Effect of azomethine structural modification of electrochromic performance

Abstract

Towards understanding the structural requirements for extending the anodic reversibility, a series of conjugated azomethine triads end-capped with amides were prepared. The effect of a phenyl and two triphenylamine end-capping groups on the oxidation potential, electrochemical reversibility, and color were assessed. The core aromatic of the conjugated framework was also varied to include thiophene, EDOT, and two triphenylamine derivatives. With the amide end group, the anodic potential increased by 270 mV relative to the amine termini counterpart. An irreversible anodic process was observed with the unsubstituted triphenylamine end group, resulting in an electroactive film deposited on the working electrode. In contrast, desired reversible oxidation was possible with an 4,4′-dimethyltriphenylamine end-capping group. The anodically induced reversible perceived color change was from orange to violet when the central aromatic was a triphenylamine. The absorption of the oxidized intermediate was contingent on the capping-group and the central aromatic and it was upwards of 220 nm red-shifted from the neutral state. Similar spectral shifts were found with the EDOT and thiophene cores. In these cases, the red color of the intrinsic chromophore bleached upon oxidation and it was replaced with a blue color with a broad absorption at ca. 650 nm. Consistent differences in the transmittance with repeated oxidation/neutralization cycles were possible upwards of 6 hours when end-capping with the 4,4′-dimethyltriphenylamine, regardless of the aromatic core.