Femtosecond fluorescence studies of Auramine O in hybrid sol-gel derived films
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Femtosecond fluorescence studies of Auramine O in hybrid sol-gel derived films P. Prospositoa, A. Quatela, H. Zhangb and M. Glasbeekb a Physics Department, University of Rome Tor Vergata Via della Ricerca Scientifica 1, 00133 Rome – Italy b Laboratory for Physical Chemistry, University of Amsterdam Nieuwe Achtergracht 129, 1018 WS Amsterdam - The Netherlands ABSTRACT Femtosecond fluorescence upconversion experiments have been performed on Auramine O (a diphenylmethane dye) in polymethylmethacrylate (PMMA) and in hybrid organic/inorganic solgel 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 O 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. INTRODUCTION In recent years, great attention has been devoted to organic-inorganic composites as new advanced materials. The sol-gel method is a typical procedure to produce such composites in an easy, economic and fast way. Optical studies on hybrid materials doped with organic molecules are interesting for many reasons, e.g., the investigation of the fundamental spectroscopic properties of the dye molecules isolated in the sol-gel environment, the study of dye molecules/matrix energy transfer, the use of luminescent molecules as probes of the sol-gel process and the development of materials with specific optical behavior [1,2]. Quite a few chromophores exhibiting intramolecular conformational changes after ultrafast optical excitation were shown to be of great importance for many processes in biology and chemistry [3-5]. For such molecules, the excited-state dynamics may be strongly influenced by the solvent viscosity or more generally, by the microscopic environment around the dye molecules. The phenomenon has been studied for many chromophores such as photoexcited diand triphenylmethane dye molecules [6-8]. An interesting example of a chromophore showing the above mentioned properties is Auramine O, a diphenylmethane dye, which is weakly fluorescent in low viscosity solvents, such as water, and highly fluorescent in viscous solvents, polymers, protein systems and micellar structures [913]. Recently, time-resolved fluorescence up-conversion and transient absorption investigations have been carried out and more inf
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