Supramolecular Interaction-Assisted Fluorescence and Tunable Stimuli-Responsiveness of l-Phenylalanine-Based Polymers

Supramolecular host–guest interactions between randomly methylated β-cyclodextrin (RM β-CD) and side-chain phenylalanine (Phe) and Phe–Phe dipeptide-based homopolymers have been employed for the amplification of fluorescence emission of otherwise weakly fluorescent amino acid Phe. The host–guest complex has been characterized by 1 H and 13 C NMR spectroscopy, two-dimensional rotating-frame overhauser spectroscopy, Fourier-transform infrared spectroscopy, UV–visible spectroscopy, and fluorescence spectroscopy. To gain insights into the origin of fluorescence in homopolymers, density functional theory calculations were performed where phenyl moieties inside the less polar core of β-CD were observed to form a π–π coupled complex resulting in an enhanced emission. Furthermore, the complex-forming ability of Phe, the guest molecule, has been employed in tuning the cloud point temperature ( T CP ) of statistical copolymers derived from side-chain Phe/Phe–Phe-based methacrylate monomers and N -isopropylacrylamide. By varying the co-monomer feed ratios in the statistical copolymer and hence the concentration of RM β-CD throughout the polymer chain, host–guest interaction-assisted broad tunability in T CP of the supramolecular polymeric complex has been achieved.