The Synergistic Effect of Triazine and Phosphaphenanthrene Units on the Physico-Chemical Behavior of Polyimides

With the aim to develop polymers with appealing, multifunctional characteristics, a series of polyimides were designed by anchoring 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) units on the main polymer chains containing 1,3,5-triazine and several flexible moieties, such as ether, hexafluoroisopropylidene, or isopropylidene. A detailed study was conducted to establish structure–property correlations, with a focus on the synergistic effectiveness of triazine and DOPO moieties on the overall features of polyimides. The results evidenced good solubility of the polymers in organic solvents, their amorphous nature with short-range regular-packed polymer chains, and high thermal stability with no glass transition temperature below 300 °C. Spectrophotometric measurements revealed the existence of a strong charge transfer complex in these polymers that led to a “black” appearance, which generated broad absorption bands spanning on the overall visible range. Nevertheless, these polymers displayed green light emission associated with 1,3,5-triazine emitter. The electrochemical characteristics of the polyimides in solid state demonstrated their strong n-type doping character induced by three different structural elements with electron-acceptance capability. The useful properties of these polyimides, including optical, thermal, electrochemical, aesthetics, and opaqueness, endow them with several possible applications in the microelectronic field, such as protecting layers for the inner circuits against UV light deterioration.


Introduction
Polyimides belong to the well-known class of high thermostable polymers which have continuously attracted the attention of academia and industry due to their unique and appealing combination of physical and chemical properties. Since their discovery in 1908, polyimides have been subjected to various modifications in order to render the designed polymeric architectures with targeted properties for a wide range of applications. Relevant reviews that have described the synthetic pathways for the development of polyimides, their physico-chemical characterization, as well as the diverse range of applications have been published over the last 10 years [1][2][3][4][5][6][7]. The characteristics of classical all-aromatic polyimides include high-temperature resistance, mechanical and chemical stability, elevated glass transition temperature, variable optical transmittance, as well as low dielectric constants. All these properties can be tailored by altering the architecture of the polymer chains, and they strongly depend on the functional groups inserted in the macromolecular backbones which also influence the molecular weight, solubility, crystallinity, intermolecular forces, or electron conducting/storage ability in addition to the above-mentioned properties. All these characteristics make the newly developed polyimides valuable candidates for a wide range of applications in different forms, such as film, fiber, nanofiber, membrane, foam, adhesive, or coating in aerospace, electronics, automotives, medicine, information recording and new imaging technology, modern paper, solar cells, or other green energy fields [8]. Thus, researchers have customized the polyimide macromolecular Despite the numerous articles based on the synthesis of polyimides published so far, none of them have focused on the incorporation of triazine and DOPO heterocyclic units in the same macromolecular chain. Therefore, this study attempts to highlight the synergistic effect of DOPO and triazine on the physico-chemical properties of a series of polyimides which also incorporate several flexible linkages. To this aim, a diamine monomer containing DOPO-substituted triazine was used in polycondensation reaction with several dianhydrides which incorporate bridges of variable flexibility. We explored, in detail, the influence of this bulky unit on the overall polyimide properties, including thin films morphology, photo-optical characteristics, thermal stability, and electrochemical features.

Results and Discussions
With the aim to develop polymers with useful characteristics for nowadays technologies, the classical polyimide structure was modified by anchoring DOPO on the 1,3,5triazine unit of the main polymer chains. To enhance the solubility and polymer processability by wet methods, additional moieties with various degrees of flexibility were incorporated into the polymer structure, taking care to maintain the high thermal stability and other inherent properties of classical polyimides. To this aim, the diamine TE-DOPO containing DOPO-substituted triazine was used in the polycondensation reaction with three commercial aromatic dianhydrides to synthesize the desired polyimides, PI1-PI3, according to Scheme 1. Due to the diamine insolubility in NMP at room temperature, the polycondensation reaction was conducted in a single step, without the isolation of the intermediate polyamidic acid. Scheme 1. Synthesis of DOPO-substituted triazine-containing polyimides PI1-PI3 (the inset shows the images of the obtained polyimide powders).

Structural Characterization
The structural identification of the obtained polyimide powders was accomplished by FTIR and NMR spectroscopies. The FTIR spectra revealed the existence of the characteristic absorption bands of the newly formed imide cycle at: 1777-1764 cm −1 (C=O asymmetric stretching vibration), 1704-1700 cm −1 (C=O symmetric stretching vibration), 1378-1377 cm −1 (C-N stretching vibration), and 754-717 cm −1 (imide ring deformation). The absorption bands identified at about 1542-1540 cm −1 were attributed to the heterocyclic Scheme 1. Synthesis of DOPO-substituted triazine-containing polyimides PI1-PI3 (the inset shows the images of the obtained polyimide powders).

