Covalent Topochemical Photo-Cross-Linking of Diketopyrrolopyrrole-Based Polymers with Polydiacetylenes toward Tunable Mechanical and Electronic Properties

A solvent-less and additive-free approach was developed for the cross-linking of π-conjugated polymers through the formation of polydiacetylene (PDA) cross-links. Different ratios of conjugation-breaking spacers incorporating 1,3-butadiyne motifs were inserted into semicrystalline diketopyrrolopyrrole-based polymers and reacted under UV light to trigger a topochemical photopolymerization. Upon photo-cross-linking, the formation of PDA cross-links was confirmed by Raman spectroscopy, and the resulting materials were further characterized through various techniques including Fourier transform infrared spectroscopy, atomic force microscopy, X-ray scattering, and UV–vis spectroscopy to evaluate their photophysical and optical properties. Furthermore, the mechanical properties of the PDA-containing semiconducting materials were evaluated via quantitative nanomechanical mapping, which confirmed that an increased cross-linking density rigidified the polymer network in thin films, leading to more brittle yet robust semiconductors. The incorporation of PDA cross-linking bridges across adjacent polymer chains was also shown to preserve the solid-state morphology, leading to uniform cross-linked thin films. Finally, the electronic properties of the cross-linked materials were investigated by the fabrication of organic field-effect transistors, which confirmed that the new cross-linking approach does not significantly alter the properties of the semiconducting materials. The novel cross-linking method for semiconducting polymers described is a promising approach for the processing of organic electronic materials and for the modulation of their thermomechanical properties, thus opening new avenues to design and develop solvent-resistant and rigid materials for emerging organic electronics.