Triazine and Porphyrin-Based Cross-Linked Conjugated Polymers: Protonation-Assisted Dissolution and Thermoelectric Properties

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Nov 2021Triazine and Porphyrin-Based Cross-Linked Conjugated Polymers: Protonation-Assisted Dissolution and Thermoelectric Properties Ling-Ling Wang†, Qing-Lin Jiang†, Duo-Kai Zhao, Qing-Lei Zhang, Yan-Hua Jia, Cheng Gu, De-Hua Hu and Yu-Guang Ma Ling-Ling Wang† Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 †L.-L. Wang and Q.-L. Jiang contributed equally to this work.Google Scholar More articles by this author , Qing-Lin Jiang† Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 †L.-L. Wang and Q.-L. Jiang contributed equally to this work.Google Scholar More articles by this author , Duo-Kai Zhao Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Qing-Lei Zhang Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Yan-Hua Jia Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Cheng Gu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , De-Hua Hu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author and Yu-Guang Ma *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000528 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail As two pivotal functional segments, triazine and porphyrin can be coupled to form a highly cross-linked conjugated polymer. Although the obtained conjugated polymers are almost insoluble in most solvents, a small amount of protonic acid can cause the formation of a colloidal solution of polymers. The dissolution process has proved to be a surface charge effect induced by protonation of porphyrin units in the polymer framework. High-quality conjugated polymer films have been prepared on diverse substrates by solution drop-casting and then used for thermoelectric (TE) applications. The films exhibit remarkable TE performance with high Seebeck coefficients (−6650 μV K−1) and electronic conductivities (0.042 S cm−1), yielding a power factor of 185 μW m−1 K−2, which, collectively, are the highest among all reported n-type organic TE materials. This work may pave the way for the design and development of solution-processed, cross-linked polymer films as promising TE materials. Download figure Download PowerPoint Introduction Cross-linked conjugated polymers are a unique class of polymers that are inherently endowed with extended π-conjugated skeletons1–4 and display outstanding electrical and optical properties in luminescence,5,6 energy storage,7,8 and catalysis.9 As a special class of conjugated cross-linked polymers, donor–acceptor polymers provide strong electron coupling between the electron donor and acceptor on the organic skeleton framework. Porphyrins, as common donor molecules, are well-known aromatic macrocyclic ligands that form neutral square-planar complexes, which participate in electronic exchange and play an important role in redox processes.10 In contrast, triazines, as acceptor molecules, are a class of nitrogen-containing heterocycles that exhibit an electron-withdrawing state. Through the superiority of donor–acceptor structures, porphyrins and triazines can be induced into cross-linked conjugated polymers capable of switching their electronic properties between electron-rich and electron-poor states.11–14 Inspired by these strategies, we introduced a porphyrin core and four cyano-peripherals into the synthesis of triazine and porphyrin-based cross-linked conjugated polymer (TPCP) through trimerization. The porphyrin has the ability to donate electrons to enhance the intramolecular charge transfer, while the triazine plays an important role in electron withdrawing to increase intermolecular interactions, which may yield excellent electrical conductivity in the TPCP. The strong electron affinity caused by the electron-withdrawing triazine-groups allows the TPCP to form relatively stable radical anions through chemical reduction.15 Nevertheless, such polymers are intrinsically cross-linked and insoluble, which impedes their film-forming ability and restricts their application in optoelectronic applications. A successful strategy is to introduce solubilizing substituents into the skeleton16–19; however, electronically inactive side chains can induce negative effects on intermolecular charge transfer or exciton migration because of the elongated intermolecular distance.20 The case of polyaniline dissolution induced by counter ions21,22 encouraged us to explore soluble cross-linked conjugated polymers through protonation. By protonating polyaniline with a properly functionalized protonic acid, the counterion