Acidity of All-Silica MCM-41—Studied by Laser Spectroscopy of Adsorbed Fluorescent Probe Compounds

The polarity within the channel environment of all-silica MCM-41 and the surface acidity were probed by the adsorption of rhodamine-B lactone (RhB-L) and 1-naphthylamine (NA) respectively. The photochemical properties of these probe compounds were studied by laser-induced fluorescence spectroscopy and compared with their properties in solution. The environment within the channels was found to be as polar as methanol. The fluorescence wavelength and decay rate have shown that when adsorbed on all-silica MCM-41, the lactone ring in RhB-L was opened to form the zwitterion (RhB-Z). It was not possible to assess whether RhB-L was protonated by the surface of MCM-41 because the fluorescence of both the RhB-Z and the cationic form RhB-C are almost identical. NA was protonated at the ground state on the surface of all-silica MCM-41. The proton, however, was donated back to the surface upon excitation with laser. From the pKa of the conjugate acid of NA and the fluorescence decay time of the excited state of NA (NA*), the acidity of the surface of the all-silica MCM-41 was estimated to be equivalent to an aqueous perchloric acid with pH 1.8–2.5, depending on the level of loading of NA. The origin of the acid surface was concluded to be the silanol groups known to be present on the surface. The nature of these groups and the influence of the polarity and the solvation effect of the framework on the acidity were discussed.