Impact of Regiochemistry and Isoelectronic Bridgehead Substitution on the Molecular Shape and Bulk Organization of Narrow Bandgap Chromophores

A comparison of two classes of small molecules relevant
to the
field of organic electronics is carried out at the molecular and supramolecular
levels. First, two molecules that differ only in the position of a
pyridyl N-atom within an acceptor fragment are compared and contrasted.
X-ray investigation of single crystals reveals that positioning the
pyridyl N-atoms proximal to the molecules center
changes the molecular shape by bending the molecule into a banana
shape. Second, we demonstrate that the banana shape of the molecule
can be controlled by replacing a Si atom within the dithienosilole
fragment with a C or Ge atom. Here, utilization of cyclopentadithiophene
or dithienogermole as the internal electron-rich unit leads to a decrease
or an increase in the bending of the conjugated backbone, respectively.
Such molecular shape changes alter intermolecular packing and thus
affect bulk properties, leading to large differences in the optical,
thermal, and crystallization properties.