can induce the processing ability of the resulting polyaniline complex. The use of similar methods may achieve solution processing of conjugated cross-linked polymers. Furthermore, considering the nature of cross-linked conjugated polymers, this approach may tune the electronic conductivity by regulating protonation. Herein, we report a highly soluble TPCP by introducing a small quantity of protonic acid into the porphyrin segments of the TPCP framework (Figure 1a). The experimental results demonstrate that the porphyrin segments can be reduced to a dianion state using excess Bronsted acid, resulting in well-dispersed charged polymers in solution. Therefore, the TPCP displays obvious solubility in various organic solvents to form colloidal solutions, allowing us to obtain high-quality TPCP films by drop-casting. Considering their high electrical conductivity, the TPCP films were explored for thermoelectric (TE) applications, and it was found that the TPCP films exhibit excellent TE properties when optimizing the degree of protonation. Figure 1 | (a) Schematic representation of protonated TPCP. (b) Photographs of the TPCP dissolved in various solvents. Upper: concentrated solutions under sunlight; lower: dilute solutions under 365 nm UV light showing the Tyndall effect. Download figure Download PowerPoint Experimental Methods Synthesis of TPCP Concisely, 2.0 mL of trifluoromethanesulfonic acid (TFSA) was trickled down to 300 mg of 5,10,15,20-tetraki(4-cyanophenyl)porphyrin (TPPCN) at −10 °C under nitrogen in ∼5 min. The resulting purple solution was stirred for 3 h and then left at 60 °C overnight. The crude product was then quenched in cold water and stirred in a NaOH solution for ∼2 h and washed with a NaOH solution for several hours to remove the redundant TFSA. The resulting powder was collected by centrifugation and subjected to Soxhlet extraction with NaOH solution, water, and methanol for 12 h for each solvent. After Soxhlet extraction, the obtained dark purple powder was dried in a 10 mL degassed Pyrex tube, which was then heated at 85 °C for 5 h and the yield was 92%. Preparation of TPCP solutions Briefly, the TPCP powder was added into various organic solvents and TFSA was dropped into the mixture. The mixture was then manually shaken and a TPCP solution was obtained. To obtain more homogenous solutions, the solutions underwent ultrasonication for 10 min. In addition, the solutions were filtered with a 0.2 μm syringe filter and stored for 24 h before use. TPCP film formation for thermoelectricity The preferable solubility of TPCP allowed us to prepare thin films in a solution-processed manner. The TPCP films were prepared by drop-casting the above solutions onto the desired substrate, followed by thermal evaporation of the solvents, which afforded continuous and high-quality films. We adopted the simplest means, for example, drop-casting the solutions onto the substrates. About 100 μL of the TPCP solutions was dropped onto the glass substrate, and the remaining solvent was evaporated at 150 °C for 4 h. This facile method enables us to choose various substrates, conductive or insulating ones, which is beneficial to the fabrication of TE devices with different configurations. TE measurements We adopted a two-dimensional geometric structure using an electrode configuration to test the Seebeck coefficient. The films were prepared with a length and width of 15 and 10 mm, respectively. In addition, two electrodes patterned by photolithography covered each side of the films (positions H and C) in the length direction. The films were fixed to a heat source and sink using a heat conducting paste for better performance. We chose position H as the heat side of the TPCP films and position C as the room side. Position H was heated to a high temperature (TH) by a standard hotplate while maintaining position C with a standard four-point probe technique. Impedance spectroscopy measurements of TPCP films The ionic conductivities of the TPCP films were measured using impedance spectroscopy (Metrohm Autolab B.V., Utrecht, The Netherlands). The devices for impedance spectroscopy were prepared on the basis of the film with prepatterned silver electrodes using a two-point probe approach and connected to the impedance spectrometer through their electrodes. The frequency was swept from 1 MHz to 0.1 Hz from 50% to 90% relative humidity (RH) at room temperature. Furthermore, the measurements were carried out inside a climate chamber. Results and Discussion The TPCP was synthesized from a TPPCN monomer through superacid-catalyzed trimerization of triazine