Femtosecond fluorescence studies of Auramine O in hybrid sol-gel derived films

Femtosecond fluorescence upconversion experiments have been performed on Auramine 0 (a diphenylmethane dye) in polymethylmethacrylate (PMMA) and in hybrid organic/inorganic sol-gel based films. All the investigated samples showed a fast decay (few picoseconds) and a long decay (hundreds of picoseconds). The fast components are representative of the rapid cooling of the excess excitation energy to the matrices. The long components are representative of the excited-state lifetime of the probed molecules. Auramine 0 in solid matrices showed lifetimes longer than in liquid solutions. Torsional diffusion motions of the two phenyl rings of the molecule are held responsible for the excited-state dynamics. A dynamic Stokes shift has been observed for all samples. The lack of a rise time when detection was on the red side is explained in terms of an adiabatic coupling between emissive and nonemissive excited states, as is the case of liquid solutions. Different spectral shifts in PMMA and hybrid glasses have been measured. A different coupling between the emissive and nonemissive excited states for the two types of matrices is considered.