.52, 67.53, 83.08, 171.42. (Pale yellow solid; yield=94 %).

2.3 General method for the preparation of DA1-DA4: Intermediates MA1 to MA4 (1.0 g, 1.0 mol equiv), Cu(I)Cl (2.5 mol equiv) and NH4Cl were added to 50 mL of THF. Then, the reaction mixture was purged with air for 10 min to saturate the oxygen atmosphere in the reaction mixture, and the solution was stirred for 24 h in the dark. After 24 h, 1N HCl was poured into the reaction mixture, and the settled white precipitate was filtered through filter paper. The white precipitate was dried under reduced pressure to obtain the pure product (DA1, DA2, DA3, and DA4) as a white solid in yields of 70-73 %.

Synthesis of N1,N14-dihexyltetradeca-6,8-diynediamide (DA1): 1H NMR (CDCl3, 600 MHz), δ(ppm): 0.82 (t, 6H, J = 7.2 Hz), 1.23 (m, 12H), 1.49 (m, 8H), 1.69 (m, 4H), 2.20 (m, 8H), 3.18 (m, 4H), 5.64 (bs, 2H). 13C NMR (CDCl3, 150 MHz), δ(ppm): 13.08, 17.99, 21.64, 23.89, 25.92, 26.80, 28.22, 28.48, 28.66, 30.83, 35.18, 38.56, 64.60, 171.45. ESI-MS (m/z): calcd. 416.3, found 417.3 (M + H).



Synthesis of N1,N14-diheptyltetradeca-6,8-diynediamide (DA2): 1H NMR (CDCl3, 600 MHz), δ(ppm): 0.81 (t, 6H, J = 7.2 Hz), 1.23 (m, 16H), 1.42 (m, 4H), 1.48 (m, 4H), 1.67 (m, 4H), 2.12 (m, 4H), 2.21 (m, 4H), 3.17 (m, 4H), 5.59 (bs, 2H). 13C NMR (CDCl3, 150 MHz), δ(ppm): 13.04, 17.99, 21.57, 23.92, 25.88, 26.81, 27.95, 28.63, 30.72, 35.13, 38.59, 64.65, 171.57. ESI-MS (m/z): calcd. 444.3, found 445.2 (M + H).

Synthesis of N1,N14-dioctyltetradeca-6,8-diynediamide (DA3): 1H NMR (CDCl3, 600 MHz), δ(ppm): 0.81 (t, 6H, J = 6.6 Hz), 1.22 (m, 20H), 1.42 (m, 4H), 1.49 (m, 4H), 1.67 (m, 4H), 2.11 (m, 4H), 2.21 (m, 4H), 3.17 (m, 4H), 5.43 (bs, 2H). 13C NMR (CDCl3, 150 MHz), δ(ppm): 13.06, 17.99, 21.62, 23.91, 25.92, 26.81, 28.18, 28.66, 30.78, 35.17, 38.55, 64.63, 171.37. ESI-MS (m/z): calcd. 472.4, found 473.2 (M + H).

Synthesis of N1,N14-dinonyltetradeca-6,8-diynediamide (DA4): 1H NMR (CDCl3, 600 MHz), δ(ppm): 0.81 (t, 6H, J = 7.2 Hz), 1.22 (m, 24H), 1.42 (m, 4H), 1.48 (m, 4H), 1.66 (m, 4H), 2.10 (m, 4H), 2.20 (m, 4H), 3.17 (m, 4H), 5.43 (bs, 2H). 13C NMR (CDCl3, 150 MHz), δ(ppm): 13.08, 17.99, 21.64, 23.89, 25.92, 26.80, 28.22, 28.48, 28.66, 30.83, 35.18, 38.56, 64.60, 171.45. ESIMS (m/z): calcd. 500.4, found 501.2 (M + H).

2.4 Preparation of PDA-Embedded PEO Films: A solution of diacetylene monomer (2.5 wt%) and PEO powder (12.5 wt%) in 20 mL of CHCl3 was stirred at RT in the dark until a clear homogeneous solution was obtained. This


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homogeneous solution was transferred into a glass petri dish and cast at 10° C for 12 h. UV light was irradiated onto the dried film, and a blue film was obtained and then cut to the proper size. 2.5 Preparation of 3. Result and Discussions: Generally, DA monomers have two types of alkyl chains, namely, the inner alkyl chain, which acts as the linker between the diacetylene and functional groups such as amides and the peripheral alkyl chain. To study the pronounced odd/even effect of the peripheral alkyl chain, we synthesized a series of diacetylenes (DA1-DA4) with different peripheral alkyl chain lengths (Scheme 1).