under low-temperature reaction conditions. Specifically, 2.0 mL of TFSA was trickled down to 300 mg of TPPCN at −10 °C under nitrogen in ∼5 min. The purple solution was stirred for 3 h and then left at 60 °C overnight. The TPCP was obtained as a dark purple powder (yield of 92%), which was slightly soluble in polar solvents and insoluble in nonpolar solvents. Various analytical techniques were employed to demonstrate the structure of the TPCP powder ( Supporting Information Figures S1–S3 and Table S1). The formation of triazine rings inside the TPCP was indicated by Fourier transform infrared (FT-IR) spectroscopy. The characteristic peak at 1712 cm−1 was assigned to the stretching vibration of C=N in the triazine ring group, while the vibration of the triazine ring was at 1279 cm−1 ( Supporting Information Figure S1). In the 13C nuclear magnetic resonance spectra, the resonance signal at ∼172 ppm can be assigned to the sp2 carbon in the triazine ring ( Supporting Information Figure S2). These results confirm that the TPCP power was synthesized successfully through superacid catalysis trimerization. When a small amount of TFSA was added to the TPCP suspension, a clear TPCP solution in N,N-dimethylformamide (DMF) was obtained by manual shaking. The solubility of the TPCP in DMF was 11.6 mg mL−1 at a TFSA molar ratio of 3.8% (relative to the porphyrin segment in the TPCP). It was found that such solutions displayed clear Tyndall effects, illustrating the colloidal properties of the solutions (Figure 1b). The solutions were stable for several months under air conditions, and there was no separation of the sedimentation. We further confirmed that the solubility depends on the types of solvents and the amounts of TFSA ( Supporting Information Figure S4 and Table S2). At the TFSA molar ratio of 3.8%, the solubilities of the TPCP in 1,3-dimethyl-2-imidazolidinone and dimethyl sulfoxide were 7.4 and 4.6 mg mL−1, respectively. Therefore, the TPCP in DMF exhibited the highest solubility. We performed UV–vis spectroscopy to investigate the change in TPCP solubility with TFSA molar ratios of 3.8%, 8.1%, 12%, 16.2%, 20%, and 24.3%, respectively ( Supporting Information Figure S5 and Table S3). It can be seen that the solubility was further improved with increasing molar ratios of TFSA, showing that TFSA plays a crucial role in the solubility. To confirm the integrity of the TPCP structure under different protonation degrees, FT-IR measurements were conducted and there were no new characteristic peaks observed ( Supporting Information Figure S6), indicating that the TPCP structure had not undergone degradation and that it had retained its initial structure. Therefore, protonation-assisted dissolution can be attributed to the protonation of porphyrin segments in the TPCP23,24 ( Supporting Information Figure S7) and the electrostatic repulsion effect. To investigate the protonation process, the UV–vis spectra of the TPCP with the quantitative addition of TFSA were measured (Figure 2a). With increasing TFSA, the absorption peak at 420 nm decreases while the peak at 446 nm increases, consistent with the spectra variation tendency of the porphyrin monomer, in which the porphyrin skeletons were gradually charged with protons.20 Similarly, for the TPCP structure, the spectra indicate that the proton-exfoliated TPCP was obtained. The protonation process follows the principle of the lowest energy, which starts at the surface of the TPCP without solubility at low TFSA molar ratios. However, with increasing TFSA, the TPCP powder gradually becomes soluble, accompanied by protonation. When the TFSA molar ratio reaches 3.8% with a solubility of 11.6 mg mL−1, the TPCP begins dissolving into the solvents. The Dichtel group25 reported that after acid treatment, the reductions occurred at the surface due to disordered stacking of the protonated polymer sheets, which minimizes electrostatic repulsion. Therefore, we hypothesize that the protonation and solubility start at the surface and the soluble mechanism is due to the colloidal properties with surface-charged TPCP solutions (Figure 3).20 Figure 2 | (a) UV–vis spectral changes with the titration of TFSA into TPCP solution (left arrow: the decreasing of H2P; right arrow: the increasing of H4P2+). (b) UV–vis spectral changes at 446 and 420 nm. Download figure Download PowerPoint Moreover, we propose that the proton density of the TPCP surface at the TFSA molar ratio of 3.8% results in the lowest density of soluble properties. With the TFSA concentration further increasing, the proton density of the TPCP surface