Synergetic Effect between Structural Manipulation and Physical Properties toward Perspective Electrochromic n-Type Polyimides

Abstract

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.

Assessing the Electrical Characteristics of p–n Heterojunction Prototype Diodes Realized with n-Type Polyimide Materials

“…Since both six-membered imide cycles are not part of the same fused aromatic system, the 1,1-dichloroethene unit does not provide the necessary conjugation strength so that the first oxidation potential is notable lower. All of the above statements are supported by the results reported by our research group in previews works ,, or by other authors …”

“…According to the spectroelectrochemical diagrams shown in Figure S12, these processes are associated with the oxidation of the triphenylamine unit, which led to radical cations ( P1 – P3 ) or dication ( P3 ) species. As a first observation, the oxidation potential values of TPA are higher compared with others reported for triphenylamine-based derivatives , or polyimides, , but in the same range as those containing fused aromatic systems . This hampered oxidation is due to the presence of the strong electron-withdrawing units, namely para -substituted trifluoromethyl groups and ortho -substituted imide cycles, which strongly deplete electrons from the triphenylamine core, making the nitrogen atom less susceptible to oxidation.…”

“…The most important criteria in the evaluation of the thermal stability of these polyimides are provided by the initial decomposition temperature (IDT), which was found in the range of 380–406 °C, the temperatures of 10% gravimetric loss ( T 10% ), estimated to range between 426 and 451 °C, and the temperature corresponding to the maximum rate of decomposition that falls around 500 °C. The recorded values are slightly lower compared with those of other polyimides from the same class, ,, but high enough to meet the requirements for the targeted p–n heterojunction-based applications. This can be attributed to the lower molecular weights of our polyimides.…”

D-A-D Compounds Combining Dithienopyrrole Donors and Acceptors of Increasing Electron-Withdrawing Capability: Synthesis, Spectroscopy, Electropolymerization, and Electrochromism

“…Thus, thiadiazole, if combined with dithienopyrrole in a polymer of the -(D-π-A-π-D)- type, turns out to be an interesting electrochromic material of excellent switching stability, superior to that reported for electrochromes containing oxadiazole acceptors. 76 , 77 …”

Opto-Electronic Properties Modulation Through Iodine Doping of Imine- and Triphenylamine-Based Oligomers

“…For this reason, the conjugated organic compounds offer a great alternative to conventional inorganic-based derivatives. [5][6][7][8][9][10] Among them, poly(imine)s, known as Schiff base polymers or poly(azomethine)s, are of great interest for both industry and academia due to their easy preparation and potential applications in electronics and opto-electronics. 11 Polyimines display interesting properties, like high thermal stability, fluorescence and electrochemical features, para-magnetism, resistance to high energy, liquid crystalline properties, and intrinsic conductivity, as well as complexation capability and biological activity.…”