Construction of Fluorescent Conjugated Polytriazole Containing Double-Decker Silsesquioxane: Click Polymerization and Thermal Stability

This study synthesized two azide-functionalized monomers through p-dichloro xylene and double-decker silsesquioxane (DDSQ) units with NaN3 to form DB-N3 and DDSQ-N3 monomers, respectively. In addition, five different propargyl-functionalized monomers were also prepared from hydroquinone, bisphenol A, bis(4-hydroxyphenyl)methanone, 2,4-dihydroxybenzaldehyde (then reacted with hydrazine hydrate solution) and 1,2-bis(4-hydroxyphenyl)-1,2-diphenylethene with propargyl bromide to form P-B, P-BPA, P-CO, P-NP, and P-TPE monomers, respectively. As a result, various DDSQ-based main chain copolymers could be synthesized using Cu(I)-catalyzed click polymerization through DDSQ-N3 with different propargyl-functionalized monomers, of which the chemical structure and molecular weight could be confirmed by using Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC) analyses. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscope (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy analyses also could characterize the thermal stability, morphology, and optical behaviors of these DDSQ-based copolymers. All results indicate that the incorporation of an inorganic DDSQ cage could improve the thermal stability such as thermal decomposition temperature and char yield, because of the DDSQ dispersion homogeneously in the copolymer matrix, and this would then affect the optical properties of NP and TPE units in this work.


Synthesis of DDSQ-Based Main Chain Type of Copolymer through Click Reaction
A mixture of DDSQ-N 3 (0.309 g, 0.0002 mol) and CuBr (0.029 g, 0.08 mol) was stirred under a nitrogen atmosphere. After that, DMF (20 mL) was injected into the P-B (0.446 g, 0.0024 mol), P-BPA (0.730 g, 0.0024 mol), P-CO (0.696 g, 0.0024 mol), P-NP (0.836 g, 0.0024 mol), and P-TPE (1.057 g, 0.0024 mol). After using the freeze-thaw cycle method three times, PMDETA (0.0417 mL, 0.08 mol) was injected into the reaction. The mixture reacted under a nitrogen atmosphere and was heated under reflux at 70 • C for 48 h. Then, the solvent of DMF was removed by the reduced pressure distillation system. After removing DMF, the obtained copolymer in the THF solution was passed through an alumina column to remove the CuBr catalyst, and finally, the THF was removed by a rotary evaporator to obtain the desired product.

Synthesis of DB-N 3 and DDSQ-N 3
The synthesis scheme of DB-N 3 Figure 1e,f also indicate that the substitution changed the thermal properties of the azide unit for the DB-N 3 monomer. Furthermore, Figure 2c,d display 1 H and 13 C NMR spectra to confirm the complete substitution of DDSQ-Cl by the azide unit; the DDSQ-CH 2 connected to the Cl atom was slightly shifted to a higher field from 2.91 to 2.90 ppm based on 1 H NMR ( Figure 2c) and 28.65 to 28.63 ppm based on 13 C NMR (Figure 2d). The remaining DDSQ units were confirmed using the 29 Si NMR spectrum as shown in Figure 2e, where the peaks were observed near −21.45, −28.04, and −79.08 ppm, respectively attributed to the Si-CH 2 , Si-Phenyl, and Si-O-Si units in DDSQ-N 3 . Most importantly, the matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectra of the DDSQ-N 3 at 1272 g mol −1 for [DDSQ-N 3 + Na] + was observed, as shown in Figure 2f, and a good correlation existed between the experimental and calculated molecular mass, indicating the well-defined DDSQ structure. Taking into account all data from the FITR, NMR, and MALDI-TOF mass spectra analyses, we could confirm the successful synthesis of DB-N 3 and DDSQ-N 3 compounds.

