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-propi onyl-2(dimethylamino)naphthalene) are used as in situ fluorescence probes t o monitor the molecular mobility and chemical evolution during sol-gel sili ca 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 i s monitored by interferometry. Results of the spatially resolved fluorescen ce depolarization for films withdrawn from a fresh sol show that the tumbli ng motion of both probes is retained until the final deposition stage when an extensive cross-linking silica framework forms. Intermediate probe mobil ity occurs at an earlier time in films pulled from an aged sol. A compariso n between thin film deposition and monolith formation is made.