Spectral and photophysical studies of inclusion complexes of some neutral 3H-indoles and their cations and anions with beta-cyclodextrin

Interactions between 3H-indole derivatives, their cations and anions, and microheterogeneous environments like micelles and vesicles have been studied extensively in our laboratory recently. We report herein the interactions of 2-(p-aminophenyl)-3,3-dimethyl-5-cyano-3H-indole (1) and 2-[p-(dimethylamino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (2) with aqueous solutions of beta-cyclodextrin (beta B-CD), studied by absorption and fluorescence steady-state and time-resolved measurements. The stoichiometries of the cyclodextrin:guest inclusion complexes have been determined by steady-state fluorescence measurements. The data reveal that two types of complexes, i.e., 1:1 and 2:1 types are formed. Thermodynamic parameters are calculated at six different temperatures. Spectral characteristics, bandwidths, and photophysical parameters indicate that molecule 2 is better protected against hydrophilic interactions. Protonation reactions carried out at different concentrations of beta-CD show that the protonation is inhibited at the indolic nitrogen, contrary to what was observed in other microheterogeneous media. Time-resolved measurements and global analysis of the results are best described by a discrete triple exponential decay law clearly indicating that the guest molecules experience three different environments in aqueous solutions: bulk water and a stepwise 1:1 and 2:1 (beta-cyclodextrin:guest) inclusion complexation. The effective polarity of the cyclodextrin cavity is equivalent with the polarity of an 80:20 methanol-water mixture at the beta-CD rim where the indolic (tertiary) nitrogen is likely to be located near the ''alcoholic'' secondary rim of the macrocycle.