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.