Spectroscopic studies on the supramolecular interactions of methyl benzoate derivatives with p-sulfocalix[6]arene macrocycles.

ABSTRACT

This paper is a continuation of our previous research and aims to further investigate and elucidate the nature and mechanisms of noncovalent supramolecular interactions between four methyl benzoate derivatives (I-IV), which are capable of exhibiting Twisted Intramolecular Charge Transfer (TICT) and/or Excited State Intramolecular Proton Transfer (ESIPT)-type behavior, and chemical and biological nanocavities. Photophysical and photochemical properties of molecules I-IV in aqueous solution in the presence of well-recognized macrocyclic host p-sulfocalix[6]arenes (SCA[6]) have been studied using steady-state, time-resolved and 1H NMR spectroscopic techniques. The changes in the ground- and excited-state spectroscopic characteristics (absorption and fluorescence spectra, time-resolved fluorescence spectra, fluorescence decay times and 1H NMR spectra) undergo significant modifications upon encapsulation of the investigated methyl benzoate derivative in the macromolecular cavity. For the two compounds (I and II), the interactions with the macrocycles with a hydrophobic SCA[6] cavity lead to the formation of stable inclusion complexes with 11 stoichiometry, both in the ground and excited state, while the stoichiometry of the III-SCA[6] and IV-SCA[6] complexes in the ground and excited states is 12. The values of the equilibrium constants have been determined from the spectroscopic data using Benesi-Hildebrand and nonlinear regression procedures. The location of the organic molecule inside the SCA[6] has been investigated by 1H NMR experiments. The changes in macrocyclic compound-induced NMR chemical shifts clearly indicate that the chemical structure of inclusion complexes is very different for methyl benzoate derivative-SCA[6] and methyl benzoate derivative-CB[7] systems. Finally, we have shown, using time-dependent fluorescence Stokes shift, that very fast solvation dynamics of pure water is markedly different from that of the confined water molecule in SCA[6] system.