Benzoyl chloride (99%), N-boc-ethylenediamine (96%), di-tert-butyl dicarbonate (99%), 1-ethyl-3-(3-dimethyl- aminopropyl)carbodiimide hydrochloride (EDC, 99%), 3,6-dioxaoctamethylenediamine (98%), 4-hydroxybenzoic acid (99%) and 4-aminobenzoic acid (99%) were purchased from Energy Chemical. 10,12-Pentacosadiynoic acid (PCDA, 98%) was purchased from Alfa Aesar. 4-Amino-1-butanol (98%) and 6-amino-1-hexanol were purchased from Aladdin (Shanghai) Chemical Reagent Co., Ltd. Benzoic acid, benzoic anhydride (98%), sodium sulfate, hydrochloric acid, sodium chloride, sodium hydroxide, ethyl acetate, triethylamine (Et₃N), dichloromethane (DCM), chloroform, ethylene glycol, triethylene glycol, methanol, trifluoroacetic acid (TFA), citric acid, 4-dimethylaminopyridine (DMAP), dimethyl sulfoxide (DMSO), N,N’-dimethylformamide (DMF), 1-hydroxybenzotriazole (HOBt), benzophenone, oxalyl chloride and tetrahydrofuran (THF) were purchased from Sinopharma Chemical Reagent Co., Ltd. Chloroform and THF were dried using calcium hydride and sodium respectively before use. Other reagents were used as received. Deionized water was used for all experiments.

Three series of m-PCDA monomers were synthesized by esterification or amidation reactions as shown in Fig. 1, using PCDA (labeled as PCDA-0 in this paper) as the starting reactant. The detailed synthesis processes and characterizations including ¹H-NMR, ¹³C-NMR and HRMS for all m-PCDA monomers are described in the electronic supplementary information (ESI).

Fig 1  The syntheses of three series of m-PCDA monomers (PCDA-1−PCDA-9) derived from 10,12-pentacosadiynoic acid (PCDA-0).

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The as-synthesized diacetylene monomer was dissolved in chloroform at a concentration of 2 mg·mL⁻¹. A filter paper (3 cm × 3 cm) was immersed in the solution for 10 s, then dried naturally in laboratory hood. After the immersion-dry process of the filter paper was repeated three times, the dried filter paper was exposed under 254-nm UV light for 3 min at 25 °C, using a UV lamp with a power of 6 W. The products after the light exposure were labeled according to the corresponding monomers, for example, the product of PCDA-1 after UV irradiation was labeled PDA-1.

¹H-NMR and ¹³C-NMR spectra were recorded on a Bruker NMR spectrometer (Avance, 400 MHz) at 25 °C. High resolution mass spectra were obtained on Autoflex Speed TOF mass spectrometer. The morphologies of the samples were observed by scanning electron microscope (SU8220, 3.0 kV). The samples were sprayed gold for 80 s before observation. The differential scanning calorimetry (DSC) curves were obtained on DSC Q2000. The samples were directly heated from the room temperature to 150 °C at a rate of 10 °C·min⁻¹ under the flow of N₂, and the heating curves were recorded correspondingly. Raman spectra were recorded on a LabRamHR Evolution Raman spectrometer with a resolution of 0.35 cm⁻¹ (785 nm laser source).

Thermochromic behavior of the PDA sample was observed and recorded according to the following process. The filter paper loaded with PDA product was placed on Linkam CSS-450-CRY0 hot stage, and then heated to the pre-set temperature at a heating rate of 2 °C·min⁻¹. The color change of the sample on the filter paper was captured in real-time using a digital camera.