3.1 Odd/Even effect on polymerization of diacetylene monomers: The prepared diacetylene monomers DA1–DA4 were polymerized under shorter UVlight irradiation for 10 min. PDA2 and PDA4 were obtained by topochemical polymerization of DA2 and DA4 respectively, which possess an odd-numbered peripheral alkyl chain. The DA1 and DA3 monomers, which have even-numbered peripheral alkyl chains (Scheme 2), could not be polymerized in the solid state even when irradiated for a long time. To obtain better insight into the odd/even effect of the peripheral alkyl chain on topochemical polymerization, we determined the optimized geometry of the dimers of DA3 and DA4 through ab initio density functional theory (DFT) calculations as implemented in the Gaussian 09 package using the HF exchange functional with the 3-21g basis sets.25 The optimal packing geometry for DA3 and DA4 is depicted in Scheme 2. For DA4 (n= odd number), the internuclear distance between the two acetylenic carbons is 4.22 Å. This value approximately matches the requirement for



topochemical polymerization, and therefore, monomers with odd-numbered peripheral alkyl chains readily undergo topochemical polymerization.25 By contrast, for DA3 (n=even number), the internuclear distance between the diyne carbons is 5.29 Å (scheme 2). DA3 was not polymerized, which may be because the intermolecular distance between the two monomer units of DA3 is greater than the 4.9 Å separation essential for topochemical polymerization.25,26 This result reveals that polymerization of diacetylenes also depends on the length of the alkyl chain on the peripheral position of the DA monomers. Because of the absence of polymerization, we did not study DA1 and DA3 in further thermochromic investigations. Raman spectroscopy was employed to confirm the formation of PDA2 and PDA4. As depicted in Figure 1, for DA2 and DA4, the C≡C stretching band appeared at 2258 and 2252 cm-1, respectively. For PDA2 and PDA4, the C≡C stretching band was shifted to 2079 and 2074 cm-1, respectively, and another new peak appeared at approximately 1450 cm−1 (C=C) due to the existence of the yne-ene structure of the blue-colored PDAs.27

3.2 Thermochromic property of polymer in solid state To examine the solid-state thermochromic property of the prepared polydiacetylenes, PDA2 and PDA4 were gradually heated from 20 °C to 100 °C. When the temperature increased to 90 °C, PDA2 showed a drastic color change from blue to intense red, and upon subsequent recooling to room temperature, the blue color reappeared for the solid-state PDA. A temperaturedependent UV-Vis absorption spectroscopic study was performed to understand the thermochromic behavior of PDAs in solid state. PDA2 exhibits high reversibility, and therefore,


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to monitor the thermochromic property of PDA2, the blue solid was heated to 90° C for 10 min and was kept on the sample holder with UV-Vis spectra recorded each 15 seconds. Figure 2 shows the UV-Vis absorbance spectrum of PDA2 on different temperature. For a warm red solid sample, the absorption peak appeared at 553 nm. This broad absorption peak shifted to a longer wavelength at 634 nm with gradual diminishing of the peak at 553 nm during the cooling process. The appearance of the broad absorption peak at 634 nm is mainly due to the existence of an extended π-conjugation system of the blue phase PDA2. The broad peak at 553 nm is caused by the twisting of the polymer backbone and the changes in the degree of conjugation in the system.10 The appearance of a clear isosbestic point at 583 nm indicated that thermal equilibrium was attained between the blue phase and the red phase without forming any significant new intermediates. PDA4 shows the same UV-Vis absorption results as PDA2 (Figure S1 and S2). The thermochromic reversibility of PDA2 and PDA4 solids was investigated in up to 10 cycles of the heating (90 °C) and cooling (20 °C) process. This result reveals that PDA2 and PDA4 exhibit excellent thermochromic reversibility and stability in solid state (Figure S3).