Structural Characterization
The structural identification of the obtained polyimide powders was accomplished by FTIR and NMR spectroscopies. The FTIR spectra revealed the existence of the characteristic absorption bands of the newly formed imide cycle at: 1777-1764 cm −1 (C=O asymmetric stretching vibration), 1704-1700 cm −1 (C=O symmetric stretching vibration), 1378-1377 cm −1 (C-N stretching vibration), and 754-717 cm −1 (imide ring deformation). The absorption bands identified at about 1542-1540 cm −1 were attributed to the heterocyclic triazine unit. Evidence supporting the maintenance of the cyclic DOPO groups in the polyimides was provided through the appearance of some characteristic absorption bands, as follows: P-Ar at about 1475-1472 cm −1 , P-CH 2 at 1436-1434 cm −1 , P=O at 1146-1140 cm −1 , and P-O-Ph at 920-916 cm −1 . The absorption bands due to C-H aliphatic bonds from the ethylidene bridge which connects the two heterocycles were found in the ranges of 2965-2951 cm −1 and 2894-2898 cm −1 , providing further evidence for the existence of the unaltered bulky DOPO-substituted triazine unit in the macromolecular chains of the polyimides. Aromatic C-H and C-C linkages of the aromatic rings were identified at approximatively 3080-3060 cm −1 and 1608-1606 cm −1 , respectively. The specific flexible group from the diimide segment of each polymer was identified at 1227 cm −1 due to the absorption of aromatic ether units in PI1, 1209 cm −1 due to the absorption of CF 3 moieties in PI2, and 2965 cm −1 and 2898 cm −1 due to the absorption of aliphatic C-H linkage belonging to the C(CH 3 ) 2 group of PI3 ( Figure 1). triazine unit. Evidence supporting the maintenance of the cyclic DOPO groups in the polyimides was provided through the appearance of some characteristic absorption bands, as follows: P-Ar at about 1475-1472 cm −1 , P-CH2 at 1436-1434 cm −1 , P=O at 1146-1140 cm −1 , and P-O-Ph at 920-916 cm −1 . The absorption bands due to C-H aliphatic bonds from the ethylidene bridge which connects the two heterocycles were found in the ranges of 2965-2951 cm −1 and 2894-2898 cm −1 , providing further evidence for the existence of the unaltered bulky DOPO-substituted triazine unit in the macromolecular chains of the polyimides. Aromatic C-H and C-C linkages of the aromatic rings were identified at approximatively 3080-3060 cm −1 and 1608-1606 cm −1 , respectively. The specific flexible group from the diimide segment of each polymer was identified at 1227 cm −1 due to the absorption of aromatic ether units in PI1, 1209 cm −1 due to the absorption of CF3 moieties in PI2, and 2965 cm −1 and 2898 cm −1 due to the absorption of aliphatic C-H linkage belonging to the C(CH3)2 group of PI3 ( Figure 1). An important aspect provided by the FTIR spectra relates to the absorption bands which appeared in the ranges of 3409-3401 cm −1 and 1637-1623 cm −1 which can be attributed to carboxylic and amide groups absorptions, respectively. These suggest the occurrence of an incomplete imidization of the intermediary polyamidic acids. We presumed that when polyamidic acids were formed, certain intermolecular interactions occurred between the carboxylic acid units from the main chains and DOPO pendant unit (through -OH and P=O), which impeded the formation of a fully-cyclized polyimide, despite the long reaction times and catalyst used in the polycondensation reactions. The incomplete cyclization of the intermediary polyamidic acids to the corresponding fully cyclized polyimide forms is a known phenomenon encountered for polyimides bearing bulky pendant groups where a 100% degree of imidization is rarely found [47,48].
The NMR spectra provided additional support for the FTIR data discussed above. In the case of the polyimides PI1 and PI2, which display a more rigid structure of the diimide segment, the aromatic protons coming from the diamine and diimide segments appear as broad overlapped signals, in the ranges of 7.31-8.16 ppm (PI1) and 7.25-8.16 ppm (PI2). Polyimide PI3, which contains a longer flexible unit, exhibits a slightly clearer spectrum with peaks separated in two regions: 8.3-7.18 ppm and 7.36-7.12 ppm (Figure 2a). The different pattern of the 1 H-NMR spectrum of this polymer may be associated with the higher flexibility induced by the presence of 6H and ether units, which reduce the chain packing to a lower degree compared to PI1 and PI2, where intermolecular interactions are predominant. The 1 H-NMR spectra of the polyimides were compared to that of the starting diamine monomer and the results indicated its full incorporation into the corresponding polymers due to the disappearance of characteristic amine protons from 6.55 ppm. An important aspect provided by the FTIR spectra relates to the absorption bands which appeared in the ranges of 3409-3401 cm −1 and 1637-1623 cm −1 which can be attributed to carboxylic and amide groups absorptions, respectively. These suggest the occurrence of an incomplete imidization of the intermediary polyamidic acids. We presumed that when polyamidic acids were formed, certain intermolecular interactions occurred between the carboxylic acid units from the main chains and DOPO pendant unit (through -OH and P=O), which impeded the formation of a fully-cyclized polyimide, despite the long reaction times and catalyst used in the polycondensation reactions. The incomplete cyclization of the intermediary polyamidic acids to the corresponding fully cyclized polyimide forms is a known phenomenon encountered for polyimides bearing bulky pendant groups where a 100% degree of imidization is rarely found [47,48].