was enhanced, which represents a positive correlation between the protonation and solubility. When the proton density of the TPCP surface becomes saturated, the reductions occur inside of the TPCP. From Figure 2b, we conclude that for a TFSA molar ratio of 240%, the TPCP segments can be completely protonated. In conclusion, for the lower TFSA molar ratio of 3.8%, the reductions most likely occur at the TPCP surface. Figure 3 | Schematic diagram of thermoelectricity and illustration of three-dimensional spherical structure. Download figure Download PowerPoint It should be noted that the intensity of absorption reaches equilibrium with the addition of excess TFSA (Figure 2b), indicating that the porphyrin in the TPCP skeletons was completely protonated.23 As a result, in the case of a small addition of TFSA, the protonation degree of the TPCP is in proportion to the TFSA molar ratio, while it will achieve equilibrium under excess TFSA addition ( Supporting Information Figure S8 and Table S4). As shown in Supporting Information Figure S8, from the determination of the protonation constant plot, we could acquire the protonation degree at different quantities of protonic acid, for example, when the TFSA molar ratio is 3.8%, the protonation degree is ∼1.7%. Herein, we illustrate that among the six protonation degrees of TFSA, the TPCP solution with the lowest protonation degree of 1.7% presents excellent solubility performance, which demonstrates that the TPCP solution can be obtained with lower protonation, and it is not necessary to have full protonation to achieve solubility. The high solubility of the protonated TPCP allows us to prepare films through solution drop-casting and high-quality protonated TPCP films with a dark purple color can be obtained on diverse substrates ( Supporting Information Figures S9 and S10). Scanning electron microscopy images revealed continuous films and the continuity gradually decreased with increasing protonation degree ( Supporting Information Figures S11 and S12), which can be attributed to the surface charge distribution of the TPCP. With highly charged surfaces, the repulsion grew stronger and each TPCP particle moved farther. Protonated TPCP films with large extended π-conjugated skeletons and strong electron coupling may produce high electrical conductivity. First, we measured the electrical conductivity (0.008 S cm−1) of the TPCP film with protonation of 1.7% under 50% RH. It was found that the measured electrical conductivities were significantly enhanced with increasing protonation degree ( Supporting Information Figure S13). Furthermore, another important phenomenon was that the measured electrical conductivity increased significantly with increasing humidity, for example in the TPCP film with 7.2% protonation, the measured electrical conductivity could be improved from 0.004 S cm−1 at 40% RH to 0.8 S cm−1 at 90% RH ( Supporting Information Figure S14). It should be noted that such high electrical conductivity includes electronic conductivity (from the electron transfer attributed to the n-type TPCP) and ionic conductivity (from the movement of protons in the film). Impedance spectroscopy was used to investigate ion transport in the films with various protonation degrees at different humidities ( Supporting Information Figure S15). Based on the results, we calculated the electronic and ionic conductivity ( Supporting Information Figure S16 and Table S5). As shown in Supporting Information Figure S16, the ionic conductivity increased with increasing humidity at all protonation degrees, which is a typical characteristic of ionic conduction. For electronic conduction, when the TPCP film was at 50% RH, the electronic conductivity reached the highest value of 0.042 S cm−1 at 7.2% protonation (Figure 4a). Figure 4 | TE properties of different protonated TPCP films at 50% RH: (a) electronic conductivity; (b) Seebeck coefficient; (c) power factor; (d) proposed mechanism. Download figure Download PowerPoint Considering such high electrical conductivity compared with typical organic n-type films, we propose that the TPCP film is suitable for TE applications. Generally, the TE performance of a material is quantified by the dimensionless figure of merit ZT = (S2σκ−1)T and power factor PF = S2σ, where S, σ, T and κ are the Seebeck coefficient, electronic conductivity, absolute temperature, and thermal conductivity, respectively.26 Good TE materials should have high ZT, which accordingly requires a large Seebeck coefficient, a high electrical conductivity and a low thermal conductivity.27–30 The films and device