The interaction of rhodamine B with sulfobetaine tetrapentylcalix[4]resorcinarene in the range of millimolar concentrations

The effective binding of Rhodamine B (RhB), a xanthene dye, in solutions of amphiphilic sulfobetaine tetrapentylcalix[4]resorcinarene (C₅-SB) at millimolar concentrations has been found. Using the FT-PGSE NMR technique, it was determined that the binding fraction (P_b) of RhB in the solution with a С(RhB)/С(С₅-SB) ratio of 3.75 mM/5 mM was 0.57, whereas in a solution with an order of magnitude lower concentration, the P_b was only 0.12. This significant difference in interaction is also supported by 2D NOESY and ¹H NMR spectra. Based on fluorimetry data, it appears that the influence of C₅-SB on the spectral properties of RhB increases as the C₅-SB/RhB molar ratio increases, but decreases as the concentration of C₅-SB and RhB decreases. By comparing the spectral properties of RhB in the presence of C₅-SB and tetrapentylcalix[4]resorcinarene with carboxylic (C₅-COO⁻) and trimethylammonium (C5-N⁺Me₃) peripheral groups, an assumption about the driving forces behind RhB, which led to an enhancement of the emission properties of the dye, was made. The study demonstrates the possibility of controlling the emissive properties of a xanthene dye by binding it to a supramolecular associate with amphiphilic calix [4] resorcinarene.