Variations of fluorescent molecular sensing for organic guests by regioselective anthranilate modified β- and γ-cyclodextrins

Abstract

Flexible hosts, regioselectively modified, namely disodium 6^A,6^B-, 6^A,6^C- and 6^A,6^D-dianthranilato-β-cyclodextrins (β-1, β-2 and β-3, respectively) and γ-cyclodextrin analogues, disodium 6^A,6^B-, 6^A,6^C-, 6^A,6^D- and 6^A,6^E-dianthranilato-γ-cyclodextrins (γ-1, γ-2, γ-3 and γ-4, respectively) have been synthesized as a sensor for organic guests including terpenoids and bile acids. These host compounds show pure monomer fluorescence, in which β-1 shows an increase in fluorescence intensity on accommodation of guest species. On the other hand, β-3 exhibits an increase in intensity on complexation of bile acids and a decrease in intensity for smaller guests such as terpenoids. Host β-2 exhibits a mixed type of β-1 and β-3. The extent of fluorescence variation with a guest is employed to display the sensing abilities of those hosts. The sensing parameter (ΔI/I ⁰) to describe the sensing ability of the hosts was used. Host β-1 can detect both small and large guests with high sensitivity. Hosts β-2 and β-3 show a similar sensing pattern for guests, while the monoderivative (β-4) can detect small guests with higher sensitivity, but cannot detect larger guests such as bile acids. In the case of larger hosts such as γ-1, γ-2 and γ-3, they show positive parameter values for small guests such as the terpenoids examined, which means the fluorescence intensity increases on accommodation of a guest, whereas γ-4 shows negative parameter values. Host γ-3 exhibits the highest sensitivity for bile acids. The sequence of the binding ability of these hosts is γ-3 > γ-4 > γ-2 > γ-1. The behavior of the appended moieties of those hosts during a host–guest complexation are studied by induced circular dichroism (ICD) spectra and fluorescence spectra. The ICD spectral patterns of β-1, β-2 and β-3 are quite different. On the other hand, the ICD patterns of γ-cyclodextrin analogs are similar. For example, the spectrum of γ-2, alone or in the presence of a guest is very similar to that of γ-3, indicating that the movements of the appended moieties are very similar. The guest-induced variations in the fluorescence or ICD intensity suggest that the appended moieties act as a spacer or hydrophobic cap which enables the cyclodextrin to form a 1∶1 host–guest complex.