Molecular Motion and Environmental Rigidity in the Framework and Ionic Interface Regions of Mesostructured Silica Thin Films

Abstract

The rigidochromic probe molecule ClRe(CO)3-2,2‘-bipyridine is used to study the properties of mesostructured silica thin films. The probe serves dual functions: its emission band maximum shifts to higher energy with increasing rigidity of its environment and simultaneously the fluorescence depolarization measures the mobility of the probe molecule itself. Dip-coated, mesostructured 2-d hexagonal thin films doped with ClRe(CO)3-2,2‘-bipyridine are prepared by a one-step, one-pot synthesis procedure using tetraethoxysilane as the silicon source and cetyltrimethylammonium bromide as the structure directing agent. Amorphous silicate films prepared without the surfactant are used as controls. In the freshly prepared mesostructured films, the probe molecules reside in both the silica framework (in a rigid environment) and the ionic interface region near the surfactant headgroups (in a nonrigid environment) as shown by the rigidochromic shift. The fluorescence depolarization confirms the rigidity; the molecule is fixed in position in the silica framework but rotates in the interface region. The spectra of the probe in dried and frozen films show that maximum rigidity is achieved at low temperature.