The NMR spectra provided additional support for the FTIR data discussed above. In the case of the polyimides PI1 and PI2, which display a more rigid structure of the diimide segment, the aromatic protons coming from the diamine and diimide segments appear as broad overlapped signals, in the ranges of 7.31-8.16 ppm (PI1) and 7.25-8.16 ppm (PI2). Polyimide PI3, which contains a longer flexible unit, exhibits a slightly clearer spectrum with peaks separated in two regions: 8.3-7.18 ppm and 7.36-7.12 ppm (Figure 2a). The different pattern of the 1 H-NMR spectrum of this polymer may be associated with the higher flexibility induced by the presence of 6H and ether units, which reduce the chain packing to a lower degree compared to PI1 and PI2, where intermolecular interactions are predominant. The 1 H-NMR spectra of the polyimides were compared to that of the starting diamine monomer and the results indicated its full incorporation into the corresponding polymers due to the disappearance of characteristic amine protons from 6.55 ppm. Still, in line with FTIR findings, 1 H-NMR spectra also revealed the existence of residual polyamidic acid groups in all polymers due to the broad peak found at about 13.15-12.10 ppm and the sharp peak centered at 11.64-11.31 ppm, which were attributed to the carboxylic and amide protons, respectively.
Molecules 2023, 28, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molecules boxylic and amide protons, respectively. The 31 P-NMR spectra also confirmed the correct structure of the synthesized polyimides through the presence of a main sharp peak at about 36.98-37.04 ppm associated with the phosphorus of the polyimide chains, and a broad, weak shoulder-like peak in the range of 32-33.08 ppm, which was tentatively attributed to the non-cyclized polyamidic acid segments (Figure 2b). The broadness of the latter provides evidence for the involvement of the P=O unit in intermolecular interactions.  Due to the incorporation of the bulky pendant DOPO segment in the side chains and of flexible ether, hexafluoroisopropylidene, or isopropylidene units in the main chains, improved organo-solubility and processability of the synthesized polyimides is expected. To verify this, solubility tests were performed using 5 mg of polyimides in 1 mL solvent. The results evidenced a very good polymer solubility in polar organic solvents (NMP, DMAc, DMF, and DMSO). In the case of less polar solvents, such as THF, chloroform, acetone, or dichloromethane, PI1-PI3 were insoluble even on heating ( Table 1). The good solubility at room temperature in organic polar solvents is attributed, on one hand, to the polymers non-coplanar structure induced by the presence of the voluminous DOPO-substituted ethylidene unit from the side chain and, on the other hand, to the flexible groups from the diimide segment. The synergetic effect of these structural modifications led to The 31 P-NMR spectra also confirmed the correct structure of the synthesized polyimides through the presence of a main sharp peak at about 36.98-37.04 ppm associated with the phosphorus of the polyimide chains, and a broad, weak shoulder-like peak in the range of 32-33.08 ppm, which was tentatively attributed to the non-cyclized polyamidic acid segments ( Figure 2b). The broadness of the latter provides evidence for the involvement of the P=O unit in intermolecular interactions.
Due to the incorporation of the bulky pendant DOPO segment in the side chains and of flexible ether, hexafluoroisopropylidene, or isopropylidene units in the main chains, improved organo-solubility and processability of the synthesized polyimides is expected. To verify this, solubility tests were performed using 5 mg of polyimides in 1 mL solvent. The results evidenced a very good polymer solubility in polar organic solvents (NMP, DMAc, DMF, and DMSO). In the case of less polar solvents, such as THF, chloroform, acetone, or dichloromethane, PI1-PI3 were insoluble even on heating ( Table 1). The good solubility at room temperature in organic polar solvents is attributed, on one hand, to the polymers non-coplanar structure induced by the presence of the voluminous DOPO-substituted ethylidene unit from the side chain and, on the other hand, to the flexible groups from the diimide segment. The synergetic effect of these structural modifications led to polymer conformations which enabled the penetration of small solvent molecules through the chains and polymer solubilization. Owing to the good solubility in polar organic solvents, evaluation of the average molecular weights of polyimides was possible by using GPC measurements. The data revealed relative high values for the investigated parameters: Mn had values between 151,700 and 301,700 g/mol, Mw was in the range of 354,500-693,900 g/mol, while the polydispersity index was between 2.19 and 2.34 ( Table 2). Apart from this, GPC diagrams evidenced signals for the residual polyamidic acid segments with low Mn values, in the range of 6500-7700 g/mol (Mw of about 7100-8200 g/mol and polydispersity index between 1.07 and 1.1). Thus, contents of about 12, 10, and 8 repeating units of polyamidic acid segments relative to approx. 565, 912, and 716 imide segments were found, which means a conversion of 97.9%, 98.9%. and 98.9% of the polyamidic acid to the imide structure for PI1, PI2. and PI3, respectively.

