Competitive ultrafast electron and proton transfer reactions within titania and silica mesoporous materials

[EN] We report on UV-visible absorption and emission and ultrafast emission dynamics of 7-hydroxyquinoline (7HQ) encapsulated within titania-doped silicates (mesoporous and unstructured) and purely TiO2 nanomaterials. When titania is incorporated as larger (similar to 50 angstrom) nanoparticles (TiO2-MCM41), the stationary spectra and fluorescence decay times are very similar to those of the regular MCM-41 sample (R-MCM41). In turn, the presence of titania domains in MCM-41(Ti-MCM41) leads to a large increase of the ground state population of anionic (A) and zwitterionic (Z) forms of the adsorbed dye. This is explained in terms of the cooperative effect of Ti-OH groups and the MCM-41 framework interacting with the confined structures. The emission spectra in Ti-MCM41 show a partial fluorescence quenching, whereas a complete quenching occurs when the dye is interacting with pure TiO2 materials (mesoporous TiMeso and nonporous P25). A femtosecond emission study revealed a very short lifetime (10-14 ps) of the Z form. This strong emission quenching is assigned to an electron transfer process from the excited dye to the conduction band of TiO2 material. For 7HQ caged within TiMeso pores, this process occurs in 0.1-0.4 and 0.8-1.2 Ps for A and Z forms, respectively. However, for 7HQ/Ti-MCM41, the emission decays become longer (0.3-5 ps). This result is due to the presence of mixed dynamics gated from 7HQ molecules located close to and far from TiO2 domains, reflecting the heterogeneity of the sample and the competition between proton- and electron-transfer processes. Our results suggest negligible contribution of a direct titania excitation in the observed dynamics of the used materials.
This work was supported by the MICINN through projects: Consolider Ingenio 2010 (CSD2009-0050, MULTICAT), MAT2008-01609, MAT2011-25472, MAT2009-14528-C02-01, and CTQ2010-17988 (PPQ subprogram). We thank Mr. Mikko Ojala for his help at the early stage of this work.