Inclusion complexation of 4′,6-diamidino-2-phenylindole (DAPI) with cucurbit[7]uril and cyclodextrins (native β-, 2,6-di-O-methylated β-, and γ-cyclodextrin): characteristic inclusion behaviour and fluorescence enhancement

Herein, we investigated the inclusion complexation of a fluorescent probe [4′,6-diamidino-2-phenylindole (DAPI)] using various cyclodextrins (CDs) and cucurbit[7]uril (CB7). Using the continuous variation method, DAPI was found to form a 1:1 inclusion complex with CDs and CB7. 1H-NMR and 2D ROESY (1H-1H rotating frame nuclear Overhauser effect) spectroscopy indicated that the inclusion of DAPI by the CDs and CB7 occurs via the encapsulation of the phenyl group and indole moiety of DAPI. The stability of the CB7 inclusion complex was higher than that of the CDs: the stability was attributed to the extra interaction formed between the dicationic dye (DAPI) and polar carbonyl groups at the portals of CB7. Furthermore, we determined the fluorescence quantum yield (Φ) of the inclusion complexes. The Φ values of DAPI were significantly enhanced upon its inclusion by CB7 and increased as the empirical solvent polarity parameter (ET(30)) decreased. Based on these results, we concluded that the polarity of the microenvironment and protonation ability of the excited state of DAPI play important roles in emission efficiency.