  The processability and optoelectronic properties of organic semiconductors
can be tuned and manipulated via chemical design. The substitution of the alkyl
side chains by oligoethers has recently been successful for applications such
as bioelectronic sensors and photocatalytic water-splitting. The carbon-oxygen
bond in oligoethers is likely to render the system softer and more prone to
dynamical disorder that can be detrimental to charge transport for example. We
use neutron spectroscopy, X-Ray diffraction (XRD), differential scanning
calorimetry (DSC) and polarized optical microscopy to study the effect of the
substitution of n-hexyl (Hex) by triethylene glycol (TEG) on the structural
dynamics of two organic semiconductors: a phenylene-bithiophene-phenylene
(PTTP) molecule and a fluorene-co-dibenzothiophene (FS) polymer.
Counterintuitively, inelastic neutron scattering (INS) reveals a softening of
the modes of PTTP and FS with Hex side chains, pointing towards an increased
dynamical disorder in these systems. However, T-dependent X-Ray and neutron
diffraction, INS and DSC evidence an extra reversible transition close to room
temperature (RT) for PTTP with TEG side chains. The observed transition, not
accompanied by a change in birefringence, can also be observed by quasi-elastic
neutron scattering. A fastening of the TEG side chains dynamics is observed in
the case of PTTP and not FS. We therefore assign this transition to the melt of
the TEG side chains which are promoting dynamical order at RT, but if
crystallising, may introduce an extra reversible structural transition above RT
leading to thermal instabilities. A deeper understanding of side chain polarity
and structural dynamics can help guide materials design and navigate the
intricate balance between electronic charge transport and aqueous swelling,
sought for a number of emerging organic electronic and bioelectronic
applications.