XRD and SEM Studies
Due to the black appearance of the polyimide powders (as can be seen in Scheme 1), XRD measurements were registered to analyze the possible crystalline nature of the polymers. The broad X-ray diffractogram of the polyimides ( Figure 3) in the range of 15-25 • indicate their amorphous nature. The two overlapped halos identified in this region may be associated with the mixture of the two polymer structures, i.e., polyimide and polyamidic acids. The peak found at smaller diffraction angles in all polyimides (approx. 7 • ) is tentatively attributed to the changes in local order (regular repeating distances), i.e., short-range regular packing of polymer chains with no crystalline features [49]. The amorphous nature of these polyimides is due to the presence of the bulky unsymmetrical DOPO-substituted triazine moiety in the diamine segment and of flexible units (ether, hexafluoroisopropylidene, or isopropylidene) in the diimide segment. All of these structural units disturb the overall regularity and hinder the arrangement in crystalline domains of polyimide chains [50]. Starting from the XRD results, it was anticipated that some solid-state organization may occur in thin films obtained from these polymers. Thus, coatings of PI1-PI3 were obtained on glass plates from polymer solutions in NMP by using the drop-casting tech nique, after gradual annealing until 250 °C. To further evaluate the films morphology optical microscopy (OM) visualizations were first involved. The photographs taken at a magnification of 20× evidenced the formation of continuous and homogeneous films, with similar morphologies for all polymers (Figure 4, top). Although the polyimides structures contain different diimide segments, no obvious influence of this structural parameter on the morphological features of the films was noticed. The capability of the polymer chains to arrange into more defined assemblies was observed by performing scanning electron microscopy (SEM) investigations. Thus, when Starting from the XRD results, it was anticipated that some solid-state organization may occur in thin films obtained from these polymers. Thus, coatings of PI1-PI3 were obtained on glass plates from polymer solutions in NMP by using the drop-casting technique, after gradual annealing until 250 • C. To further evaluate the films' morphology, optical microscopy (OM) visualizations were first involved. The photographs taken at a magnification of 20× evidenced the formation of continuous and homogeneous films, with similar morphologies for all polymers (Figure 4, top). Although the polyimides' structures contain different diimide segments, no obvious influence of this structural parameter on the morphological features of the films was noticed. Starting from the XRD results, it was anticipated that some solid-state organization may occur in thin films obtained from these polymers. Thus, coatings of PI1-PI3 were obtained on glass plates from polymer solutions in NMP by using the drop-casting technique, after gradual annealing until 250 °C. To further evaluate the films morphology, optical microscopy (OM) visualizations were first involved. The photographs taken at a magnification of 20× evidenced the formation of continuous and homogeneous films, with similar morphologies for all polymers (Figure 4, top). Although the polyimides structures contain different diimide segments, no obvious influence of this structural parameter on the morphological features of the films was noticed. The capability of the polymer chains to arrange into more defined assemblies was observed by performing scanning electron microscopy (SEM) investigations. Thus, when the surfaces of thin films were analyzed, similar morphological features were found for The capability of the polymer chains to arrange into more defined assemblies was observed by performing scanning electron microscopy (SEM) investigations. Thus, when the surfaces of thin films were analyzed, similar morphological features were found for all polymers, as in the case of OM. The rigid rods of the polymer macromolecules generated by the imide-triazine framework mostly experienced π-π stacking interactions during film preparation under heating, leading to a grain-like morphology (Figure 4, bottom). However, from a certain level, the primary morphological entities arranged in bigger assemblies, as well-observed in the case of PI2. Most likely, surface tension constraints occurred that forced the grains to organize into clusters having a sheet-like shape and variable dimensions. It should be noted that this type of solid-state organization has not been previously reported for DOPO-containing polyimides, which prompted us to consider that a strong contribution to it originates from triazine that through the synergetic effect with imide and DOPO induced a close macromolecular packing.

