Polymorphism in Bulk and Thin Films: The Curious Case of Dithiophene-DPP(Boc)-Dithiophene

Polymorphism
is an interesting phenomenon critical to our understanding
of structure–property relationships in solid-state functional
materials. We report the synthesis and structural characterization
of two polymorphic forms of a DPP-Boc derivative, DPP4T-α and
DPP4T-β, as well as the study of their optical properties. Thin
film studies have been carried out to identify the specific polymorphic
form that exists in contact with the substrate and also to obtain
a better understanding of the interface morphology with respect to
crystal packing. The two polymorphs, DPP4T-α and DPP4T-β,
have been structurally characterized using single crystal/powder X-ray
diffraction data with a detailed analysis of Hirshfeld surfaces and
fingerprint plots facilitating a comparison of the type and nature
of intermolecular interactions in the supramolecular architectures.
DPP4T-α crystallizing in a space group P21/c with Z′ = 0.5
interlinked via C–H···O/π and π···π
interactions forms 2D herringbone sheets. In the polymorph DPP4T-β
with space group P1 and Z′ = 1, the crystal
packing is stabilized by CH···π and π···π
interactions forming a columnar network. From considerations of density
and from lattice energy calculations, it can be concluded that the
α-form is more stable in bulk. In thin films the β-form
was found to be more stable. This work gives a unique example where
the polymorphism could be identified and separated both in the bulk
and in thin films. This study on the structural effects of polymorphism
is useful for further development of DPP-based materials and also
for targeted design of other functional materials.