for measuring the Seebeck coefficient are shown in Supporting Information Figure S17. The thin film geometry and electrode configuration also have significant effects on the S measurement results.31 To check the set-up in this work, we also measured the TE performance of PEDOT:PSS (PH1000), with an S value of 15.8 ± 1.21 μV K−1, in agreement with the literature.32 We measured the Seebeck coefficient of the film as a function of protonation. As shown in Figure 4b, the film exhibits a negative Seebeck coefficient, further indicating its n-type characteristics. With increasing protonation degree, the Seebeck coefficient experiences a continuously decreasing trend (from −7860 to −5970 μV K−1). Such a high Seebeck coefficient inevitably leads to suspicions of ionic effects; however, we have relatively strong evidence that this arises from the electronic Seebeck effect. First, the Seebeck effect of protons can only produce a positive Seebeck coefficient.33 Second, we also found that with increasing humidity, the Seebeck coefficient decreases ( Supporting Information Figure S18). It is clear that under higher humidity, the movement of protons is accelerated, and its positive Seebeck effect will partially offset the negative Seebeck effect of the electrons, leading to a reduction in the electron Seebeck effect (Figure 4d). This was proved by the decreasing Seebeck coefficient with increasing humidity. Finally, the stability of the output of the external circuit further proved that the large Seebeck effect came from electrons ( Supporting Information Figures S19 and S20). Therefore, when the TPCP films are used for TE application, the electrons and protons together provide the measured electrical conductivity, while electrons provide the measured Seebeck effect (Figure 4b). The calculated power factor of the TPCP film reaches a maximum value of 185 μW m−1 K−2 at a protonation degree of 7.2% (Figure 4c), which is much higher than that of current n-type organic TE materials ( Supporting Information Table S6). Recently, while the performance of p-type organic TE materials has been significantly improved, the development of n-type organic TE materials is behind both in the performance and variety of materials. There are few examples of high performance n-type organic TE materials, such as vapor-doped fullerene34 (PFmax = 30 μW m−1 K−2) and metal bismuth interfacial-doped TDPP35 (PFmax = 113 μW m−1 K−2) ( Supporting Information Table S6). The first example of a covalent organic framework being used as a TE material was a fluorene-based two-dimensional system doped with iodine.36 The material showed a large Seebeck coefficient as high as 2450 μV K−1, with a low power factor of 0.063 μW m−1 K−2, mainly due to its lower electronic conductivity. In this work, our strategy of protonation can effectively improve the electrical conductivity of cross-linked conjugated polymers, and maintain a high Seebeck coefficient to thereby obtain a significantly enhanced power factor, which may promote the development of n-type organic TE materials. Conclusions A highly soluble proton-exfoliated TPCP was synthesized by introducing protons into the TPCP skeletons, representing a novel tactic for the preparation of TPCP solutions with superior TE properties. Our design doctrine predicts that isolated pairs of like charged colloidal spheres experience electrostatic repulsive interactions. The charged TPCP incorporating ionic building blocks promotes itself by dispersion into various solvents and gains remarkable solubility. In addition, its high solubility presented the possibility of solution processability into high-quality films. The TE properties of the TPCP films were explored and the films exhibited the highest power factor. The remarkable results presented here provide new avenues for high-performance TE materials, particularly for the use of reduced and protonated porphyrin electronic structures. The consequence is a novel method for the investigation of cross-linked conjugated polymer solution-processability for new opportunities in TEs. Supporting Information Supporting Information is available and includes additional figures and results. Conflict of Interest The authors declare no competing financial interests. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (nos. 21975078 and 91833304), the Fundamental Research Funds for the Central Universities, the Introduced Innovative R&D Team of Guangdong Province (no. 201101C0105067115), the Natural Science Foundation of Guangdong Province (no. 2019B030301003), the 111 Project, and the Thousand Youth Talents Plan.