The method of B3LYP-D3(BJ) with 6-311G(d,p) basis set in DFT was used to optimize the geometric configurations of the bimolecular aggregates, which was carried out using Gaussian 16 C.01 software package^[

PCDA-0 is a well-known diacetylene monomer that contains only one carboxyl group at one end in addition to the straight carbon chain. It usually appears in a white crystalline state (Fig. S67 in ESI) with a melting point of 62−63 °C, and can be polymerized rapidly under the irradiation of 254-nm UV light to produce the blue PDA-0, as shown in Fig. 2(a). The Raman spectra of PCDA-0 and PDA-0 are shown in Fig. 3. It is seen that the characteristic absorption band at 2256 cm⁻¹ of ―C≡C―C≡C― group in PCDA-0 disappears on the spectrum of PDA-0, accompanied by the appearance of two new bands at 1448 and 2095 cm⁻¹, which are assigned to ―C=C― and ―C≡C― groups respectively, indicating PCDA-0 molecules are polymerized induced by the UV light and form the conjugated ene-yne polymer molecular chains.^[

Fig 2  (a) Digital photos of PCDA-0−PCDA-6 before and after 254-nm UV irradiation (insets are SEM images of the samples. PCDA-5 and PCDA-6 are colorless liquids); (b) Optimized geometries of bimolecular aggregates of PCDA-0−PCDA-4 obtained by DFT calculations.

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In order to investigate the effect of the hydrogen bond interaction on the solid phase polymerization behavior of the diacetylene monomers, five new monomers (PCDA-2−PCDA-6) with specific structures were synthesized. Among them, PCDA-2−PCDA-4 samples contain an end benzene ring and one amide group with different distance from diacetylene unit, while PCDA-5 and PCDA-6 samples contain benzoate end groups with different chain lengths, which means that there is no hydrogen bond interaction between these two monomer molecules compared with PCDA-0−PCDA-4 molecules. As can be seen from Fig. 2(a), PCDA-2−PCDA-4 are all white solids crystalline powder (Figs. S69−S71 in ESI) and quickly turn blue under 254-nm UV irradiation. The Raman spectra of PCDA-2−PCDA-4 before and after the UV irradiation are also displayed in Fig. 3, and their changes are almost identical with those of PCDA-1. The r_1,4s of PCDA-2−PCDA-4 obtained by DFT calculations are 3.47, 3.87 and 3.82 Å, respectively (Fig. 2b), which theoretically meet the space conditions for the solid phase polymerization. However, PCDA-5 and PCDA-6 are liquid at room temperature and do not show any changes under the same UV irradiation condition. This result shows that even if there exists π-π interaction originated from the benzene ring, but because there is no hydrogen bond interaction, such diacetylene monomer molecules have too weak intermolecular interaction to form crystallization at room temperature. The DSC curves of PCDA-5 and PCDA-6 in Fig. S87 (in ESI) prove that the melting temperatures of them are both lower than 25 °C, and the r_1,4s of PCDA-5 and PCDA-6 obtained by DFT calculations are 6.15 and 5.56 Å, respectively (Fig. S75 in ESI), which theoretically do not meet the space conditions for the solid phase polymerization.

Fig 3  Raman spectra of PCDA-0−PCDA-4 (a) before and (b) after 254-nm UV irradiation.

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To further investigate the impact of functional groups capable of forming intermolecular hydrogen bonding interactions on the polymerization properties of monomers and the thermochromic characteristics of the corresponding polydiacetylenes, three new monomers (PCDA-7−PCDA-9) containing two functional groups that can form intermolecular hydrogen bond were synthesized. As seen in Fig. 4, regardless of the distance between the two functional groups (amide or carboxyl groups), PCDA-7, PCDA-8, and PCDA-9 are all white solids crystalline powders (Figs. S72−S74 in ESI) at room temperature, and can be rapidly polymerized under the irradiation of 254-nm UV light. There is no discernible difference in the morphologies of the three monomers before and after UV irradiation observed by SEM (insets of Fig. 4a). But the Raman spectra of PCDA-7−PCDA-9 before and after the UV irradiation displayed in Fig. 5 prove the formation of the corresponding PDA, just like the changes of PCDA-1−PCDA-4. Moreover, the r_1,4s in the optimized conformation of the bimolecular aggregates of PCDA-7−PCDA-9 calculated by DFT method are 3.66, 3.45 and 3.47 Å, respectively, which all theoretically meet the space conditions for the solid phase polymerization. The above results indicate that functional groups that can form intermolecular hydrogen bond interaction must be introduced in the design of the molecular structure of diacetylene monomer, which is the most critical factor to promote the monomer to be in the crystalline phase at room temperature, and then carry out solid phase polymerization to obtain new PDA materials.

