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.
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.
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