Synthesis of P-B, P-BPA, and P-CO Monomers
Scheme S1 summarizes the synthesis of P-B, P-BPA, and P-CO monomers from hydroquinone, 4,4 -(propane-2,2-diyl)diphenol, and bis(4-hydroxyphenyl)methanone with propargyl bromide at 80 • C under N 2 atmosphere for 48 h. Figures S1-S3 show their corresponding FTIR, 1 H, and 13 C NMR spectra recorded at room temperature. The C≡C-H and C≡C stretching vibration at 3270 and 2130 cm −1 for the P-B monomer, 3278 and 2115 cm −1 for the P-BPA monomer, and 3270 and 2125 cm −1 for the P-CO monomer and the other characteristic absorption peaks including C=C and C=O units are summarized in Figures S1b, S2b, and S3b, respectively, based on FTIR analyses. 1 H NMR spectra display the peaks of O-CH 2 and C≡C-H at 4.65, and 2.50 ppm for the P-B monomer, 4.65 and 2.51 ppm for the P-BPA monomer, and 4.78 and 2.56 ppm for the P-CO monomer, and the other characteristic peaks, including aromatic protons and CH 3 protons, are also summarized in Figures S1c, S2c, and S3c. Furthermore, their corresponding 13 C NMR in Figures S1d, S2d, and S3d exhibited peaks of O-CH 2 and C≡C at 56.53, 79.42, and 75.46 ppm for P-B monomer, 55.90, 79.21, and 75.60 ppm for P-BPA monomer, and 56.10, 77.78, and 76.40 ppm for P-CO monomer. The other carbon peaks including C=O, aromatic carbon, and CH 3 were also summarized. All these results are able to confirm the successful synthesis of P-B, P-BPA, and P-CO monomers in this study.

Synthesis of P-TPE Monomer
The synthesis of P-TPE is summarized in Figure 4a. Here, 4-dydroxybenzophenone is reacted with Zn and TiCl 4 to form TPE-2OH based on the McMurry reaction. Similarly, the TPE-2OH reacted with propargyl bromide to form a P-TPE monomer. The OH and C=O stretching vibrations were seen at 3142 and 1637 cm −1 for the 4-dydroxybenzophenone compound, and both peaks disappeared and formed the ≡C-H and C≡C stretching vibration at 3289 and 2118 cm −1 for the P-TPE monomer based on FTIR analyses in Figure 4b. Figure 4c shows the 1 H NMR spectra of 4-dydroxybenzophenone at 7.65 ppm, and the other aromatic protons ranging from 7.13 to 6.56 ppm. The OH unit disappeared and formed the O-CH 2 and C≡C-H units at 4.61 and 3.47 ppm of P-TPE. In addition, 13 C NMR spectra of 4-dydroxybenzophenone and the P-TPE monomer are shown in Figure 4d. The C=O carbon at 197.29 ppm was observed and the aromatic carbon ranged from 162.27 to 115.88 ppm; the C=O carbon disappeared and the O-CH 2 and C≡C carbon were observed at 56.09, 78.91, and 75.77 ppm for the P-TPE monomer, also indicating the successful synthesis of the P-TPE monomer in this study.  13 C-NMR analyses of TPE-2OH and P-TPE. * is the CDCl 3 peak.

Synthesis of TPE-DB-Based Main Chain Type of Copolymer through Click Reaction
In this study, we first synthesized the TPE-DB copolymer through click reaction from P-TPE and DB-N 3 as the model reaction as shown in Figure 5a under an N 2 atmosphere at 70 • C for 48 h. The chemical structure of the P-TPE and DB-N 3 monomers has already been discussed in detail above, as shown in Figures 1 and 4, based on FTIR, 1 H, and 13 C NMR analyses. The total disappearance of the characteristic acetylene unit at 3142 cm −1 and 2118 cm −1 of P-TPE, and the azide unit of the DB-N 3 unit at 2101 cm −1 based on FTIR analyses in Figure 5b, suggest that both acetylene and azide units participated in the click reaction. In addition, Figure 5c shows their 1 H NMR spectra; we could also observe the appearance of a new signal at 8.04 ppm, corresponding to the proton of the triazole unit from the click reaction. Furthermore, the CH 2 unit from DB-N 3 was significantly down-field shifted from 4.35 to 4.61 ppm. The disappearance of the C≡C-H unit at 3.47 ppm from P-TPE and the corresponding 13 C NMR spectra are also summarized in Figure 5d; the CH 2 unit from DB-N 3 was significantly down-field shifted from 45.91 to 56.09 ppm, and there was a decrease in acetylene units, also indicating that the synthesis of the TPE-DB copolymer was successful.

