Synthesis, Electronic Structure, Molecular Packing/Morphology Evolution, and Carrier Mobilities of Pure Oligo-/Poly(alkylthiophenes)

Monodispersed conjugated oligothiophenes are receiving
attention
in fundamental and applied science due to their interesting optical,
optoelectronic, and charge transport properties. These “low
molecular weight” polymers serve as model structures for the
corresponding polymer analogues, which are inherently polydispersed.
Here we report the synthesis, electronic structure, molecular packing/morphology,
and charge transport properties of monodispersed oligothiophenes with
up to six didodecylquaterthiophene (DDQT) building block repeat units
(i.e., 24 thiophene units). At the point where the effective conjugation
length is reached, the electronic structure showed convergence behavior
to the corresponding polymer, poly­(3,3″-didodecyl-quaterthiophene)
(PQT-12). X-ray crystal structure analysis of the dimer (DDQT-2) showed
that terminal thiophenes exhibit syn-conformations,
similar to the terminal syn-conformations observed
in the trimer (DDQT-3). The dimer also exhibits a rare bending of
the terminal alkyl side chains in order to prevent steric hindrance
with neighboring hydrogens attached to core thiophenes. Grazing incidence
X-ray scattering measurements revealed a morphology evolution from
small molecule-like packing to polymer-like packing in thin films,
with a morphology transition occurring near the effective conjugation
length. Charge transport measurements showed a mobility increase with
decreasing chain length. We correlated the molecular packing and morphology
to charge transport and determined that carrier mobilities are most
sensitive to crystallinity and crystal grain misorientation. This
indicates that molecular weight is not a decisive factor for improved
carrier mobility in the low molecular weight region, but rather the
degree in crystallinity and in-plane crystal orientation. These results
represent a fundamental advancement in understanding the relationship
between conjugation length and carrier mobilities in oligothiophene
semiconductors.