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Femtosecond Fluorescence Dynamics of a Proton-Transfer Dye Interacting with Silica-Based Nanomaterials

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journal contribution
posted on 15.04.2010, 00:00 by Boiko Cohen, Shufeng Wang, Juan Angel Organero, Leandra Franciscato Campo, Felix Sanchez, Abderrazzak Douhal
2-[5′-N-(3-triethoxysilyl)propylurea-2′-hydroxyphenyl]-benzothiazole (HBTNH2) was covalently bonded to the inner surface of amorphous silica nanoparticles and unmodified and Al-doped mesoporous structured silicate, MCM-41 and (Al)MCM-41, respectively. The photodynamics of these materials was investigated using steady-state and femtosecond fluorescence up-conversion techniques. The guest molecule shows emission dependence on the excitation wavelength. Covalently bonding HBTNH2 to silica nanoparticles affects both the spectral and the dynamical behavior of the dye. The dual-band fluorescence and ultrafast dynamics indicate the possibilities for an excited-state intermolecular proton-transfer reaction between the dye and the silica framework. The observed behavior is assigned to contribution from excited populations of enol, keto, and anion forms. The excited-state dynamics and the emission spectra of the HBTNH2−MCM-41 material clearly reflect the confinement effect imposed by the MCM-41 nanochannels. Furthermore, we find that the photophysical behavior of HBTNH2 is strongly influenced by the Al content in the MCM-41 framework. We also show that, in the mesoporous silica materials with a low Al/Si ratio, the electronically excited-state intramolecular proton-transfer process is prevented and the resulting signals are due to the phenolate-type anion formed in the ground state. Our results demonstrate how the nature of the confinement affects both the steady-state and the time-resolved emission properties of a dye, which will shape the nanophotonics of this kind of nanomaterial.