Proton transfer and supramolecular complex formation between Nile Blue and tetraundecylcalix[4]resorcinarene—a fluorescence spectroscopic study

Abstract

The interactions between a calixarene host comprising dissociable protons, and an organic dye guest existing in a neutral base and in a protonated form, have been studied on model systems obtained by dissolving Nile Blue base (1) and tetraundecylcalix[4]resorcinarene (2) in two apolar solvents, dichloromethane and toluene. Steady state absorption and fluorescence spectroscopy and fluorescence lifetime measurements were applied as experimental methods. It has been established that the equilibrium composition of these systems is controlled predominantly by three reactions: (1) formation of the weakly bound complex 1·2; (2) simultaneous formation of the tightly bound ionic species with zero net charge 1⁺·2⁻ (1⁺ = protonated form of 1, 2⁻ = monophenolate form of 2); (3) protonation of the latter product in a subsequent step yielding 1⁺·2. The equilibrium constants for these reactions have been determined from the absorption spectra by an iterative procedure. The high value of the equilibrium constant for reaction (2) in toluene (9.8 × 10⁶) is in accordance with the strongly solvatophobic nature of cation 1⁺ in this solvent. The fluorescence lifetime of neutral base 1 increases whereas that of its protonated form, 1⁺, markedly decreases upon complexation. These effects have been interpreted in terms of intra- and intermolecular decay channels.