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

The inclusion of a series of luminescent Re(l) complexes in alpha- and beta-cyclodextrin in water is described. The complex's general structure is [L2Re(CO)(3) (4-R-pyridine)] [ClO4] where L-2 represents the alpha-diimine ligands 2,2'-bipyridine or 1,10 phenanthroline. The R groups were selected to span a range of hydrophobicity from -H to -(C(H)2)(12)CH3. These R's showed a variety of binding constants of the complexes to the hydrophobic interior of the P-cyclodextrin. Binding to P-cyclodextrin was accompanied by shifts in the emission lambda(max) to shorter wavelengths, and increases in both the excited state lifetime and luminescence quantum yield. No such effects were observed for alpha-cyclodextrin. ne 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 O-2. Solvent exposure experiments suggest that binding effectively blocks about half the solvent access to the chromophore. (c) 2006 Elsevier B.V. All rights reserved.