Surface photochemistry: p-Hydroxystilbazol within nanochannels of Na+ and H+ ZSM-5 zeolites

This work reports the surface photochemistry study of p-hydroxystilbazol adsorbed within nanochannels of MFI zeolites, namely in a series of NaZSM-5 and HZSM-5 zeolites where the surface acidity of the host was varied systematically.Laser-induced room temperature luminescence of air equilibrated solid powdered samples of p-hydroxystilbazol adsorbed onto the two sets of zeolites, revealed the existence of multiple emissions of p-hydroxystilbazol, namely as a broad band centered at about 470 nm all cases where the host surface was highly acidic, while in other cases emissions with maxima at about 410, 530 and 660 nm were also detected.The decay times of the p-hydroxystilbazol emission vary greatly going from solution to entrapment within the nanochannels of the ZSM-5 zeolites and roughly increase by an order of magnitude upon inclusion. A lifetime distribution analysis has shown that the longest lifetime for the p-hydroxystilbazol fluorescence emission exists in the case where the 530 nm type emission was detected, i.e. whenever the hydrophobic character of the host increases. p-Hydroxystilbazol entrapped in the more acidic zeolites emits mainly in the protonated form. However, p-hydroxystilbazol may emit from neutral, protonated and zwitterionic forms in the case of the more hydrophobic zeolites.High energy (266 nm excitation and high fluences of the excitation laser (30 mJ per pulse) enabled us to evidence a 660 nm emissive form assigned to the quinoid derived from the zwitterionic form of p-hydroxystilbazol. Delayed fluorescence of p-hydroxystilbazol was also detected for the HZSM-5 and NaZSM-5 entrapment both at room temperature.

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► p-Hydroxystilbazol (STB) was used to study the acidity of H+ and Na+ ZSM-5 zeolites.
► Neutral, protonated and zwitterionic forms of STB fluorescence were detected.
► The cationic form of STB revealed strongly Brönsted acid sites.
► Neutral and zwitterionic forms of STB exist onto the hydrophobic surfaces.