Long-Chain 3,4-Ethylenedioxythiophene/Thiophene Oligomers and Semiconducting Thin Films Prepared by Their Electropolymerization

A series of soluble H-terminated conjugated oligomers
incorporating
3,4-ethylenedioxythiophene (EDOT) combined with a small number of
thiophene units and ranging in length from four to eight EDOT/thiophene
groups was prepared with the ultimate goal to investigate if facile
formation of a reactive trication radical species would enable electrochemical
polymerization of such long-chain oligomers. Spectroscopic and electrochemical
studies of the oligomers revealed some general dependencies of their
electronic properties on the total number and position of EDOT groups.
It was the number of consecutive EDOT units rather than total number
of these units which was found to have the most profound effect on
electronic energy gap and conjugation length. This influence originates
from the especially strong planarization induced in the conjugated
backbone by the incorporation of EDOT units. In contrast, incorporation
of thiophene units was found to result in loss of the conformational
stabilization. This phenomenon was analyzed using the natural bond
orbital computational approach, which revealed the predominantly hyperconjugative
nature of the EDOT-induced conformational stabilization. Whereas shorter
oligomers, in agreement with the general consensus, were found to
be inert toward electrochemical polymerization due to low reactivity
of electrochemically generated cation radical and dication species,
the longest oligomer showed an unprecedentedly efficient electropolymerization
to yield a stable thin film of an electroactive polymer. The efficient
electropolymerization of the long-chain oligomer was found to be in
agreement with the formation of a reactive trication radical species.
The electronic and spectral properties of the resulting semiconducting
polymer film were studied by various electrochemical and spectroelectrochemical
methods, as well as conductive probe AFM technique, and revealed a
number of unusual features (such as electrical rectifying switching
behavior) consistent with the possibility of increased molecular order
in this material.