Abstract
This work reports the use of benzophenone, a very well characterized probe, to study new hosts (i.e., modified celluloses grafted with alkyl chains bearing 12 carbon atoms) by surface esterification. Laser-induced room temperature luminescence of air-equilibrated or argon-purged solid powdered samples of benzophenone adsorbed onto the two modified celluloses, which will be named C12-1500 and C12-1700, revealed the existence of a vibrationally structured phosphorescence emission of benzophenone in the case where ethanol was used for sample preparation, while a nonstructured emission of benzophenone exists when water was used instead of ethanol. The decay times of the benzophenone emission vary greatly with the solvent used for sample preparation and do not change with the alkylation degree in the range of 1500–1700 micromoles of alkyl chains per gram of cellulose. When water was used as a solvent for sample preparation, the shortest lifetime for the benzophenone emission was observed; this result is similar to the case of benzophenone adsorbed onto the “normal” microcrystalline cellulose surface, with this latter case previously reported by Vieira Ferreira et al. in 1995. This is due to the more efficient hydrogen abstraction reaction from the glycoside rings of cellulose when compared with hydrogen abstraction from the alkyl chains of the modified celluloses. Triplet-triplet transient absorption of benzophenone was obtained in both cases and is the predominant absorption immediately after laser pulse, while benzophenone ketyl radical formation occurs in a microsecond time scale both for normal and modified celluloses.
Highlights
Diffuse reflectance laser flash photolysis and laser-induced luminescence, both in time-resolved mode or ground-state absorption spectroscopy in the diffuse reflectance mode, are important techniques that have been used by several research groups to study opaque and crystalline systems [1,2,3,4,5,6,7,8]
Ground-state diffuse reflectance absorption spectra for BZP adsorbed onto C12-1500, C12-1700, and microcrystalline cellulose [7] were obtained with the use of an integrating sphere [1, 7]
The photochemistry of BZP onto the modified celluloses is determined by the nature of the adsorption site
Summary
Diffuse reflectance laser flash photolysis and laser-induced luminescence, both in time-resolved mode or ground-state absorption spectroscopy in the diffuse reflectance mode, are important techniques that have been used by several research groups to study opaque and crystalline systems [1,2,3,4,5,6,7,8]. These solid-state photochemical methods have been applied by us to study several organic compounds adsorbed onto different hosts such as microcrystalline cellulose [7, 8], p-tertbutylcalix[n]arenes (n = 4, 6, and 8) and their derivatives [10,11,12], silicalite, cyclodextrins [7, 12, 13], and silica [14]. Apart from the triplet-state luminescence observed in all cases, in the case of “normal” silica the emission of an excited form of hydrogen-bonded benzophenone was detected [14]
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