Localizing molecular probes: Inclusion of Re(I) complexes in β-cyclodextrin

The inclusion of a series of luminescent Re(I) complexes in α- and β-cyclodextrin in water is described. The complex's general structure is [L2Re(CO)3(4-R-pyridine)][ClO4] where L2 represents the α-diimine ligands 2,2′-bipyridine or 1,10 phenanthroline. The R groups were selected to span a range of hydrophobicity from –H to –(CH2)12CH3. These R's showed a variety of binding constants of the complexes to the hydrophobic interior of the β-cyclodextrin. Binding to β-cyclodextrin was accompanied by shifts in the emission λmax to shorter wavelengths, and increases in both the excited state lifetime and luminescence quantum yield. No such effects were observed for α-cyclodextrin. The binding data could be fit by a simple two state model and binding constants ranged from 0.2 to 6.5 mM−1. In contrast to previous studies, the mechanism responsible for the changes in photophysics is attributed to shielding the complexes from solvent interaction rather than from quenching by O2. Solvent exposure experiments suggest that binding effectively blocks about half the solvent access to the chromophore.