What is the role of planarity and torsional freedom for aggregation in a π-conjugated donor–acceptor model oligomer?

Abstract

We compare the aggregation properties of two model compounds, differing in their stiffness, in solution by a combined spectroscopic and computational approach.

Role of Torsional Flexibility in the Film Formation Process in Two π-Conjugated Model Oligomers

“…Concomitant with the disappearance of the 1.95 eV peak, the extinction of the remaining broad band centered at 2.15 eV increases discontinuously and then reduces subsequently while it broadens as expected from increased torsional motion. This is clear spectroscopic evidence that the structured feature at 1.95 eV can be assigned to an aggregate, consistent with the one observed in a CT hexane solution of 2.5 × 10 –4 M at 1.85 eV and 180 K. …”

“…The loss of OD at 2.15 eV in this time range is due to throw-off of excess solution. The shape of the emission spectra differs from the spectra of a dilute solution . This is due to significant overlap of absorption and emission, which results in strong reabsorption.…”

“…CT comprises a cyclopentadithiophene, making it planar and stiff. In contrast, TT has two thiophenes with hexyl side chains connected in a more flexible head-to-head fashion, which induces a twist of about 70° in the otherwise planar structure. , …”

“…We emphasize that the delayed crystallization in TT is a kinetic effect. The molecular geometry in solution is strongly twisted, as the central dihedral angle is about 70°. , The energetic barrier to planarization is around 0.3 eV, which is too large to be overcome spontaneously. However, the molecules adopt a planar conformation in the solid state, , as well as in solution upon aggregation .…”

“…The molecular geometry in solution is strongly twisted, as the central dihedral angle is about 70°. , The energetic barrier to planarization is around 0.3 eV, which is too large to be overcome spontaneously. However, the molecules adopt a planar conformation in the solid state, , as well as in solution upon aggregation . The decisive difference between aggregate formation in solution at low temperatures and film formation upon spin coating is the temporal duration of these processes.…”