3.3 Single component thermochromic ink from DA. The majority of DA functional materials described to date have been successfully applied to paper substrates by converting them to a vesicle solution. Thus, very few studies have reported the use of PDAs as inks in inkjet printers.28,29 Inkjet devices need a specific ink form which were made of vesicle solutions of the corresponding monomers and surfactants/co-surfactants.28 Even after the PDA vesicles are successfully obtained, the realization of water compatibility and their use in a printing application are still challenging. Therefore, the preparation of three component



(DA monomer, surfactant and co-surfactants) inks requires the use of a tedious procedure. To address this problem, we specifically designed monomers that possess three units in a single DA monomer: (i) a diacetylene unit to induce topochemical polymerization, (ii) a bis-amide for enhanced self-assembly of the polymer backbone, and (iii) peripheral alkyl chain for increased ethanol compatibility to make the material ideally suited for viscous ink formation (Figure 3). In this investigation, the suitability of the single component ink for a pen was explored using a fountain pen. The single component ink solution, containing 2.5 wt% of DA2 in ethanol, was loaded in the fountain pen. It was then used to write on unmodified A4 sized paper. After writing, the paper was dried at RT for 10 min to apply strain to DA2 on the paper. As shown in Figure 3, there are no observable letters on the paper, which is mainly due to the stability of the monomer species on the paper. After UV irradiation, blue-colored letters appeared on the paper due to the topochemical polymerization of the DA2 monomers. This result reveals that the attached diacetylene monomer meet the requirement for topochemical polymerization on paper. Furthermore, the blue letters on paper changed to red when heated at 60° C for a minute. This indicates that the thermochromic behavior of PDA2 still present even on paper and that this DA2 ink solution will be highly suitable for use as anti-counterfeiting materials. As shown in video S1, the prepared DA2 monomer solution was utilized as an efficient single component ink for fountain pen which could involve the sequential process such as writing, topochemical polymerization and thermochromic behavior of PDA2 on a conventional paper (Video S1). This video clip demonstrates the efficiency of DA2 ink solution in fountain pen.

3.4 Thermochromic property of PDA1 in film state


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The explosion in the demand for thermochromic commercial products has motivated the effort to develop smart materials which could work in real field and are easily accessible.30-32 We have chosen four different polymer material for preparing polymer matrix such as polyethylene oxide (PEO), poly (methyl methacrylate) (PMMA), polystyrene (PS), and polyvinyl alcohol (PVA). Out of these four polymer matrix, PEO based polymer matrix tend to form a soft and flexible PDA2 and PDA4-embedded PEO films (see in SI). Thermoplastic PEO is an appropriate material for thin film fabrication due to its crystallinity and ease of making any molded shape.33 DA2/DA4 monomers and the PEO polymer matrix in chloroform were mixed with the suitable weight ratio as described in the experimental section. This homogeneous solution of diacetylenes and PEO polymer was involved in the casting process for 12 h at 10 °C and successively yielded transparent and flexible DA2- and DA4-embedded PEO thin films. The obtained transparent DA2- and DA4-embedded PEO films then underwent topochemical polymerization by irradiation with shorter- UV light, resulting in the appearance of blue color in the PDA2- and PDA4-embedded PEO flexible strips (Figure 4a). As depicted in Figure 4b, the noticeable color change from blue to red was observed in the film state in the same manner as that in the powder state. This result indicates that the PEO film does not affect the thermochromic behavior of PDA2 and PDA4. Thermochromic properties of the thin films were investigated by obtaining UV-Vis absorbance spectra at RT and 60 °C. The λmax of the UV-Vis absorption spectrum for the blue-colored film was at approximately 634 nm at RT. When the film was heated to 60 °C, the λmax shifted to 580 nm (Figures 5a and 5b), and consequently, the twisting of the PDA backbone produced a dark red-colored film. To further investigate the reversibility of PDA2- and PDA4embedded PEO films, the film was subjected to heating from 20 to 100 °C. At each temperature,



the film was incubated for 2 min, and the thermochromic property was investigated as shown in Figures 5c and 5d. The PEO film slowly lost the blue pattern with simultaneous enhancement in red color, and the red color was enriched at 60 °C. Upon the removal of the heat source, the red film quickly returned to the blue color. The same results were obtained for more than 10 cycles of the heating/cooling process (Figure S2), showing that the thermochromic reversibility of the film remains stable and indicating that the film is highly suitable for temperature monitoring. Obtaining a different color in the PDA system is another interesting research direction. As discussed in the experimental section, the achievement of green color in PDA systems has not be reported to date. In this work, we use the subtractive color mixing concept to achieve a dark green PDA. Disperse yellow-3 (DY) (0.25 wt%) was mixed with DA2 monomer (2.25 wt%) and PEO (12.5 wt%) mixture in 20 mL of CHCl3, and subsequently, the casting process was used to obtain a yellow strip DY-DA2 (Figure 6a). As depicted in Figure 6b, DY-DA2 was involve irradiation process for 5 min, the appear blue PDA2 was mixed with yellow dye (DY) to produce green color, thus in line with the subtractive color mixing phenomenon the solid yellow strip was converted into a green-colored flexible strip (DY-PDA2). The thermochromic property of DYPDA2 was investigated by measuring UV-Vis absorbance spectra at RT and 60 °C (Figure 7a). As depicted in figure 7a, the DY-PDA2 film showed two λmax at 410 nm and 635 nm, and the shorter wavelength absorbance at 410 nm occurs mainly due to the existence of the disperse yellow dye, while the longer wavelength absorbance at 635 nm results from the presence of blue PDA2. The appearance of two significant peaks at 410 and 635 nm clearly indicated that the hybridization of yellow and blue gave green color. Meanwhile, the λmax of the film was shifted to 538 nm at 60 °C, consequently producing a dark red film. The thermal reversibility of the DY-


