Fluorescence decay study of anisotropic rotations of substituted pyrenes physisorbed and chemically attached to a fumed silica surface

Optical polarization spectroscopy has been used to investigate molecular dynamics of the fluorescent probes 1-pyrenebutanol and 1-pyrenebutyric acid at the solid/air interface of cab-o-sil (fumed silica nanoparticles). Pyrenebutanol was chemically attached to the surface of cab-o-sil through a silyl ether bond, while pyrenebutyric acid was physisorbed on the surface. Dynamics of fluorescence depolarization for both molecules was studied under steady-state and time-resolved conditions. Low and high loadings of the probe molecules were used in our studies to examine the dynamics of the probes motion and excimer formation. Our data indicate that excimer formation is mostly static in nature for the adsorbed probe, but shows more dynamic character for the chemically attached probe with spacer molecules present. Fluorescence lifetimes were dependent on the concentration of the probe molecule and became shorter at higher surface loadings for both the chemically attached and physisorbed probes. For the chemically attached probe, the presence of excess co-attached biphenyl molecules was found to provide a 2-D solvent-like environment.