Manifold Fluorescence Enhancement of a Styryl(pyridinium)-chromene Hybrid Dye upon Binding with an Elongated β-Cyclodextrin Cavity.

The interaction of a styryl(pyridinium)-chromene hybrid dye (DSP-C) with the 2-hydroxypropyl-β-cyclodextrin (HPβCD) macrocycle leads to a remarkably large increase (∼310-fold) in its fluorescence intensity, in contrast to the relatively smaller (∼45-fold) enhancement observed with native β-cyclodextrin (βCD). Both macrocycles (βCD and HPβCD) bind with the styryl(pyridinium) as well as the chromene fragments of the hybrid dye, with the simultaneous formation of 1:1 and 2:1 host:guest complexes. However, the binding constant (K) is more than an order of magnitude higher for HPβCD than for βCD. The improved binding affinity of HPβCD is attributed to its elongated and deeper hydrophobic cavity. This is supported well by the optimized geometries of the host-guest complexes. Theoretical calculations also reveal that the energy change due to the release of high-energy water molecules from the host nanocavity is more favorable for HPβCD than βCD, resulting in greater stability of the HPβCD:DSP-C complex. Interestingly, though the fluorescence of DSP-C arises from its styryl(pyridinium) fragment, the complexation of the chromene unit with the host plays a major role in augmenting the fluorescence enhancement of the hybrid dye. The large fluorescence change in the HPβCD:DSP-C system has been utilized for the detection of bile salts by the indicator displacement strategy.