Synthesis of TPE-DDSQ-Based Main Chain Type of Copolymer through Click Reaction
Similar to the TPE-DB copolymer, the synthesis of the TPE-DDSQ main chain type of copolymer with an inorganic DDSQ cage structure from P-TPE with DDSQ-N 3 by using click reaction is displayed in Figure 6a. The total disappearance of the characteristic acetylene unit at 3142 cm −1 and 2118 cm −1 of P-TPE and the azide unit of the DDSQ-N 3 unit at 2095 cm −1 based on FTIR analyses is shown in Figure 6b. The remaining Si-O-Si stretching absorption at 1095 cm −1 of the TPE-DDSQ copolymer suggests the successful synthesis of the TPE-DDSQ copolymer, and both acetylene and azide units participated in the click reaction. Furthermore, the appearance of a new signal at 8.01 ppm corresponds to the proton of the triazole unit from the click reaction based on 1 H NMR analysis in Figure 6c. The CH 2 unit from DDSQ-N 3 was slightly down-field shifted from 28.70 to 31.81 ppm, and the decrease in acetylene units based on 13 C NMR analyses in Figure 6d also implies that the synthesis of the TPE-DDSQ copolymer was successful.

Synthesis of Other DDSQ-Based Main Chain Types of Copolymer through Click Reaction
The synthesis of other DDSQ-based copolymers, such as B-DDSQ, BPA-DDSQ, CO-DDSQ, and NP-DDSQ organic/inorganic hybrids is summarized in Figure 7a. The disappearance of the characteristic acetylene unit at 2115-2130 cm −1 of propargyl-functionalized monomers and the azide unit of the DDSQ-N 3 unit at 2095 cm −1 based on FTIR analyses is shown in Figure 7b. The remaining Si-O-Si and Si-CH 3 stretching absorption at 1095 cm −1 and 1263 cm −1 of the other four DDSQ-based copolymers also suggested that both acetylene and azide units participated in the click reaction. In addition, the small signal at ca. 8.01 ppm is due to the protons of triazole units of these DDSQ-based copolymers from the click reaction based on 1 H NMR analysis in Figure 7c. Both CH 2 (a for O-CH 2 and b for N-CH 2 ) units were located at ca. 4.77-4.65 ppm and 3.75-3.74 ppm, respectively, and other aromatic protons also ranged from 7.85 to 6.81 ppm. Furthermore, the NP-DDSQ copolymer shows further CH=N and OH protons at 8.61 and 6.61 ppm, implying that the synthesis of the NP-DDSQ copolymer was successful. The corresponding 13 C NMR spectra are also summarized in Figure 7d. The C=O unit was observed at 195.44 ppm for the CO-DDSQ copolymer, and the C=N unit was located at 164.17 ppm for NP-POSS. Both CH 2 (a for O-CH 2 and b for N-CH 2 ) units were located at ca. 56.70-56.06 ppm and 31.03-30.29 ppm, respectively, and other aromatic carbon signals are also observed in Figure 7d. All results from FTIR and NMR analyses indicate that all DDSQ-based copolymers were successfully synthesized in this study. Table 1 and Table S1 summarize the molecular weights, PDI [by GPC and MALDI-TOF analyses, Figures S11-S15], and thermal properties of all DDSQ copolymers synthesized in this study.

