Pore selectivity and electron transfers in HZSM-5 single crystals: a Raman microspectroscopy mapping and confocal fluorescence imaging combined study

Abstract

After mixing HZSM-5 single crystals and solid t-stilbene, micro Raman mapping and micro fluorescence emission imaging provide evidence of the adsorption, spontaneous ionization and diffusion of the guest into the pores of zeolite. The experiments provide evidence of both radical cation and subsequent charge transfer complexes (CTC). Using a set of excitation wavelengths, Raman spectra of different charge separated states (CSS) were identified by taking advantage of the resonance effect; the correct assignment of the species stabilized in the single crystal is confirmed by analysis of the reference CSS stabilized in powder samples. This assignment is also supported by the correlation of fluorescence emission images obtained for single crystals at different excitation wavelengths and the fluorescence excitation–emission matrix (EEM) plots obtained for the powder reference samples. Thanks to complementary fluorescence and Raman imaging techniques, the spatial distribution of the CSS was found to be mainly located at the tips and in the intergrowth after long times of contact which are correlated with high loading rates. Unusual Al zoning at the crystal extremities and higher polarity in the sinusoidal pores might be put forward to explain these features.