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PDA2-embedded PEO film was evaluated using the same procedure as that used for the PDA2embedded PEO film. The PEO film slowly lost the green color with simultaneous enhancement of the red color by heating up to 100 °C. When the heat source was removed, the red film quickly returned to the green color (Figure 7b). This result reveals that the prepared diacetylene DA2 is well suited for achieving different colors by subtractive color mixing method. 4. Conclusion This work is the first direct demonstration of the odd/even effect of the length of the peripheral alkyl chain in diacetylene monomers on their polymerization, and it provides clear guidance for rational molecular design in the development of PDAs. The distance between two monomers (≈4.0 Å) that is favorable for facilitating topochemical polymerization depends on the number of methylene units present in the peripheral alkyl chain, as proven by DFT theoretical calculations. Writing on conventional paper has been achieved with DA2 ink solution by using a fountain pen. The excellent thermochromic reversibility of PDA2 and PDA4 in the thin-film state proved the suitability of the prepared PDAs for use as thermal indicators for a wide range of temperatures (20 to 100° C). Together with the gradual temperature-dependent color changes, these results indicate that these PDAs are highly suitable for the detection of minute temperature changes in any system. Finally, the subtractive color mixing concept was demonstrated to work well in PDA2 and was used to obtain a different color pattern on PDA materials.

Supporting Information Supporting Information contains 1H NMR, 13C NMR, Solid UV, and thermochromic reversibility cycle. This material is available free of charge via the Internet at http://pubs.acs.org.



Acknowledgments This study was supported by Korea Agency for Infrastructure Technology Advancement (Grant No. 17RDPPC13618801000000).


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Scheme 1. Synthesis of diacetylene monomers DA1-DA4.



Scheme 2. Odd/Even effect of peripheral alkyl chain on topochemical polymerization and optimized geometry of DA3 and DA4 dimers.


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Figure 1. Raman spectra of (a) DA2 (top) and PDA2 (bottom), (b) DA4 (top) and PDA4 (bottom).



0.06

0.05

Absorbance

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0.04

0.03

0.02

0.01

0.00 400

450

500

550

600

650

700

Wavelength Figure 2. UV-Vis absorption spectra of PDA2 at different temperature in solid state.


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Figure 3. Photograph of written letters on A4 size paper. Top: Schematic diagram of self assembly DA2 monomers on paper (left), after topochemical polymerization (middle) and thermochromic behavior of PDA2 on paper (right). Bottom: Written letters invisible in DA2 state (left), blue letters appears after polymerization (middle) and red letters appear by heating process (right).



Figure 4. (a) Schematic representation of preparation of PDA embedded PEO film (b) thermochromic reversibility of PDA2 and PDA4 imbedded PEO film.


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Figure 5. UV-Vis absorption spectra of (a) PDA2 (b) PDA4 imbedded PEO film at RT and 60° C. photograph of thermochromic behavior of (c) PDA2 (d) PDA4 imbedded PEO film at different temperatures (after 2 min incubation at each temperature).



Figure 6. (a) Illustration of polymerization of DY-DA2 embedded PEO film, (b) Schematic presentation for subtractive color mixing phenomenon of blue (PDA-2) and yellow dye, (c) thermochromic behavior of DY-PDA2


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(a)

(b)

Figure 7. (a) UV-Vis absorption spectra of DY-PDA2 embedded PEO film at RT (green spectra) and 60° C (red spectra), (b) Photograph of thermochromic behavior of DY-PDA2 embedded PEO film at 20 to 100° C.



TOC Graphic


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Tuning of the Topochemical Polymerization of Diacetylenes Based on

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