J-type dimer of Auramine O dye upon encapsulation in cucurbit[8]uril host showing intense excimer emission

Abstract

The host−guest interactions between Auramine O (AuO) dye and cucurbit[n]uril (CB[n], n = 7, 8) were investigated in aqueous solution by 1H NMR, UV/Vis, fluorescence spectroscopic techniques and single crystal diffraction. The results indicate the simultaneous formation of 1:1 and 2:1 inclusion complexes between CB[7] and AuO with two binding constants as 1.17 × 105 M−1 and 3.99 × 102 M−1 by UV/Vis spectroscopy titration (1.20 × 105 M−1 and 5.26 × 103 M−1 by fluorescence spectroscopy) in phosphate buffer (pH 7.4). And a 1:2 homoternary inclusion complex CB[8]·AuO2 with binding constant as 2.95 × 109 M−2 (by fluorescence spectroscopy titration) is detected. Interestingly, the appearance of intense new broad emission band with big red-shift (100 nm) with respect to that of AuO proves the formation of excimer upon encapsulation of AuO molecules in CB[8]. Determination of single crystal structure of CB[8]·AuO2 further discloses the existence of J-type dimer with intermolecular CH···π interactions between two included phenyl groups on AuO molecules. Upon host−guest complexation of AuO with CB[7] or CB[8], the pKa shifts of AuO were measured.