Inclusion complexes of cationic xanthene dyes in cucurbit[7]uril

Abstract Improvements in optical properties of organic xanthene fluorophores through molecular encapsulation in cucurbit[7]uril (CB7) should enable better molecular probes and devices to be designed. Although the interactions of several dyes with CB7 have been studied, the data have often been incomplete. Uniformly applied ensemble spectroscopic studies are presented herein, including 1H NMR, UV–Vis, and fluorescence titration experiments of aqueous CB7 complexes with the monocationic xanthene dyes rhodamine 6G (Rh6G, 1), rhodamine B (RhB, 2), rhodamine B benzyl ester (RhBBE, 3), pyronin B (PyB, 4) and pyronin Y (PyY, 5). All of these cationic xanthene dyes formed 1:1 complexes with cucurbit[7]uril as evidenced by NMR data and Job’s plots of fluorescence changes upon addition of CB7. Non-linear regression analysis of the fluorescence titration curves gave precise Ka’s for RhB, RhBBE and PyB between 1.1 × 105 M−1 and 9.1 × 106 M−1. The fluorescence emission intensity of Rh6G was lowered 0.8-fold in the presence of CB7 while the other dyes examined showed an increase between 1.3 and 4.7-fold. NMR titration experiments from 0 to 2.0 equivalents of CB7 per equivalent of xanthene gave in only some cases very clear evidence of inclusion complexation. Non-specific adsorption of these xanthene dyes onto borosilicate glass was very pronounced and could be inhibited by dye inclusion into CB7. Graphical Abstract Pyronin B-Cucurbit[7]uril complex