REPORT 2022
Stage III (2022)
SCIENTIFIC AND TECHNICAL DESCRIPTION
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In Stage III of the EngEChrom project, 1 diamines based on phenothiazine (PTZ) was synthesized and structurally characterized. This diamine integrate triphenylamine (TPA) into a double electron donor topology to reduce the oxidation potential. The chemical structure of diamines, as well as the intermediates, was confirmed by 1H‒ (proton) and 13C‒ (carbon) NMR (nuclear magnetic resonance) spectroscopy and Fourier Transform infrared spectroscopy (FTIR). Proton and carbon NMR confirmed the structure and a purity of 98%. In the proton NMR spectrum, the diamine showed typical signals attributed to the presence of amino groups (at around 4.5 ppm) and other aromatic and aliphatic structural elements that highlighted the presence of PTZ and TPA units. The carbon NMR spectrum highlights the amino groups by the presence of the signal at about 145 ppm. Moreover, in the FTIR spectrum, diamine showed the absorption bands characteristic of the amino functional group. Based on them, polymers were synthesized as electroactive layers for prototype EC devices, which were synthesized by the polycondensation reaction to obtain 1 polyimide, 1 polyazomethine and 4 polyamides (6 polymers). These were grouped into tow series with the aim: (i) to identify the most advantageous class of polymers from a physico-chemical and electrochromic point of view and (ii) to identify the reaction counterparts from the same type of polymers that would lead to the most advantageous results (Figure 1).
Figure 1. Schematic representation of the design of the electrochromic polymers.
The chemical structure of the polymers was confirmed by spectral methods such as proton NMR and FTIR, and the structural elements typical of each class of polymers were identified. The thermal properties of the polymers were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The values ​​of the glass transitions of the polymers were in the temperature range typical of the classes to which they belong. Also, all polymers shown excellent thermal stability, as expected for these classes of polymers. Therefore, the recorded values ​​successfully meet the requirements for using these polymers as electroactive layers in EC devices. The quality and morphology of thin polymer films were studied by scanning electron microscopy (SEM), and the images showed that they were smooth and homogeneous, without cracks or pinholes. Both ground and excited state geometries were obtained by DFT and TD‒DTF, involving the functional B3LYP together with the basic set 6-31G++(d,p), in order to identify the distribution of HOMO and LUMO orbitals and to evidence the main transitions from the experimental UV-Vis spectra. Thus, the n-π * transitions corresponding to the TPA PTZ and POZ units were identified, as well as those from two transitions associated with the charge transfer complex (CTC) between the TPA and POZ/PTZ units and the electron acceptors. Except for polyazomethines, all synthesized polymers showed low absorption in the visible range of the electromagnetic spectrum, making them ideal for EC devices. In anodic scanning, the polymers showed two reversible REDOX waves with excellent stability even after 100 repetitive cycles, corresponding to the oxidation processes of POZ, PTZ and TPA, respectively. Spectroelectrochemistry measurements showed the variation of the absorption bands as the potential increased, being associated with the formation of cationic radicals and dicationic species. These spectral changes were also accompanied by the change of the films' color to green and blue. Based on 2 polymers with the best performance and physico-chemical characteristics, 2 EC prototype devices were assembled (Figure 2).
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Figure 2. The structure of the electrochromic prototype devices.
The obtained results were rigorously analyzed, the structure-property-efficiency correlations were established, the construction technology of the devices was optimized, and the electrochromic performances and stability were evaluated. The devices showed an electrochromic efficiency of up to 3 209 cm2/C. Also, the recovery of the electrochromic effect of the devices was up to 96% after 100 cycles (Figure 3).
Figure 3. Characteristics of the electrochromic prototype devices.
The results of EngEChrom project were disseminated by 2 oral presentations and 1 poster at 2 international scientific events, and 3 papers published in ISI-indexed journals and 1 patent application submitted to OSIM. The rigorous analysis of all the results generated during this project, as well as a result of the discussions and conclusions were drawn at the workshop which was held at the end of the project with the participation of the members of the implementation team. The dedicated web page on the official website of the host institution, offered high visibility and served as a tool for the dissemination of the results throughout the project.