Rapid molecular motion of pyrene and benzene moieties covalently attached to silica surfaces

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. 1-¹³ C 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.