Methylene blue–calixarenesulfonate supramolecular complexes and aggregates in aqueous solutions

The complexation of methylene blue (MB) by calix[n]arenesulfonates (SCAn, n = 4, 6, 8) was studied in aqueous solutions with various ionic strengths, using absorption and fluorescence spectroscopy and laser flash photolysis. The simultaneous reactions MB + SCAn ↔ MB·SCAn (1) and MB·SCAn + MB ↔ MB2·SCAn (2) were observed. The equilibrium constants K(1) and K(2) are high (105–107), their dependence on the ionic strength can be described by the extended Debye–Hückel law. K(2) > K(1) for all the three systems, in accord with the strongly aggregating nature of MB. In MB–SCA8 mixtures complexes with more than two MB units were also detected. The fluorescence of the dye is partially or completely quenched in the complexes, the formation of triplet state complexed MB cations is negligible. At low SCAn concentrations and ionic strengths the formation of colloidal aggregates was detected by resonance Rayleigh scattering.