Complexation of a cationic pyrene derivative with sulfobutylether substituted β-cyclodextrin: Towards a stimulus-responsive supramolecular material

Abstract

Abstract The self-aggregation of the aggregation prone molecules are avoided by employing a macrocyclic host molecule, such as β-cyclodextrin, which commonly favours the inclusion complex formation with the guest molecule and prevents the self-aggregation of the guest molecule. On the contrary, the substituted host molecules, with multiple charges are reported to promote self-aggregation of guest molecules, specifically when the guest molecules are oppositely charged. One such interesting anionic host molecule is sulfobutylether substituted β-cyclodextrin (SBE7-β-CD), which possesses both extended hydrophobic cavity as well as relatively high negative charge which makes this host capable of forming both stable inclusion complexes with the incoming guest molecule as well as inducing aggregation of guest molecules. However, there are only limited reports of this host inducing aggregation of guest molecules. In the present work, we report the interaction of SBE7-β-CD with a versatile cationic fluorescent probe, 1-pyrenemethyl amine (PMA), using ground-state absorption, steady-state emission, and time-resolved emission measurements. Interestingly, in contrast to various reports, SBE7-β-CD is found to promote the aggregation of PMA molecules. These aggregates are confirmed by the changes in the emission spectra of PMA by emergence of a distinct broad emission band at around 493 nm, which is largely red-shifted from the monomer emission peaks of PMA which appear at 375 and 395 nm. The detailed investigation of the photophysical properties of this assembly discloses that association of PMA molecules in their ground-state is induced by SBE7-β-CD instead of forming excimers in the excited state. These aggregates are found to show significant and reversible response to external stimuli such as ionic strength of medium and temperature, which makes this system to be responsive aggregate assembly to be explored in sensing applications of biochemically important analytes, which has been demonstrated by sensing Lysine and Arginine, important basic amino acids, using this PMA-SBE7-β-CD assembly.