In situ fluorescence probing of molecular mobility and chemical changes during formation of dip-coated sol-gel silica thin films

Pyranine (trisodium 8-hydroxy-1,3,6-pyrenetrisulfonate) and PRODAN (6-propionyl-2(dimethylamino)naphthalene) are used as in situ fluorescence probes to monitor the molecular mobility and chemical evolution during sol-gel silica thin film deposition by the dip-coating process;Measurements are made on the evolving film as it emerges from the sol at thicknesses changing from <4 mu m for the starting film to 300 nm for the final film. The thickness is monitored by interferometry. Results of the spatially resolved fluorescence depolarization for films withdrawn from a fresh sol show that the tumbling motion of both probes is retained until the final deposition stage when an extensive cross-linking silica framework forms. Intermediate probe mobility occurs at an earlier time in films pulled from an aged sol. A comparison between thin film deposition and monolith formation is made.