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A novel aromatic diamine containing 1,3,4-oxadiazole-substituted triphenylamine (TPA) was successfully synthesized and structurally characterized. Based on this, a series of three aromatic polyimides with high thermal stability were prepared and processed in thin coatings with a grain-like morphology. Their ability to form a dual intramolecular charge transfer complex between TPA and 1,3,4-oxadiazole in the side chain and TPA and phthalimide in the main chain was proven. Both the optical effects and electronic properties were found to be modulated by the competing electron-withdrawing character of 1,3,4-oxadiazole/imide units. The electrochemical investigation suggested their n-type redox capability and electron transporting characteristics, a statement supported by current–voltage measurements of some indium tin oxide/poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate/polyimide/eGaIn prototype devices. Thereby, the present polyimides enable the pathway toward alternative n-type materials currently used in optoelectronics devices.

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This work focuses on the development of novel six-membered polyimides, with special concern on the effect of the structural variation on their optoelectronic characteristics and electrochromic performances in electrochromic devices. To this aim, a new aromatic diamine incorporating diphenylamine substituted at the nitrogen atom with a 2-(4-methoxyphenyl)-1,3,4-oxadiazole segment was synthesized and fully characterized. This diamine was used in the one-step high-temperature polycondensation reaction with naphthalene- or perylene-based dianhydrides to produce highly conjugated polyimides. To highlight the effect of structural variation in the diamine segment on the main physical properties, {4-[5-(4-fluorophenyl)-1,3,4-oxadiazol-2-yl]-phenyl}-bis-(4-amino-phenyl)-amine was reacted with the same dianhydrides to obtain related polyimides. The obtained polymers displayed high thermal stability, up to 423 °C, and could be processed into thin coatings with morphologies contingent on the macromolecular architecture. With the ability to promote a dual intramolecular charge transfer complex between the diphenylamine/triphenylamine unit and both 1,3,4-oxadiazole in the side chain and naphthylimide or peryleneimide in the main chain, these polyimides experienced challenging photo-optical properties, which were driven by the competition between the electronic effects. The HOMO–LUMO energies evaluated from cyclic voltammetry experiments suggested ambipolar electronic transport characteristics, with a prevalent n-doping capability. Since the oxidation processes of perylene-based polyimides were accompanied by color switching between neutral and oxidized states, preliminary testing in electrochromic devices was carried out. Besides the stable anodic coloring with good response times, the recovery of electrochromic devices was ∼90% after 30 cycles. This excellent stability is mainly promoted by the more extended conjugation provided by the peryleneimide system.

Three polyazomethines and their corresponding model compounds were protonated with trifluoroacetic acid, and its effect on their optical (UV–vis absorption and photoluminescence) properties and electrochemical behavior has been studied, in the context of the presence and elongation of alkoxy side groups. Moreover, the effect of environment dielectric constants (i.e., polarity of the solvent) was considered on the doping process. It has been proven that the presence of alkoxy side groups is necessary for protonation to occur, while unsubstituted compounds undergo hydrolysis to constitutive units. Acid doping of imines consisting of alkoxy side chains has resulted in a distinct bathochromic shift (>200 nm) of the low-energy absorption band. Even the length of alkyl chains has not affected the position of shifted bands; it has been observed that azomethines with smaller, methoxy side groups undergo the protonation process much faster than their octyloxy-substituted analogues, due to the absence of steric hindrance. The electrochemical studies of these alkoxy-substituted imines have indicated a better p-type behavior after protonation induced by the capability of the protonated form to easily oxidize in acetonitrile and to generate the native molecules. The environmental polarity has also had impact on the doping process, which took place only in low-polar media.

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