X‑ray
Structural Investigation of Nonsymmetrically
and Symmetrically Alkylated [1]Benzothieno[3,2‑<i>b</i>]benzothiophene Derivatives in Bulk and Thin Films
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Abstract
A detailed structural study of the
bulk and thin film phases observed
for two potential high-performance organic semiconductors has been
carried out. The molecules are based on [1]benzothieno[3,2-<i>b</i>]benzothiophene (BTBT) as conjugated core and octyl side
groups, which are anchored either symmetrically at both sides of the
BTBT core (C<sub>8</sub>–BTBT–C<sub>8</sub>) or nonsymmetrically
at one side only (C<sub>8</sub>–BTBT). Thin films of different
thickness (8–85 nm) have been prepared by spin-coating for
both systems and analyzed by combining specular and grazing incidence
X-ray diffraction. In the case of C<sub>8</sub>–BTBT–C<sub>8</sub>, the known crystal structure obtained from single-crystal
investigations is observed within all thin films, down to a film thickness
of 9 nm. In the case of C<sub>8</sub>–BTBT, the crystal structure
of the bulk phase has been determined from X-ray powder diffraction
data with a consistent matching of experimental and calculated X-ray
diffraction patterns (<i>R</i><sub>wp</sub> = 5.8%). The
packing arrangement of C<sub>8</sub>–BTBT is similar to that
of C<sub>8</sub>–BTBT–C<sub>8</sub>, that is, consisting
of a lamellar structure with molecules arranged in a “herringbone”
fashion, yet with lamellae composed of two head-to-head (or tail-to-tail
as the structure is periodic) superimposed molecules instead of only
one molecule for C<sub>8</sub>–BTBT–C<sub>8</sub>. As
for C<sub>8</sub>–BTBT–C<sub>8</sub>, we demonstrate
that the same phase is observed in bulk and thin films for C<sub>8</sub>–BTBT whatever the film thickness investigated