Surface Photochemistry: Benzophenone as a Probe for the Study of Modified Cellulose Fibres

Hindawi Publishing Corporation Research Letters in Physical Chemistry Volume 2007, Article ID 18278, 5 pages doi:10.1155/2007/18278 Research Letter Surface Photochemistry: Benzophenone as a Probe for the Study of Modified Cellulose Fibres L. F. Vieira Ferreira,1 A. I. Costa,1, 2 I. Ferreira Machado,1, 3 T. J. F. Branco,1 S. Boufi,4 M. Rei-Vilar,5 and A. M. Botelho do Rego1 1 Centro de Quı́mica-Fı́sica Molecular - Complexo Interdisciplinar, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 2 Laboratório de Quı́mica Orgânica, Departamento de Engenharia Quı́mica, Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emı́dio Navarro 1, 1950-062 Lisboa, Portugal 3 Escola Superior de Tecnologia e Gestão, Instituto Politécnico de Portalegre, Lugar da Abadessa, APT 148, 7301-901 Portalegre, Portugal 4 Laboratoire Sciences des Matériaux et Environnement (LMSE), Faculté des Sciences de Sfax, BP 802-3018 Sfax, Tunisia 5 Laboratoire ITODYS, Université Paris Diderot, CNRS, 75005 Paris, France Correspondence should be addressed to L. F. Vieira Ferreira, Received 18 July 2007; Accepted 27 October 2007 Recommended by Bern Kohler 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 airequilibrated 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. Copyright © 2007 L. F. Vieira Ferreira et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION 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 [1–8]. 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 [10–12], silicalite, cyclodextrins [7, 12, 13], and silica [14]. Benzophenone (BZP) is an extremely useful molecule for probing new hosts. The n → π ∗ absorption transition was found to be very sensitive to the environment characteristics and also exhibits a photochemistry which depends on the host properties [10, 12, 14]. In a recent paper [14], we reported a comparative study of the luminescent properties of BZP adsorbed onto reversed phase silicas, “normal” silica, and silicalite (a de-aluminated zeolite). 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 also detected [14]. 2 Research Letters in Physical Chemistry With this work we intend to contribute to the study of the mechanism by which modified cellulose fibres are able to trap dissolved organic pollutants from water. Indeed, as we have shown previously [15, 16], grafting of linear alkyl chains on the fibre’s surface boosts its capacity to uptake organic solutes from aqueous solution. By ensuring grafted alkyl chains, one gives rise to hydrophobic domains on which organic solutes can be accumulated. The adsorption process occurs by transfer of the sparingly soluble organic molecules from water to organic zones of the modified cellulose where the most significative adsorbent-adsorbate interactions occur. This work reports the use of BZP, a very well characterized probe, to study new hosts (i.e., modified celluloses). The modification consists in grafting with alkyl chains, bearing 12 carbon atoms, by surface esterification with a high density of alkyl chains [15], therefore, transforming the polar surface of the normal cellulose into surfaces with a certain degree of nonpolar character. A comparison of the photochemical behaviour of BZP in modified and nonmodified celluloses was made. 2. EXPERIMENTAL 2.1. Materials Microcrystalline cellulose (Fluka DS0) was used as powdered solid support, as received. Benzophenone (Koch-Light, Scintillation grade) and ethanol (Merck, LiChrosolv grade) were also used as received. The preparation of the modified cellulose fibres started with the use of microcrystalline cellulose and involved an acylation reaction based on a solvent exchange procedure, as described in detail in [15]. The aliphatic anhydrides have 12 carbon atoms (C12) per alkyl chain and the final modified cellulose has 1500 (C12-1500) or 1700 (C12-1700) micromoles of alkyl chains per gram of cellulose [15]. 2.2. Sample preparation Benzophenone adsorption on samples was performed using two methods: the solvent evaporation method for the case of ethanol [14], and, also, adsorption from water [15]. The former method consists in the addition of an ethanolic solution containing the probe to the previously dried powdered solid substrate, followed by solvent evaporation from the stirred slurry in a fume cupboard. In the case of water, the fibers were first swollen for at least two hours, and the addition of BZP was done by adding 500 micromoles of this probe dissolved in ethanol (saturated solution so that the added amount of ethanol was minimized). The water suspensions were kept under agitation for 24 hours and the modified cellulose (with the adsorbed BZP) was removed by filtration. From the initial 500 μmole g−1 (...truncated)