Fig 4  (a) Digital photos of PCDA-7−PCDA-9 before and after 254-nm UV irradiation (insets are SEM images of the samples); (b) Optimized geometries of bimolecular aggregates of PCDA-7−PCDA-9 obtained by DFT calculations.

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Fig 5  Raman spectra of PCDA-7−PCDA-9 before (a) and after (b) 254 nm UV irradiation.

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Before the discussion on the thermochromic properties of the prepared m-PDAs, it is important to note that the prepared m-PCDA monomers will not be polymerized during the heating process, so as to ensure that the thermochromic behavior of the m-PDAs is caused only by the structural change of the polymer, rather than the thermally-induced polymerization of the unpolymerized monomers. As seen in Fig. 6, PCDA-0−PCDA-4 and PCDA-7−PCDA-9 monomers have no color change even they were heated to 100 °C, and are polymerized only under UV irradiation.

Fig 6  Digital photos of (a) PCDA-0−PCDA-4 and (b) PCDA-7−PCDA-9 at different temperatures and after UV irradiation.

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Fig. 7 shows the thermochromic behavior of PDA-0 and PDA-1. For PDA-0, when it was heated to higher than 60 °C, its color gradually changed to red. After the temperature exceeded 65 °C, when it was cooled, the color was still red and no longer changed, as evidenced by the UV-Vis spectra (Fig. S76 in ESI) of PDA-0 at different temperatures. However, for PDA-1, it needed to be heated to close to 75 °C before it began to gradually turn red, and it had not completely transformed into red until 100 °C (see Fig. S77 in ESI). Interestingly, the red PDA-1 sample would turn blue again as it cooled. As shown in Fig. 6, even when heated to 100 °C, the monomers cannot be polymerized. It is obvious that the above thermochromism of PDA-0 and PDA-1 can only be caused by the changes in the microstructure of the polymer. As can be seen from the DSC curves of PDA-0, PDA-1 and their corresponding solid monomers in Fig. 7, both PDA-0 and PDA-1 are crystalline and have similar melting behavior with their corresponding monomer, which conforms to the characteristics of solid topological polymerization and basically maintains the aggregation structure of the corresponding monomers. However, due to the change of chain structure after polymerization, the aggregation state structure level of the polymer increases, and the melting range and melting peak of the polymer increase compared with the monomer.

Fig 7  Digital photos of (a) PDA-0 and (b) PDA-1 at different temperatures; DSC curves of (c) PCDA-0 and PDA-0, (d) PCDA-1 and PDA-1.

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PDA-0 and PDA-1 have different melting behaviors. PDA-0 has a sharp melting peak. The onset melting temperature is 57.5 °C, which is consistent with the observed initial color-change temperature of PDA-0. At the end melting temperature, i.e., 71.9 °C, PDA-0 also turns completely red. The DSC results of PCDA-0 (Fig. 7c) show that the melting temperature range of PDA-0 is different from that of PCDA-0, which means the color-change temperature of PDA-0 did not correspond to the monomer. Compared with PCDA-1, PDA-1 has more complex multiple melting behavior and higher melting points (Fig. 7d). Each melting peak is very wide, which shows that there are a variety of different crystal regions inside PDA-1. Its onset melting temperature is as high as 75.1 °C, which is also consistent with the observed initial color-change temperature of PDA-1. At the same time, its final melting temperature exceeds 125 °C. It is noted that unlike PDA-0, PDA-1 can be observed to turn red between 75 and 100 °C, and immediately return to blue after cooling. This reversible thermochromism of PDA-1 can be repeated as long as it is heated to no more than 100 °C.