Thermal Properties
Differential scanning calorimetry and thermogravimetric investigations were used to analyze the thermal stability of the polymers, and the obtained data are shown in Table 2 and Figure 5. The glass transition temperature for all polymer powders could not be detected in the temperature limit of the experimental setup, so that no Tg was registered until 300 • C for any of the polyimides. Most likely, this phenomenon is present at a higher temperature due to the high degree of aromaticity of the main polymer chains and the presence of intermolecular interactions between macromolecules in solid state.
Molecules 2023, 28, x FOR PEER REVIEW  8 of 18 all polymers, as in the case of OM. The rigid rods of the polymer macromolecules generated by the imide-triazine framework mostly experienced π-π stacking interactions during film preparation under heating, leading to a grain-like morphology (Figure 4, bottom). However, from a certain level, the primary morphological entities arranged in bigger assemblies, as well-observed in the case of PI2. Most likely, surface tension constraints occurred that forced the grains to organize into clusters having a sheet-like shape and variable dimensions. It should be noted that this type of solid-state organization has not been previously reported for DOPO-containing polyimides, which prompted us to consider that a strong contribution to it originates from triazine that through the synergetic effect with imide and DOPO induced a close macromolecular packing.

Thermal Properties
Differential scanning calorimetry and thermogravimetric investigations were used to analyze the thermal stability of the polymers, and the obtained data are shown in Table 2 and Figure 5. The glass transition temperature for all polymer powders could not be detected in the temperature limit of the experimental setup, so that no Tg was registered until 300 °C for any of the polyimides. Most likely, this phenomenon is present at a higher temperature due to the high degree of aromaticity of the main polymer chains and the presence of intermolecular interactions between macromolecules in solid state. The initial decomposition temperature of polyimides, taken as 5% weight loss (T5) registered values in the range of 336-352 °C, in close correlation with the nature of the flexible unit from the dianhydride fragment. The highest value of T5 was obtained for PI2 which incorporates 6F units, while the lowest one was registered for PI1 containing ether bridges in the main chain. However, the latter displayed an initial decomposition temperature close to that found for PI3 with isopropylidene moieties in the diiimide segment (339 °C). These values are comparable to those obtained for related DOPO-containing polyimides with the same flexible linkages [43], and are slightly lower than those reported for fully aromatic conjugated triazine-based polyimides [30]. The degradation mechanism followed the same trend for polyimides PI1 and PI3, employing one major decomposition step, while PI2, incorporating hexafluoroisopropylidene units in the polymer structure, exhibited an additional decomposition step, which, in this case, was attributed to the scission of the fluorinated moieties. A shoulder-like peak can be observed in the DTG curves at about 369, 338, and 376 °C for PI1, PI2, and PI3, respectively, which can be assigned to The initial decomposition temperature of polyimides, taken as 5% weight loss (T 5 ) registered values in the range of 336-352 • C, in close correlation with the nature of the flexible unit from the dianhydride fragment. The highest value of T 5 was obtained for PI2 which incorporates 6F units, while the lowest one was registered for PI1 containing ether bridges in the main chain. However, the latter displayed an initial decomposition temperature close to that found for PI3 with isopropylidene moieties in the diiimide segment (339 • C). These values are comparable to those obtained for related DOPO-containing polyimides with the same flexible linkages [43], and are slightly lower than those reported for fully aromatic conjugated triazine-based polyimides [30]. The degradation mechanism followed the same trend for polyimides PI1 and PI3, employing one major decomposition step, while PI2, incorporating hexafluoroisopropylidene units in the polymer structure, exhibited an additional decomposition step, which, in this case, was attributed to the scission of the fluorinated moieties. A shoulder-like peak can be observed in the DTG curves at about 369, 338, and 376 • C for PI1, PI2, and PI3, respectively, which can be assigned to the decomposition of the voluminous triazine-ethylidene-DOPO fragment [42]. The main step of decomposition in all polymers was found in the range of 490-516 • C, and was correlated with the degradation of the main macromolecular chains. The char residue values at 800 • C were higher in the case of PI1 and PI3 (W 800 = 46.44% and 53.05%, respectively), and they were comparable to the values recorded for related DOPO-containing polyimides [43,45]. As expected, the introduction of DOPO-substituted triazine unit along with flexible linkages in these polyimides preserved an increased thermal stability, and therefore, the present polymers are suitable for applications that require resistance to high temperatures.

Photo-Optical Characteristics
Spectral characterization by UV-Vis absorption and fluorescence was carried out with the aim to evaluate the electronic transitions induced by the triazine and imide heterocycles in polymers PI1-PI3, and also the presence of molecular interactions. Figure 6 illustrates the electronic absorption spectra of these polyimides recorded in DMSO solution and in solid state.  [42]. The main step of decomposition in all polymers was found in the range of 490-516 °C, and was correlated with the degradation of the main macromolecular chains. The char residue values at 800 °C were higher in the case of PI1 and PI3 (W800 = 46.44% and 53.05%, respectively), and they were comparable to the values recorded for related DOPO-containing polyimides [43,45]. As expected, the introduction of DOPO-substituted triazine unit along with flexible linkages in these polyimides preserved an increased thermal stability, and therefore, the present polymers are suitable for applications that require resistance to high temperatures.

