Preparation and Characterization of π-Stacking Quinodimethane Oligothiophenes. Predicting Semiconductor Behavior and Bandwidths from Crystal Structures and Molecular Orbital Calculations

A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has
been investigated for comparison with a previously studied quinoid oligothiophene that has demonstrated
high mobilities and ambipolar transport behavior in thin-film transistor devices. Each new quinoidal thiophene
derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one-electron second oxidation between 1.37 and 1.96 V, and a reversible two-electron reduction between −0.05
and −0.23 V. The solution UV−vis−NIR spectrum of each compound is dominated by an intense (ε ≅
100 000 M-1 cm-1) low energy π−π* transition that has a λmax ranging between 648 and 790 nm. All X-ray
crystal structures exhibit very planar quinoidal backbones and short intermolecular π-stacking distances
(3.335−3.492 Å). Structures exhibit a single π-stacking distance with parallel cofacial stacking (sulfur atoms
of equivalent rings pointed in the same direction) or with alternating distances and antiparallel cofacial
stacking (sulfur atoms of equivalent rings pointed in the opposite direction). Examples of the layered and
herringbone-packing motifs are observed for both the parallel and the antiparallel cofacial stacking. Analysis
of the X-ray structures and molecular orbital calculations indicates that all of these compounds have one-dimensional electronic band structures as a result of the π-stacking. For structures with a unique π-stacking
distance, a simple geometric overlap parameter calculated from the shape of the molecule and the slip
from perfect registry in the π-stack correlates well with the transfer integrals (t) calculated using molecular
orbital theory. The calculated valence (633 meV) and conduction (834 meV) bandwidths for a quinoid
quaterthiophene structure are similar to those calculated for the benchmark pentacene and indicate that
both hole and electron mobilities could be significant.