Thermal Property and Morphology Analyses of DDSQ-Based Copolymers
The thermal stability of these DDSQ-based copolymers under an N 2 atmosphere was measured by TGA analyses, as shown in Figure 8. Figure 8a shows TGA analyses of P-B and DDSQ-N 3 monomers, and the corresponding B-DDSQ copolymer after the click reaction. The P-B monomer exhibited very low thermal stability with T d10 = 180 • C, char yield = 1.1 wt%; however, the DDSQ-N 3 monomer displayed relatively higher thermal stability with T d10 = 383 • C, char yield = 57.5 wt% since the inorganic DDSQ cage in the DDSQ-N 3 monomer could improve the thermal resistance behavior. After the click reaction to form a B-DDSQ copolymer, it also exhibited high thermal stability with T d10 = 206 • C, which is higher than the P-B monomer and the char yield = 53.1 wt%, which is close to the char yield of the DDSQ-N 3 monomer. The difference in the T d value might be due to the effect of creating the hybrid property in the organic P-B monomer. In hybrid materials, the thermal motion is restricted, reducing the organic material's decomposition pathways. The inorganic DDSQ would provide the additional heat capacity and stabilize the higher thermal decomposition. In addition, the char yield of the B-DDSQ copolymer would be enhanced after the click reaction because the covalent bond of the DDSQ cage with the P-B monomer also restricts the thermal motion of this hybrid material, indicating that the thermal stability of organic materials could improve through the click reaction and the inorganic silsesquioxane [1,3,22]. All TGA analyses of other DDSQ-based copolymers are also summarized in Figure 8b, which all display high char yields of more than 50 wt%, and the NP-POSS displays the highest char yield with 65.1 wt%. This phenomenon may be due to the strong intramolecular hydrogen bonding of the OH-N units of the P-NP monomer compared with other propargyl-functionalized monomers, as expected. The inorganic DDSQ cage dispersion into the main chain of copolymers was investigated by TEM and SEM analyses, as shown in Figure 9. No strong aggregation and featureless morphology without phase separation suggested that the inorganic DDSQ cages were dispersed well into the DDSQ-based copolymers due to the covalent bond of the DDSQ cage in these main chain types of DDSQ-based copolymers. Furthermore, the C-, N-, O-, and Si-mapping from SEM images suggest that the DDSQ was uniformly dispersed on the copolymer surfaces. The white points correspond to the DDSQ-rich domain, also confirmed by the TEM image. Each element of composition is also summarized in Figure 9 for all DDSQ-based copolymers. The homogeneous dispersions of inorganic DDSQ cages in the copolymer could decrease the thermal motions and improve the thermal stability, which is consistent with TGA analyses.

Photoluminescence Property of NP-DDSQ and TPE-DDSQ Copolymers
In this study, we choose NP-DDSQ and TPE-DDSQ copolymers to investigate the PL emission properties in solution and solid state since these two units of DP and TPE have photoluminescence properties [48][49][50][51][52][53][54][55][56][57][58]. Firstly, we measured the PL emission of NP, P-TPE, NP-DDSQ, and TPE-DDSQ in THF concentration (10 −6 M), which exhibited PL peaks centered at 530, 480, 540, and 490 nm for NP, P-TPE, NP-DDSQ, and TPE-DDSQ, respectively [ Figure 10a]. In addition, the PL analyses in the solid state [ Figure 10b] exhibited emission bands at 510, 541, 540, and 490 nm, respectively, for NP, P-TPE, NP-DDSQ, and TPE-DDSQ. Furthermore, under a UV lamp with wavelength excitation (365 nm), both NP and NP-DDSQ copolymers emitted a green color, and P-TPE and TPE-DDSQ emitted a cyan color, as shown in Figure S16. Interestingly, the NP-DDSQ copolymer shows a strong emission peak at 540 nm due to the intramolecular hydrogen bonding of OH-N and aromatic ring units and excited-state intramolecular proton transfer (ESIPT) character [50][51][52]59]. The fluorescence spectra of two materials, namely NP-DDSQ and TPE-DDSQ copolymers, which were measured at the particular excitation wavelength of 360 nm are shown in Figure S17. It is to be noted that when these materials were prepared at different concentrations in organic solvents such as THF, they showed different emission centers. As a result, we observed that the NP-DDSQ copolymer showed strong emission at around 525 nm for the concentration 10 −5 M and TPE-DDSQ copolymer showed a strong emission center at around 425 nm for the concentration 10 −5 M. The fluorescence intensity of both the NP-DDSQ and TPE-DDSQ copolymers increased with increasing water contents as shown in Figure S18. Additionally, to support these phenomena, we also investigated the fluorescence lifetime of the corresponding samples NP-DDSQ, and TPE-DDSQ, and the values were found to be 1.468 ns and 0.885 ns, respectively (Figures S19 and S20).

Conclusions
This study showed various successfully synthesized DDSQ-based copolymers based on click reaction through DDSQ-N 3 monomer with different propargyl-functionalized monomers. FTIR, NMR, and GPC analyses were characterized by their chemical structure and molecular weight. The results indicate that the incorporation of the inorganic DDSQ cage of these copolymers could improve their thermal stability properties, such as thermal decomposition temperature and char yield, because of the homogeneity of DDSQ dispersion in the copolymer matrix, and could also affect the optical properties of the NP and TPE units in this work. In addition, both the NP-DDSQ and TPE-DDSQ copolymers could be considered good materials for metal sensing and biomedical applications.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.