Properties of oligothiophene polycations

Abstract

The different electronic states of trications and tetracations have been studied for oligomers made of n 3,4-ethylenedioxythiophene units, where n ranges from 2 to 30. Results have been compared with those obtained for neutral, monodications and dications of the same n-oligomers. For the longer oligomer (n ≥ 24) trications, the doublet and quartet states are isoenergetic, while the triplet and quintet states are isoenergetic for tetracations of the same length. Analysis of the geometry reveals three separated polarons for the doublet and quartet states of tricationic n-oligomers, even though that of the former state has been attributed to a heavy spin contamination. In contrast, the structure of tetracations shows double bipolaron character for the singlet state and two polaron pairs for both the triplet and quintet states. Analyses of both charge and spin density distributions support the polaronic or bipolaronic behavior predicted by molecular geometry for the different electronic states. Results have been also used to evaluate the ionization potentials and the π-π* lowest transition energy of infinite polymer chains. Copyright © 2014 John Wiley & Sons, Ltd.