Guest—host complexes of spin-labeled indoles with cyclodextrins in the solid phase: an esr study

Abstract

Inclusion complexes of spin-labeled pyrrolidine-(1) and piperidine-containing (2) indole derivatives with β-cyclodextrin and γ-cyclodextrin (CD) were prepared in the solid phase and studied by ESR in a wide temperature interval. For all complexes and free spin probes in solvents of different polarity, local environment polarities of the NO group of the guest molecules were determined from the outer extrema separations in the ESR spectra measured at 77 K. From analysis of the Saturation Transfer (ST) ESR spectra and temperature dependences of linear ESR spectra of the complexes it follows that both guest molecules in γ-CD undergo rapid librations. The libration amplitude of the p-orbit axis of the NO group in molecule 2 increases with temperature and reaches about 16° at 333 K. The ESR lineshape of the β-CD complexes depends on the spin probe/β-CD molar ratio (ρ) even at ρ < 0.01. Lineshape analysis of the spectra recorded at different ρ showed that they consist of two components, one of them corresponding to strong spin-spin interaction between guest molecules and the other corresponding to almost absence of this interaction. The spectral components can be attributed to microphases of the complexes and to isolated complexes in the β-CD matrix, respectively. Simulation of the ST ESR and linear ESR spectra of the magnetically diluted complexes showed that the guest motion inside the CD cavity is better described by rotational jumps rather than Brownian diffusion in an orientation potential. In the temperature range 238—333 K, the rotational frequencies of 1 and 2 are in intervals 1.8·107−6·107 s−1 and 4·107−1.3·108 s−1, respectively. The rotation occurs over the whole solid angle. Significant differences in the character of molecular dynamics in the γ-CD and β-CD complexes can be explained by different stoichiometry, namely, 1: 1 for the former and 2: 1 for the latter and by different orientation of guest molecules in the complexes. In both cyclodextrins the rotational mobility of molecules 2 is higher than that of 1 owing to intramolecular conformational transitions in the piperidine ring of 2 and steric hindrances produced by the methyl group in 1.