H. Cardy

Diffuse reflectance UV-Visible spectroscopy for the qualitative and quantitative study of chromophores adsorbed or grafted on silica

Diffuse reflectance UV-Visible spectroscopy (DRUV) is used to study the adsorption and grafting of the pyrene derivative 1 on different types of silicas. For the adsorbed silicas, the formation of dimeric aggregates is evidenced both by DRUV and by fluorescence spectroscopy. The processing of the UV spectrophotometric data allows, as in solution. the determination of apparent equilibrium constants and of the spectra of the dimer. From these results, two sets of silicas are distinguished according to the amount of dimer formed and to its UV spectrum. Lower amounts of dimeric species are found on silica with either large pore size or low particle size. The spectrum in this case is only slightly redshifted relative to that of the monomeric compound. When the pore size decreases, dimer formation is more significant and its spectrum is more characteristic. The results obtained for adsorbed silicas were quantitatively treated and the Kubelka-Munk function, F(R), plotted against the concentration. Attempts to estimate the loading of silicas grafted with 1 were undertaken. It appears that small particle size silica gives rise to the highest loading and to the best quality DRUV spectra as a consequence of a minor amount of fluorescent dimer. On the other hand, it is shown that the loading of grafted silica is difficult to control by the used grafting process and that milder conditions are required

Photo-oxidation of di-n-butylsulfide by various electron transfer sensitizers in oxygenated acetonitrile

The selective activation of different photosensitizers has been carried out under comparable conditions and their efficiency towards di-n-butylsulfide oxidation in oxygenated acetonitrile compared from the product distribution after 150 minutes of irradiation. As expected, the best selectivity towards sulfoxide is obtained with a conventional energy transfer sensitizer such as Rose Bengal (RB), but also with a quinone with a low-lying triplet state, 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil or CHLO) and with 9,10-dicyanoanthracene (DCA). More significant yields in sulfonic and sulfuric acids are obtained under sensitization with 9,10-anthraquinone (ANT) or a derivative of benzophenone, 4-benzoyl benzoic acid (4-BB), with which additional experiments were carried out in order to discuss the involvement of either singlet oxygen or superoxide radical anion. Triphenyl pyrylium tetrafluoroborate (TPT+) is inefficient under the selected conditions and sulfide photo-oxidation can only be achieved with higher TPT+ concentrations with simultaneous total TPT+ bleaching. With TPT+, 1,2,4,5-tetracyanobenzene (TCNB) and TiO2, the product distribution and the low selectivity as well as the formation of numerous common by-products are indicative of radical mechanisms. All these results are discussed according to the possible formation of activated oxygen species, such as singlet oxygen, superoxide radical anion or alkylperoxy radicals.

Oxidation of sulfides and disulfides under electron transfer or singlet oxygen photosensitization using soluble or grafted sensitizers

Two different photosensitizers, 9,10-dicyanoanthracene (DCA) and benzophenone (BzO) or a silica bound derivative (BzO-Si) have been compared for the photooxidation of di-n-butyl sulfide and di-n-butyl disulfide. With either photosensitizer, sulfide photooxidation in acetonitrile leads very efficiently to sulfoxide, with sulfone and disulfides as by-products. Although an electron transfer mechanism has previously been established starting with DCA, our results are indicative of two competitive mechanisms using BzO as the photosensitizer, instead of singlet oxygen addition and electron transfer. The more sluggish photooxidation of disulfides leads to a complex mixture of products, among which n-butyl butanethiosulfonate and strong acids (alkylsulfonic and sulfuric) are the major ones. The relative ratio thiosulfonate: acids depends, among other factors, on the medium polarity with acid formation favored starting with BzO or BzO-Si in a methanol-water mixture. An electron transfer mechanism only can account for the observed products Superoxide anion, the formation of which is much easier starting from BzO than from DCA, is suggested to play a crucial role in this oxidative radical pathway. Starting from disulfides, grafted benzophenone is more efficient for acid formation than its soluble counterpart. As this photosensitizer can easily be recycled, an easy and smooth way to acid formation is thus available, provided that the reaction solvent is properly chosen.

Organic sulfides photooxidation using sensitizers covalently grafted on silica : towards a more efficient and selective solar photochemistry

A visible light sensitized oxidation of organic sulfides using a derivative of 9,10 dicyanonoanthracene (DCA) covalently grafted on silica (DCAC-Si) is studied. The reaction is more efficient than the corresponding homogeneous reaction and some improvement in the rate of product formation is noticed. These observations are discussed. In the case of di n-butylsulfide, the sulfoxide/(sulfone + disulfide) selectivity is increased. Diphenyl sulfide leads selectively and efficiently to diphenylsulfoxide. Advantages of the process, using reusable sensitizer and solar light are emphasized

Chemisorption of HSi(OH)3 on silica surfaces: an ab initio periodic study

Abstract The study of the grafting of trialkoxysilane R′Si(OR) 3 molecules on amorphous silica has been undertaken at the Hartree–Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal silica structure with five SiO 2 groups per unit cell) have been used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH) 3 has been considered for the interacting molecule. In a first step, for each type of surface the geometrical parameters have been optimised and the surface formation energy determined. The geometrical structure of the grafted molecule is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calculated values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included.

Interpretation of spectral broadening and clustering of a pyrene derivative adsorbed on silica gels

This paper discusses a mechanism of broadening of the UV absorption spectra of an adsorbed species in a microporous material, with specific reference to a pyrene derivative adsorbed on silica gels [Lacombe et al. Micropororous Mesopororous Mater., 46 (2001) 311]. We show that broadening by resonance exchange interactions between clustered molecules explains the novel qualitative features of the data. The influences of pore size and adsorbate concentration are explored in a simple model of molecules adsorbed randomly on spherical pores. Absorption spectra are calculated from a tight binding Hamiltonian for the excited state, including inhomogeneous broadening and resonance exchange. Quantitative differences between the model and the experimental spectra show that spectral broadening is due to non-random clustering in the ground state of the adsorbate molecules.