Investigation of Triphenylamine–Thiophene–Azomethine Derivatives: Toward Understanding Their Electrochromic Behavior

A series
of thiophenoazomethines connected to a central triphenylamine
were prepared. The effect of the type of aminothiophene used to prepare
the conjugated azomethines and the number of azomethine bonds, ranging
from one to three, flanking the triphenylamine on the spectral, electrochemical,
and spectroelectrochemical properties were investigated. Both the
absorption and fluorescence of the azomethines (6–8) derived from 2,5-diaminothiophene (1) were
found to be contingent on the number of the azomethines. The spectral
properties were red-shifted with increasing the number of azomethine
bonds. In contrast, the azomethines (9–11) derived from 2-aminothiophene (2) were not perturbed
by the number of azomethines. However, their spectral properties were
red-shifted relative to their triphenylamine counterparts derived
from 1. The oxidation potentials of the azomethines were
also contingent on structure. They were shifted to more positive potentials
with increasing number of azomethines. The oxidation potentials of 6–8 were less positive than 9–11, owing to the electron-donating effect of
the terminal amine. Reversible oxidation was observed with the triphenylamine
having methyl groups in the 4′-positions. All the compounds
examined underwent color changes upon both electrochemical and chemical
oxidation. The oxidized state was shifted upward of 165 nm for the
series 6–8 relative to the neutral
state, whereas it was shifted at most by 85 nm for 9–11. The most pronounced color changes between the neutral
and oxidized states occurred with the triphenylamine–thiophene–triphenylamine
bisazomethine derivative (12).