Photo-Optical Characteristics
Spectral characterization by UV-Vis absorption and fluorescence was carried out with the aim to evaluate the electronic transitions induced by the triazine and imide heterocycles in polymers PI1-PI3, and also the presence of molecular interactions. Figure 6 illustrates the electronic absorption spectra of these polyimides recorded in DMSO solution and in solid state. According to Figure 6, the absorption bands of the studied polyimides are weak, in the form of shoulders, and difficult to localize, with maxima at wavelengths of about 270, 290, and 300 nm. These overlapped high-energy absorptions can be attributed to the π-π* and n-π* transitions in the imide, triazine, DOPO, and benzene rings, according to previous reports [51,52]. Although the structural variation in the diimide segment tailored the macromolecular conformation and planarity of PI1-PI3, this had little effect on the electronic transitions in these polymers since no obvious shifts in the absorption maxima were observed. It should be emphasized that all polyimides showed in their UV-Vis absorption spectra a tail-like band spanning the overall visible domain, which may suggest the existence of the charge transfer complex (CTC) in these polymers.
As thin films, the present polyimides showed different photo responses compared to the isolated molecules in liquid phase, with broader absorption bands spanning the overall visible absorption range, which apparently displayed maxima shifted to higher wavelengths (Figure 6b). Thus, the absorption maximum was noticed at 352, 347, and 329 nm for PI1, PI2, and PI3, respectively. These data indicate that the optical changes in solid state are considerably affected by the chemical structure of the diimide segment. Accordingly, PI1 displays the highest co-planar conformation and packed polymer chains due to the more rigid shape of the ODPA-derived segment, followed by PI2 with its bulky According to Figure 6, the absorption bands of the studied polyimides are weak, in the form of shoulders, and difficult to localize, with maxima at wavelengths of about 270, 290, and 300 nm. These overlapped high-energy absorptions can be attributed to the π-π* and n-π* transitions in the imide, triazine, DOPO, and benzene rings, according to previous reports [51,52]. Although the structural variation in the diimide segment tailored the macromolecular conformation and planarity of PI1-PI3, this had little effect on the electronic transitions in these polymers since no obvious shifts in the absorption maxima were observed. It should be emphasized that all polyimides showed in their UV-Vis absorption spectra a tail-like band spanning the overall visible domain, which may suggest the existence of the charge transfer complex (CTC) in these polymers.
As thin films, the present polyimides showed different photo responses compared to the isolated molecules in liquid phase, with broader absorption bands spanning the overall visible absorption range, which apparently displayed maxima shifted to higher wavelengths (Figure 6b). Thus, the absorption maximum was noticed at 352, 347, and 329 nm for PI1, PI2, and PI3, respectively. These data indicate that the optical changes in solid state are considerably affected by the chemical structure of the diimide segment. Accordingly, PI1 displays the highest co-planar conformation and packed polymer chains due to the more rigid shape of the ODPA-derived segment, followed by PI2 with its bulky hexafluoroisopropylidene unit that reduced these features to some extent. Conversely, PI3 is characterized by the most non-planar arrangement and the lowest solid-state packing in the series, clearly reflected in the most hypsochromic shifted UV-Vis absorption band registered for this polymer.
Another important aspect to be mentioned here is related to the obvious intensification of the CTC band for these polymers, mostly induced by the better planarity of the macromolecules in solid state, which definitely favors the CTC transitions. Usually, the colors of polyimide films are much deeper when CTC formation is stronger. As mentioned earlier, our polymer films are brown-black in appearance and this correlates well with our assumption on CTC strength. Such a characteristic of "black polyimides" was previously observed for other polyimides derived from nitrogen-containing diamines [53]. According to the transmittance spectra shown in Figure 7, these PI films, especially PI1 and PI3, experience a significant decrease in the optical transmittance on overall UV-Vis spectral range. For instance, the thin film of PI1 is almost opaque at the wavelength of 500 nm, showing 18% transmittance at 500 nm and 31% at 600 nm. The highest transmittance of PI2 can be explained by the presence of low polarizability trifluoromethyl groups that reduces the CTC formation through steric hindrance and inductive effect leading to weakening chain-to-chain cohesive forces.
Molecules 2023, 28, x FOR PEER REVIEW 10 of 18 hexafluoroisopropylidene unit that reduced these features to some extent. Conversely, PI3 is characterized by the most non-planar arrangement and the lowest solid-state packing in the series, clearly reflected in the most hypsochromic shifted UV-Vis absorption band registered for this polymer.
Another important aspect to be mentioned here is related to the obvious intensifica tion of the CTC band for these polymers, mostly induced by the better planarity of the macromolecules in solid state, which definitely favors the CTC transitions. Usually, the colors of polyimide films are much deeper when CTC formation is stronger. As mentioned earlier, our polymer films are brown-black in appearance and this correlates well with our assumption on CTC strength. Such a characteristic of "black polyimides" was previously observed for other polyimides derived from nitrogen-containing diamines [53]. According to the transmittance spectra shown in Figure 7, these PI films, especially PI1 and PI3, ex perience a significant decrease in the optical transmittance on overall UV-Vis spectra range. For instance, the thin film of PI1 is almost opaque at the wavelength of 500 nm showing 18% transmittance at 500 nm and 31% at 600 nm. The highest transmittance o PI2 can be explained by the presence of low polarizability trifluoromethyl groups tha reduces the CTC formation through steric hindrance and inductive effect leading to weak ening chain-to-chain cohesive forces. These appealing optical properties endow the present polyimides with a large ap plicative prospect in fields where high heat resistance, chemical inertness, aesthetics, and opaqueness are all together required, for example, to cover flexible printing circuit boards in order to protect the inner circuits from UV light deterioration. Although the presen polymers cannot be processed in free-standing films, further modification via blending technique can be promising to suppress this drawback and benefit of their valuable properties. The information on this subject will be reported in future work. Since 1,3,5-triazines as electron acceptor units form the most attractive building blocks in the development of thermally activated delayed fluorescence (TADF) materials with high efficiencies and long operating lifetimes in OLEDs [54], it is of particular interes to explore the fluorescence properties of PI1-PI3 having this heterocycle as a constitutive structural element. To the best of our knowledge, the synergistic effect of 1,3,5-triazine and DOPO units on the fluorescence properties of polyimides is still unknown. Thus, the fluorescence behavior of the present polymers in DMSO solutions and condensed state (thin films) were investigated. A well resolved fluorescence band was recorded in the spec tral pattern of all polyimides, in the range of 528-539 nm, which indicated their ability to These appealing optical properties endow the present polyimides with a large applicative prospect in fields where high heat resistance, chemical inertness, aesthetics, and opaqueness are all together required, for example, to cover flexible printing circuit boards in order to protect the inner circuits from UV light deterioration. Although the present polymers cannot be processed in free-standing films, further modification via blending technique can be promising to suppress this drawback and benefit of their valuable properties. The information on this subject will be reported in future work. Since 1,3,5-triazines as electron acceptor units form the most attractive building blocks in the development of thermally activated delayed fluorescence (TADF) materials with high efficiencies and long operating lifetimes in OLEDs [54], it is of particular interest to explore the fluorescence properties of PI1-PI3 having this heterocycle as a constitutive structural element. To the best of our knowledge, the synergistic effect of 1,3,5-triazine and DOPO units on the fluorescence properties of polyimides is still unknown. Thus, the fluorescence behavior of the present polymers in DMSO solutions and condensed state (thin films) were investigated. A well resolved fluorescence band was recorded in the spectral pattern of all polyimides, in the range of 528-539 nm, which indicated their ability to release green light (Figure 8a). This emission was associated with 1,3,5-triazine since this heterocycle is a well-known electron-deficient moiety used in green and blue TADF emitters [54]. Previous studies on related polyimides containing DOPO units have highlighted that their emission in polar solvents occurs in the UV-blue spectral range [46], which definitely provides support for our statement. Additional evidence for the origin of the fluorescence (FL) bands of our polymers associated with triazine emission comes from the position of their maxima. Changes in the structural assembly of diimide segment of PI1-PI3 affected the FL spectra in terms of emission wavelength (small shifts) and emission intensity. Since the main polyimide chains incorporating triazine heterocycle adopt different conformations, it was expected to obtain some differences in the FL maxima. In this context, it was found that PI3 with the most flexible main chains emitted more intense light at a lower wavelength (528 nm) compared to the more rigid polymers, PI1 and PI2, that released light of lower intensity at 534 and 539 nm, respectively.
Molecules 2023, 28, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molecules release green light (Figure 8a). This emission was associated with 1,3,5-triazine since this heterocycle is a well-known electron-deficient moiety used in green and blue TADF emitters [54]. Previous studies on related polyimides containing DOPO units have highlighted that their emission in polar solvents occurs in the UV-blue spectral range [46], which definitely provides support for our statement. Additional evidence for the origin of the fluorescence (FL) bands of our polymers associated with triazine emission comes from the position of their maxima. Changes in the structural assembly of diimide segment of PI1-PI3 affected the FL spectra in terms of emission wavelength (small shifts) and emission intensity. Since the main polyimide chains incorporating triazine heterocycle adopt different conformations, it was expected to obtain some differences in the FL maxima. In this context, it was found that PI3 with the most flexible main chains emitted more intense light at a lower wavelength (528 nm) compared to the more rigid polymers, PI1 and PI2, that released light of lower intensity at 534 and 539 nm, respectively. In a condensed form, the fluorescence of PI1-PI3 was almost quenched (Figure 8b), mostly due to the molecular aggregation that led to non-radiative decay by intermolecular energy transfer. This behavior is opposite to that found for related DOPO-containing polyimides [46] which displayed a strong fluorescence emission in film state, with maximum emission wavelength at about 510 nm. Thus, it can be assumed that 1,3,5-triazine brought the most significant contribution to florescence suppression of present polyimides in solid state.

