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Synthesis of Novel Periodic Mesoporous Organosilicas Containing 1,4,5,8-Naphthalenediimides within the Pore Walls and Their Reduction To Generate Wall-Embedded Free Radicals
Novel periodic mesoporous
organosilicas (PMOs) containing 1,4,5,8-Naphthalenediimide
(NDI) chromophores as an integral part of the pore walls were synthesized
in acidic conditions, in the presence of inorganic tetraethyl orthosilicate,
using triblock copolymer surfactant Pluronic P-123 as a template.
The NDI precursor, the bridged silsesquioxane <i>N</i>,<i>N</i>ā²-bisĀ(3-triethoxysilylpropyl)-1,4,5,8-naphthalenediimide,
was synthesized by reaction of 1,4,5,8-naphthalenetetracarboxylic
dianhydride with excess 3-aminopropyltriethoxysilane. A series of
samples containing up to 19% (weight %) of NDI were prepared (the
materials were labeled PMONDIs). <sup>13</sup>C and <sup>29</sup>Si
solid-state nuclear magnetic resonance revealed that the NDI moiety
was intact in the PMONDIs and efficiently grafted to the silica network.
Samples with up to 16% NDI load presented an ordered two-dimensional-hexagonal
mesoscopic structure, according to small-angle X-ray scattering, transmission
electron microscopy, and nitrogen adsorption isotherms. Fluorescence
spectra of the PMONDIs showed excimer formation upon excitation, suggesting
high flexibility of the organic moieties. Reduction of PMONDIs with
aqueous sodium dithionite led to the formation of wall-embedded NDI
anion radicals, as observed by the appearance of new visible/near-infrared
absorption bands. The PMONDIs were also shown to be efficient photocatalysts
in the degradation of sulfadiazine, an antibiotic selected here as
a model pollutant, which is usually present in water bodies and wastewater