Molecular motion and environmental rigidity in the framework and ionic interface regions of mesostructured silica thin films

The rigidochromic probe molecule ClRe(CO)(3)-2,2 ' -bipyridine is used to s tudy the properties of mesostructured silica thin films. The probe serves d ual functions: its emission band maximum shifts to higher energy with incre asing rigidity of its environment and simultaneously the fluorescence depol arization measures the mobility of the probe molecule itself. Dip-coated, m esostructured 2-d hexagonal thin films doped with ClRe(CO)(3)-2,2 ' -bipyri dine are prepared by a one-step, one-pot synthesis procedure using tetraeth oxysilane as the silicon source and cetyltrimethylammonium bromide as the s tructure directing agent. Amorphous silicate films prepared without the sur factant are used as controls. In the freshly prepared mesostructured films, the probe molecules reside in both the silica framework (in a rigid enviro nment) 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 t he probe in dried and frozen films show that maximum rigidity is achieved a t low temperature.