Cyclic Voltammetry (CV) Measurements
The investigated polymers contain strong electron-withdrawing imide, DOPO, and 1,3,5-triazine heterocycles capable of experiencing electrochemical reduction processes, hence, their electroactivity was investigated by cyclic voltammetry (CV). All CV measurements were performed on polymer-modified ITO-coated glass as the working electrode in an electrochemical cell equipped with Ag/AgCl as the reference electrode, and Pt wire as the counter electrode, in 0.1 M tetrabuthylammonium (TBAP)/acetonitrile (ACN) as the electrolyte system. The CV curves of the investigated DOPO-and triazine-containing polyimides as electrode materials at a scan rate of 50 mV/s are displayed in Figure 9  In a condensed form, the fluorescence of PI1-PI3 was almost quenched (Figure 8b), mostly due to the molecular aggregation that led to non-radiative decay by intermolecular energy transfer. This behavior is opposite to that found for related DOPO-containing polyimides [46] which displayed a strong fluorescence emission in film state, with maximum emission wavelength at about 510 nm. Thus, it can be assumed that 1,3,5-triazine brought the most significant contribution to florescence suppression of present polyimides in solid state.

Cyclic Voltammetry (CV) Measurements
The investigated polymers contain strong electron-withdrawing imide, DOPO, and 1,3,5-triazine heterocycles capable of experiencing electrochemical reduction processes, hence, their electroactivity was investigated by cyclic voltammetry (CV). All CV measurements were performed on polymer-modified ITO-coated glass as the working electrode in an electrochemical cell equipped with Ag/AgCl as the reference electrode, and Pt wire as the counter electrode, in 0.1 M tetrabuthylammonium (TBAP)/acetonitrile (ACN) as the electrolyte system. The CV curves of the investigated DOPO-and triazine-containing polyimides as electrode materials at a scan rate of 50 mV/s are displayed in Figures 9 and 10.  Molecules 2023,   Although PI1-PI3 do not contain electron donating units, their electroactivity was screened in the anodic region to survey the existence of any possible oxidation processes. According to Figure 9a, all polyimides displayed a broad CV band centered at 1.55, 1.48, and 1.37 V vs. Ag/AgCl for PI1, PI2, and PI3, respectively, that can be associated with electron release and formation of radical cations at the NH group of residual, uncyclized amidic acid groups. Similar results were obtained on related polyimides containing triazine heterocycle having hyperbranched structure [55], when clear oxidation peaks appeared in CV curves at different potentials under positive sweeps. However, no comment was provided with respect to the origin of these peaks. The radical cations of PI1-PI3 formed during oxidation were unstable and deactivated in the next CV scans (Figure 9b), denoting weak electroactivity of these polymers in the anodic region. This was expected since the C=O unit of the amide group withdraws electrons, which are no longer available to restore the neutral molecules, and thus, the oxidation processes of the studied polymers are completely irreversible.
The expected n-doping-type character of these polymers is further demonstrated and discussed on the basis of the electroactivity detected in the cathodic region. Hence, all polyimides displayed electrochemical reduction processes that were observed at relatively low potentials (Figure 10a), varying between −1.1 V and −1.48 V vs. Ag/AgCl (the first reduction peak). Although PI1-PI3 do not contain electron donating units, their electroactivity was screened in the anodic region to survey the existence of any possible oxidation processes. According to Figure 9a, all polyimides displayed a broad CV band centered at 1.55, 1.48, and 1.37 V vs. Ag/AgCl for PI1, PI2, and PI3, respectively, that can be associated with electron release and formation of radical cations at the NH group of residual, uncyclized amidic acid groups. Similar results were obtained on related polyimides containing triazine heterocycle having hyperbranched structure [55], when clear oxidation peaks appeared in CV curves at different potentials under positive sweeps. However, no comment was provided with respect to the origin of these peaks. The radical cations of PI1-PI3 formed during oxidation were unstable and deactivated in the next CV scans (Figure 9b), denoting weak electroactivity of these polymers in the anodic region. This was expected since the C=O unit of the amide group withdraws electrons, which are no longer available to restore the neutral molecules, and thus, the oxidation processes of the studied polymers are completely irreversible.
The expected n-doping-type character of these polymers is further demonstrated and discussed on the basis of the electroactivity detected in the cathodic region. Hence, all polyimides displayed electrochemical reduction processes that were observed at relatively low potentials (Figure 10a), varying between −1.1 V and −1.48 V vs. Ag/AgCl (the first reduction peak). Although PI1-PI3 do not contain electron donating units, their electroactivity was screened in the anodic region to survey the existence of any possible oxidation processes. According to Figure 9a, all polyimides displayed a broad CV band centered at 1.55, 1.48, and 1.37 V vs. Ag/AgCl for PI1, PI2, and PI3, respectively, that can be associated with electron release and formation of radical cations at the NH group of residual, uncyclized amidic acid groups. Similar results were obtained on related polyimides containing triazine heterocycle having hyperbranched structure [55], when clear oxidation peaks appeared in CV curves at different potentials under positive sweeps. However, no comment was provided with respect to the origin of these peaks. The radical cations of PI1-PI3 formed during oxidation were unstable and deactivated in the next CV scans (Figure 9b), denoting weak electroactivity of these polymers in the anodic region. This was expected since the C=O unit of the amide group withdraws electrons, which are no longer available to restore the neutral molecules, and thus, the oxidation processes of the studied polymers are completely irreversible.
The expected n-doping-type character of these polymers is further demonstrated and discussed on the basis of the electroactivity detected in the cathodic region. Hence, all polyimides displayed electrochemical reduction processes that were observed at relatively low potentials (Figure 10a), varying between −1.1 V and −1.48 V vs. Ag/AgCl (the first reduction peak).
Since the polymers contain three structural elements with electron accepting capability, namely phthalimide, 1,3,5-trizine, and DOPO, a strong competition between their reduction takes place. This is clearly seen in the case of PI3 where three redox peaks are registered. The first peak in the cathodic region is tentatively associated with phthalimide reduction and generation of radical anions upon acceptance of electrons. The reason why all electron-accepting units of PI2 are reduced in a single step can be found if the stability of the formed radical anions upon reduction is considered. The electron-withdrawing CF 3 groups mostly stabilized these electrochemical species, and therefore, further reduction did not occur. The presence of more reduction peaks in the case of PI1 and PI3 provides an obvious example for the instability of the formed radical anions induced by the electron donor effect of ether and C(CH 3 ) 2 bridging groups from their diimide segments. These experienced further reductions, most likely at the C=O group of DOPO and 1,3,5-triazine. However, their reduction happened only in the first scan with no signature in the next scans, when only a broad peak appeared (Figure 10b), which mostly could be due to the reduction in newly formed entities after radical anions coupling or involvement in secondary reactions. Nevertheless, since the electrochemical activities of PI1-PI3 are dominated by the electroactivity of phthalimide moiety as a function of the surrounding substituents (DOPO and 1,3,5-triazine), these polymers can be considered valuable candidates for use as n-type materials in optoelectronic devices.

Synthesis
The aromatic diamine 6-(2-(4,6-diamino-1,3,5-triazin-2-yl)ethyl)dibenzo[c,e][1,2] oxaphosphinine 6-oxide (TE-DOPO) was obtained following the addition reaction of DOPO to 2-vinyl-4,6-diamino-1,3,5-triazine in toluene, by using DBU as catalyst, as reported earlier [42]. The reactions were carried out in NMP as solvent, and in the presence of benzoic acid as catalyst, at a concentration of 15%, according to the illustrative example further provided for PI3. First, 2 g (0.00567 mol) of TE-DOPO and 13 mL of NMP were added into a 100 mL round-bottom three-necked flask fitted with a nitrogen inlet and outlet. After that, the reaction mixture was heated to 120 • C until the dissolution of the diamine occurred, followed by the addition of 2.949 g (0.00567 mol) of 6HDA, 0.0494 g benzoic acid, and another 15 mL of NMP. The solution reaction was continuously stirred at the reflux temperature of NMP under a strong stream of nitrogen for long time period. The polyimide PI3 was obtained after precipitation in water of the resulting solution, filtration and drying at 100 • C for 6 h in a vacuum oven. The purification of the dark brown solid was performed by Soxhlet extraction with ethanol as solvent, for 48 h. Polyimides PI1 and PI2 were obtained in a similar way by replacing 6HDA with ODPA or 6FDA, respectively, in the form of dark brown powders, as shown in Scheme 1.

