The liquid crystal induced J-type aggregation of diketopyrrolopyrrole derivatives in monolayer

The Langmuir and Langmuir-Blodgett techniques have been widely used for nearly a century for the production of molecular thin layers with controlled packing and intermolecular distances. Unfortunately, the Langmuir layers of non-amphiphilic molecules have unstable and unrepeatable parameters. The addition of chemical compounds such as liquid crystals can stabilize and prevent aggregation in the Langmuir layer. The aim of the study was to obtain and characterize Langmuir and Langmuir-Blodgett layers of selected dithiophene diketopyrrolopyrrole (DPP) derivatives and to improve their parameters by forming mixtures with 4‑octyl‑4′‑cyanobiphenyl (8CB). Two DPPs with alkyl chains symmetrically and asymmetrically attached to pyrrole unit were chosen for the investigation. The Langmuir layer of the asymmetrical DPP was unstable over time. For the symmetrically functionalized DPP, after transferring onto solid substrates, agglomerates formation was clearly seen. For DPPs/8CB mixtures, a significant improvement in stability over time was achieved. The confocal microscopy investigation revealed almost complete removal of agglomerate structures in the symmetrical DPP with 8CB. The results proved the miscibility of components at air/water interface and the stabilizing effect of 8CB on DPPs layers. Furthermore, the addition of 8CB forces DPP J-type aggregation in the Langmuir film. Above all, the stable layers with repeatable parameters, made of indistinctly amphiphilic DPP dyes, were shown to form at the air/water interface and on solid substrates.