Rapid Molecular Motion of Pyrene and Benzene Moieties Covalently Attached to Silica Surfaces

Abstract

Remarkably large rotational diffusion rates have been determined for silica-attached 1-pyrene at the solid/air interface through time-dependent fluorescence anisotropy measurements. Diffusion rates D∥ and D⊥ were determined to be 4 × 106 and 2 × 106 s-1, respectively, by treating the motion of the attached moiety as an oblate ellipsoid of rotation. 13C NMR measurements on solid, silica-attached benzene-1-13C, acquired using conventional solution NMR methods, gave remarkable, high-resolution spectra. A lower limit for the rate of phenyl group motion was estimated to be 1.5 × 105 s-1, consistent with the fluorescence anisotropy data measured for silica-attached 1-pyrene. Molecular mechanics models for phenyl attached to representative silanol surfaces predict rotational barriers for phenyl rotations in the 2−5 kcal/mol range.