Film Preparation
The thin films were obtained from 0.5 wt.% polyimide solutions in NMP which were coated onto glass plates (for use in SEM analysis), quartz (for use in UV-Vis and fluorescence measurements), and indium tin oxide-coated glass (for CV studies) by using the drop-casting technique. The solvent removal was accomplished by subjecting the as-prepared plates to a programmed thermal treatment: 50, 100, 150, 200, and 250 • C, at each temperature for 1 h.

Measurements
The Fourier transform infrared spectroscopy (FTIR) measurements were performed on a FT-IR Bruker Vertex 70 spectrophotometer (Bruker, Ettlingen, Germany) in the ATR mode, by using powder samples.
The NMR spectra were measured with a Bruker Avance III 400 spectrometer (Bruker, Rheinstetten, Germany), operating at 400 MHz for 1 H and 62 MHz for 31 P. The chemical shifts are expressed in δ units (ppm) with respect to the residual peak of the solvent 1 H: DMSO-d 6 2.512 ppm.
Gel permeation chromatography (GPC) was used to obtain the average molecular weights of the polymers by using a ParSEC Chromatography Ver. 5.67 Brookhaven Instruments Corp. (Holtsville, NY, USA) apparatus equipped with refraction and UV detectors and PL Mixed C Column. Measurements were performed with polymer solutions of 0.5% concentration in DMF as solvent. The calibration was accomplished with polystyrene standards of known molecular weights.
The UV-Vis spectra of polyimides were performed on a Shimadzu UV-1280 spectrophotometer (Kyoto, Japan), by using NMP solutions of 1 × 10 −6 M concentrations and thin films coated onto quartz plates.
The fluorescence spectra of polyimides were recorded on a Shimadzu RF-6000 spectrofluorophotometer (Kyoto, Japan). The measurements were carried out with the same polyimide solutions and thin films used for registration of the UV-Vis absorption spectra.
The wide X-ray powder diffraction (XRD) spectra were measured on a Bruker AD8 Avance diffractometer (Billerica, MA, USA), by using Ni-filtered Cu-Kα radiation (λ = 0.1541 nm) at 36 kV and 30 mA. All diffractograms were registered at room temperature, in the range of 2-40 (2θ degrees) on polyimide powder and are reported as smoothed spectra.
The morphology of the thin polyimide films was evaluated by using a Verios G4 UC Scanning Electron Microscope (SEM) from Thermo Fisher Scientific (Waltham, MA, USA). The samples were sputtered with Pt in order to obtain a better image resolution of the film surface.
The optical microscopy photographs were taken using an Olympus BH-2 polarized light microscope (Shinjuku, Tokyo, Japan) at a magnification of 40×, at room temperature.
The differential scanning calorimetry (DSC) measurements were performed on polyimide samples in the form of powders using a Mettler Toledo DSC-type device (Mettler Toledo, Greifensee, Switzerland), by heating 3-5 mg of each sample from 25 to 300 • C, with a heating rate of 10 • C/min, under nitrogen atmosphere.
The thermogravimetric analysis (TGA) was also recorded on polyimide powders, with a Mettler Toledo TGA-SDTA851e device (Mettler Toledo, Greifensee, Switzerland), in nitrogen atmosphere. The mass of the tested samples was in the range of 2.1-3.3 mg, while the heating was applied with a rate of 10 • C/min, between 25 and 800 • C.
The cyclic voltammetry (CV) experiments were performed on a Potentiostat-Galvanostat (PG581, Uniscan Instruments, Buxton, United Kingdom) using 0.1 M TBAP/ACN as the electrolyte system. The CV curves were obtained in a classical electrochemical cell equipped with three electrodes: Pt wire, from Radiometer Analytical with ø = 1 mm and surface area of 0.785 mm 2 as the auxiliary electrode, ITO coated with polyimide as the working electrode, and saturated Ag/AgCl as the reference electrode. All measurements were performed at a scan rate of 50 mV/s in air, at room temperature, after purging the electrolyte solution with N 2 stream.

Conclusions
With the aim to develop new appealing materials with multifunctional properties, we synthesized and investigated high-performance polyimides containing DOPO-substituted 1,3,5-triazine bulky moiety. This structural unit, along with some flexible groups were introduced to tailor the flexibility of the polyimide chains. This molecular design led to excellent organo-solubility in polar organic solvents, which allowed polymer processing from solution into thin films with grain-like morphology. Moreover, due to the synergetic effect of imide, DOPO, and 1,3,5-triazine, these polyimides were endowed with a black appearance, exhibiting a UV-Vis absorption spectrum extended on the overall UV-Vis domain. Accordingly, the thin polymer films were almost opaque, enabling optical transmittance greater than 18% at 500 nm. Because of the presence of versatile functional groups, the polyimides showed good thermal stability, with no glass transition temperature up to 300 • C, as well as green fluorescence under excitation with UV light. The different linkages from the diimide segment had a significant role in the modulation of the electrochemical behavior of the polyimides, mainly due to the electronic effects promoted by these units. The cyclic voltammetry evidenced complex reduction behavior, affected by the stability of the formed radical anions. Nevertheless, these polymers can be considered to be among the potential candidates for use as n-type materials. With the attractive properties reported here, these polymers may find applications as advanced materials in electronics and (opto)electronics, as well as high-temperature materials in engineering fields.

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