Systematic investigation of the synthesis and light-absorption broadening of a novel diketopyrrolopyrrole conjugated polymer of low and high molecular weight with thermo-labile groups†
To strengthen the π–π stacking interaction of conjugated polymer chains and study the absorption broadening effect, a narrow band-gap copolymer with high (DPP-Car-Boc-H) and low (DPP-Car-Boc-L) molecular weight, comprising alkyl substituted diketopyrrolopyrrole (DPP) and t-butoxycarbonyl (t-Boc)-protected carbazole (Car-Boc) units, is designed and synthesized. The absorption broadening is investigated on the basis of molecular weight, solution and solid state, solution temperature and nonsolvent effect. FTIR analysis confirmed the decomposition of t-butoxycarbonyl protecting groups after thermal annealing treatment. It is found that the removal of thermo-labile groups (t-Boc) at 180 °C and the liberation of active NH groups further broaden the light-absorption spectra in thin films due to the intensified π–π stacking interaction between polymer chains which is verified through TGA, SEM and XRD. After thermal annealing, SEM images exhibited cubic crystals spread uniformly and the increased order degree of aggregation was verified further. Additionally, the impact of electronic structure change on the photo-response broadening effect is excluded by Gaussian molecular simulation. This work provides a viable approach and convincing evidence that thermal fabrication strengthens conjugated polymer chain stacking through the large π-delocalized plane and provides broad absorption properties for use in photoelectric devices.