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 μm 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 Results of the spatially resolved fluorescence depolarization for films withdrawn from a fresh sol show that the tumbling motion of both probes is retained until the final deposition stage when an extensive cross-linking silica framework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.