Distinctive effects on fluorescence quantum yields of 4-substituted N-methylphthalimides by inclusion complexation with β-cyclodextrins

Abstract

The inclusion constants K for the complexation of 4-substituted N-methylphthalimides with β-cyclodextrins [β-CD and 2,6-di-O-methylated β-CD (DM-β-CD)] were determined from the fluorescence spectra that were enhanced by the inclusion. The structures of the 1:1 inclusion complexes were characterized using 2D ROESY NMR measurements. 4-Substituted N-methylphthalimides showed higher stability for the inclusion complexes formed with the methylated β-CD compared with those with native β-CD (KDM-β-CD/Kβ-CD = 1.2–2.1); this is attributed to the enhanced hydrophobic interaction due to the methylation of the rim OH groups. 4-Substituted N-methylphthalimides having donor–acceptor (D–A) characteristics showed large blue shifts of the fluorescence maxima. In addition, we determined the fluorescence quantum yields (Φ values) for the inclusion complexes using quantum measurement apparatus equipped with a half-moon unit. The Φ values of the D–A N-methylphthalimides were significantly enhanced by the inclusion of β-CDs. The results suggested that the hydrogen-bonding interaction of the OH groups of the β-CD rim with N-methylphthalimides is not operative and that the enhancement of the quantum yield upon inclusion is attributable to the local polarity around the phthalimides inside the hydrophobic CD cavity.