As for the color-changing mechanism of PDA materials, most current studies tend to think that it is caused by the decrease of effective conjugate chain length under various types of environmental stimulation.^[

On the other hand, PDA-0 has a very narrow melting range. By heating with a hot station, it almost melted completely at 65 °C. At this time, the polymer chains are free from the primary crystal lattice, and have enough activity to adjust to the new chain conformations. When the temperature drops below the melting point, the new polymer chain conformations are frozen, so the color no longer changes. However, PDA-1 has a very wide melting range (75−125 °C), and when it is heated to 75−100 °C, it does not melt completely. At this time, the polymer chains have a certain activity, but they have not yet broken away from the crystal lattice. So although in this temperature range, the polymer chain conformations have changed to a certain extent, resulting in the sample slowly turning red. While when it is cooled, the polymer chain conformation may return to the previous state due to the restriction of the crystal lattice, and the color of the sample will also change back to blue.

Fig. 8 exhibits the thermochromic behavior and the melting curves of PDA-2−PDA-4 that all contain one benzene ring and one amide group. The melting curves of the corresponding monomers (PCDA-2−PCDA-4) are shown in Figs. S84−S86 (in ESI), respectively, which confirms once again that topologically polymerized PDAs have more hierarchical crystal structures similar to their monomers, i.e., similar melting point, but wider melting range. By comparing the melting and the thermochromic behavior of PDA-2−PDA-4, it is also shown that the initial color-changing temperatures of PDAs are very close to the onset melting temperature. When they were heated to the temperature lower than the melting point (the temperature corresponding to the melting peak on the DSC curve), their color can be observed to return to blue after cooling. However, once the temperature exceeds their melting point, the samples remain red after cooling, and can no longer be reversible thermochromic (see Fig. S78−S80 in ESI).

Fig 8  Digital photos of (a) PDA-2, (b) PDA-3 and (c) PDA-4 at different temperatures; DSC curves of (d) PDA-2, (e) PDA-3 and (f) PDA-4.

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Fig. 9 and Figs. S88−S90 (in ESI) exhibit the thermochromic behavior and the melting curves of PDA-7−PDA-9 and their corresponding monomers (PCDA-7−PCDA-9) which contain one benzene ring and two groups that can form hydrogen bonds. Compare with PDA-2−PDA-4, PDA-7−PDA-9 have stronger intermolecular interaction due to more than one group that can form hydrogen bonds. Their onset melting temperatures are all as high as more than 60 °C. Moreover, their melting ranges are also widened, and even multiple melting behavior appears for PDA-7−PDA-9, and their end melting temperatures of the first melting peaks are above 90 °C. The end melting temperatures of PDA-7 and PDA-9 are higher than 120 °C. Similar to the reversible thermochromic behavior of PDA-2−PDA-4, the initial color-changing temperatures of PDA-7−PDA-9 are close to their onset melting temperature. When they were heated to the temperature lower than the maximum melting point, their color can be observed to return to blue after cooling. However, once the temperature exceeds their maximum melting point, the samples will remain red after cooling. This was further supported by the UV-Vis spectra of PDA-7−PDA-9 at different temperatures (see Figs. S81−S83 in ESI).

Fig 9  Digital photos of (a) PDA-7, (b) PDA-8 and (c) PDA-9 at different temperatures; DSC curves of (d) PDA-7, (e) PDA-8 and (f) PDA-9.

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To better understand the structure-properties relationship between the thermochromic properties of m-PDA and the structure of the monomers, we have summarized a diagram as shown in Fig. S91 (in ESI). From the discussion above, it can be concluded that the appropriate introduction of benzene ring into the molecular structure of linear diacetylene monomer can change the melting point of the obtained PDA, and broaden its melting range so as to obtain reversible thermochromic PDA materials that can be used in different temperature ranges.