Applied Catalysis A: General, Vol.570, 339-347, 2019
UV-vis spectroscopy combined with azastilbene probe as a tool for testing basicity of mesoporous silica modified with nitrogen compounds
N-bromobenzyl substituted (E)-4'-hydroxy-4-stilbazolium bromide (NBSB), a representative of azastilbene compounds which reveals strong solvatochromism, was used as a probe molecule for testing basicity of nitrogen organic compounds grafted on mesoporous cellular foam silica (MCF). Easy deprotonation of this compound in contact with basic species was the reason for its use for probing the basicity of AP/MCF (AP - (3aminopropyl) trimethoxysilane), Im/MCF (Im - N-trimethoxysililpropylimidazole) and Az/MCF (Az - (3-azidopropyl)(triethoxy)silane). NBSB and its deprotonated form were analyzed by UV-vis spectroscopy measurements in solutions and UV-vis in solid materials. Ab-initio calculation allowed distinction between the preference of NBSB for interactions with SiOH on the support surface and with nitrogen species of the modifier. The NBSB protonation - deprotonation equilibrium dependence on pH was indicated. The role of solvent was considered. In water indirect probing of basicity by NBSB was observed via abstraction of proton from solvent by the modifier and deprotonation of NBSB due to the increased pH of the solution. In acetonitrile direct basicity probing took place in which NBSB was adsorbed in MCF pores. AP/MCF and Im/MCF caused deprotonation of NBSB, as monitored by UV-vis reflectance spectra. This effect was stronger for AP/MCF, which showed higher basicity. Az/MCF could not deprotonate NBSB in either water or acetonitrile.