Encapsulation of rhodamine-6G within p-sulfonatocalix[n]arenes: NMR, photophysical behaviour and biological activities

Abstract

The inclusion complexes of rhodamine-6G (Rh6G) dye within p-sulfonatocalix[4]arene (SCX4), p-sulfonatothiacalix[4]arene (TSCX4), p-sulfonatocalix[6]arene (SCX6) hosts has been characterized using the ¹H NMR, 2D NMR NOESY, infrared, fluorescence experiments and scanning electron microscopy analysis. The inclusion complexes show encapsulation of the phenyl ring of the Rh6G guest within the SCXn (n = 4 and 6) or TSCX4, with its ester functionality protruding outside the cavity along the direction bisecting two adjacent sulfonato groups of the macrocyclic host; the inference that has been corroborated through the density functional theory. The aromatic protons of Rh6G confined within the host cavity of the complex reveal shielded signals in the measured NMR spectra. The formation of inclusion complexes has been confirmed through 2D NMR NOESY experiments. Fluorescence experiments demonstrated that the quenching constant of the SCX6 complex was ∼10⁴ times larger than those for the SCX4 and TSCX4 complexes. Biological activity measurements further revealed that the Rh6G⊂SCX4 complex possesses remarkable antiproliferative and antimicrobial activities.