Spectroscopic studies of the excited-state intramolecular proton and electron transfer processes of methyl benzoate derivatives in cucurbit[7]uril nanocage

Abstract

Listen

Translate article

The encapsulation of four methyl benzoate derivatives (I-IV) in aqueous solution in the presence of the macrocyclic host molecule cucurbit[7]uril (CB[7]) was studied using steady-state, time-resolved and 1H NMR spectroscopic techniques. The significant changes in the steady-state and time-resolved fluorescence spectra (and calculated from them fluorescence decay times) were attributed to the formation of a well-defined, stable 1:1 inclusion complex. To understand the mechanism of supramolecular solvation dynamics, the time-dependent fluorescence Stokes' shifts, represented by the normalized spectral response function c(t), were studied. Our studies have shown that solvation dynamics of water are significantly inhibited inside a cucurbit cavity. The experimental results were also used to calculate, according to Benesi-Hildebrand plot and nonlinear least-squares regression analysis, equilibrium constants of the formed inclusion complex. For all investigated dyes, the K1 value is significantly larger than this previously calculated for I-IV inclusion into cyclodextrins (CDs), indicating the superior host abilities of CB[7] as compared to CDs. The position of the guest molecule inside the host cavity was investigated by 1H NMR experiment. The nature of the 1H NMR shifts for different protons is qualitatively different for methoxy derivative-CB[7] complex (I-CB[7] and III-CB[7] systems) and hydroxy derivative-CB[7] complex (II-CB[7] and IV-CB[7] systems), which indicates that the inclusion complexes of hydroxy and methoxy derivatives with CB[7] are of a difference in nature. The CB[7]-induced chemical shifts clearly demonstrate that for II-CB[7] and IV-CB[7] complexes, benzene ring is fully embedded in the interior of CB[7], while for I-CB[7] and III-CB[7] complexes, only the part of the benzene ring is located within the CB[7] cavity. Finally, 1H NMR chemical shifts and evolution of the steady-state and time-resolved emission spectra clearly indicate that -NMe2 group attached to the III and IV is located outside of the CB[7] portal.