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A Synergistic Coassembly of Block Copolymer and Fluorescent Probe in Thin Film for Fine-Tuning the Block Copolymer Morphology and Luminescence Property of the Probe Molecules

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journal contribution
posted on 22.01.2013, 00:00 by Biplab K. Kuila, Chanchal Chakraborty, Sudip Malik
Here, we investigate a synergistic coassembly of a block copolymer, polystyrene-b-poly­(4-vinylpyridine) (PS-b-P4VP), and a fluorescent probe molecule, pyrenebutyric acid (PBA), in thin film using block copolymer supramolecular assembly (SMA) strategy for a wide range of compositions tuned by varying the molar ratio (r) of PBA and 4VP units. The PBA molecules form supramolecules with PS-b-P4VP through H-bonding between the carboxylic acid group of 1-pyrenebutyric acid and pyridine ring of P4VP. For compositions r = 0, 0.1, 0.25, and 0.5, the SMAs exhibit cylindrical morphology, whereas for r = 0.75 and 1, the SMAs generate lamellar morphology. Interestingly, it has been observed that the orientation of the microdomains depends on the solvent used for annealing and can be switched reversibly on exposing the SMA films to corresponding solvent. In a nonselective solvent like chloroform, the microdomains are oriented normal to the substrate, whereas in a selective solvent like 1,4-dioxane, the microdomains are oriented parallel. The synergistic coassembly of PS-b-P4VP and PBA in SMAs with higher molar ratio results in a structure-within-structure pattern characterized by two length scales from phase separation of block copolymer and parallel π–π stacking of the pyrene moiety of PBA molecules inside the comb block. The photophysical properties of PBA in different SMAs of varying composition were studied both in solution and in thin film state and compared to pure PBA. The UV–vis study shows the H type of aggregation of PBA molecules inside the comb block by parallel stacking of the pyrene units, and the PBA molecules orient parallel to the substrate when the microdomains are oriented normal to the substrate. The pure PBA molecules in thin film exhibit excimer emission extensively, whereas the PBA molecules in different supramolecular assemblies exhibit emission ranging from monomer to mixture of monomer and excimer. The SMA shows more intense fluorescence emission compared to pure PBA both in solution and in thin film.