Charge Delocalization
in a Homologous Series of α,α′-Bis(dianisylamino)-Substituted
Thiophene Monocations
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Abstract
A homologous series of three molecules containing thiophene,
bithiophene,
and terthiophene bridges between two redox-active tertiary amino groups
was synthesized and explored. Charge delocalization in the one-electron-oxidized
forms of these molecules was investigated by a combination of cyclic
voltammetry, near-infrared optical absorption spectroscopy, and EPR
spectroscopy. All three cation radicals can be described as organic
mixed-valence species, and for all of them the experimental data are
consistent with strong delocalization of the unpaired electron. Depending
on what model is used for analysis of the optical absorption data,
estimates for the electronic coupling matrix element (<i>H</i><sub><i>AB</i></sub>) range from ∼5000 to ∼7000
cm<sup>–1</sup> for the shortest member of the homologous series.
According to optical absorption and EPR spectroscopy, even the terthiophene
radical appears to belong either to Robin–Day class III or
to a category of radicals commonly denominated as borderline class
II/class III systems. The finding of such a large extent of charge
delocalization over up to three adjacent thiophene units is remarkable