Formation of Methylviologen Radical Monopositive Cations and Ensuing Reactions with Polychloroalkanes on Silica Gel Surfaces

Mao, Yun; Breen, Nancy E.; Thomas, Jacqueline

doi:10.1021/j100024a038

Cited by 31 publications

(27 citation statements)

References 19 publications

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“…19 The search for these types of complexes when MV 2+ is adsorbed in silica has been pursued unsuccessfully. 16 It is known that negative oxygens of the zeolite lattice behave as basic sites. 25 Furthermore, the basicity of the zeolites largely depends on their chemical composition and in particular on the aluminum content of the framework.…”

Section: Resultsmentioning

confidence: 99%

Intrazeolite Photochemistry. 16. Fluorescence of Methylviologen Adsorbed within Medium- and Large-Pore Zeolites

et al. 1996

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Methylviologen (MV 2+ ) incorporated within medium-and large-pore zeolites (NaZSM-5, Namordenite, NaY, and CsY) emits fluorescence at 330 and 420 nm; the latter is most prominent in zeolite NaY. The intensity ratio between these two bands depends on the structure of the zeolite. On the basis of molecular modeling and 13 C CP/MAS NMR using the dipolar dephasing technique, the first band has been ascribed to the emission of MV 2+ in a planar conformation. The unprecedented 420 nm band has been attributed to a twisted conformer probably interacting with the basic oxygen sites of the framework.There is considerable interest in the use of zeolites to control the outcome of organic reactions and as tools to examine specific spectroscopic properties. [1][2][3][4][5] In the context of the photochemical reduction of water and the production of hydrogen using visible light, a large variety of heterogeneous systems have been devised many of them having in common the presence of methylviologen (MV 2+ ) adsorbed on an inorganic support. 6-8 Zeolites 9-11 have been often the hosts of choice because their strictly regular microporous framework provides the opportunity to compartmentalize and organize a supramolecular assembly where the different stages of the overall process (light absorption, energy transfer, and water reduction) can take place in a well-defined spatial arrangement. However, the majority of these studies have not paid close attention to the possible influence that the rigid, restricted environment may play in modifying the molecular properties of MV 2+ . In the present work we have performed a study of the fluorescence of MV 2+ incorporated within different zeolites encompassing different topologies. On the basis of solid-state 13 C CP/MAS NMR spectroscopy and molecular modeling, we have found that our results can be rationalized in terms of a different degree of pyridinium ring mobility around the single C-C bond when MV 2+ is incorporated zeolites, as well as the formation of a charge-transfer-like complex between MV 2+ and the negative oxygens of the framework. Experimental SectionSamples of zeolites NaZSM-5 (Si/Al 17.5), NaMor (mordenite; Si/Al 10), NaY (Si/Al 2.6) and CsY (Si/Al 2.6, 70% ion exchange) doped with MV 2+ were obtained by room-temperature ion exchange using aqueous solution of MV 2+ hydrochloride (Aldrich, 98%). The actual loading level was determined by combustion chemical analysis (C and N) of the samples as well as by quantitative atomic absorption spectroscopy of the remaining Na + left in the zeolite after the partial exchange. Powder X-ray diffraction of the samples showed that the crystalline structure of the MV 2+ -doped zeolites was preserved during the preparation procedure.Steady-state fluorescence measurements were performed with a Perkin-Elmer LS-50 spectrofluorimeter equipped with a frontface attachment for solid samples.Semiempirical calculations were carried out using the AM1 SCF-MO method as implemented in the version 3.0 of the Figure 2. (A) Aromatic region of the 13 C CP/MA...

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Section: Resultsmentioning

confidence: 99%

Intrazeolite Photochemistry. 16. Fluorescence of Methylviologen Adsorbed within Medium- and Large-Pore Zeolites

et al. 1996

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“…For nanosecond observations, as is in Fig. 1, the detection system consisted of a fast IP28 photomultiplier and has been described previously (20). For the picosecond studies, the 532 nm light pulse synchronous with the 266 nm pulse was used to analyze absorption at this wavelength.…”

Section: Methodsmentioning

confidence: 99%

The Laser Two-photon Photolysis of Liquid Carbon Tetrachloride†

Zhang

Thomas

2006

Photochem Photobiol

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The two-photon photolysis of liquid CCl4 with 25 ps pulses of 266 nm light has been studied and compared with similar studies with high energy radiation. Both neutral and ionic species are produced from excited states and ionization. The emphasis of the study is on the ionic processes, while some data related to excited states and free radicals are presented. In both radiolysis and photolysis, a solvent separated charged pair, CCl3+ // Cl-, exhibiting a lambda(max) at 475 nm, is observed that exhibits a total growth over 38 to 100 ps. Solutes with ionization potentials less than that of CCl4 (11.47 eV) reduce the yield of the 475 nm species producing radical cations of the solute. The efficiency of this process is about 10-fold larger in radiolysis compared with photolysis. Analysis of the data suggest that the lower energy of two-photon photolysis produces a charge pair CCl4+ // CCl4-, which decays in about 3 ps to CCl4+ // Cl-. This species then decays to CCl3+ // Cl-. The lifetime of the growth of the 475 nm is measured as 46 ps. These studies clearly show areas where radiolysis and photolysis can be quite similar and also areas where the vast difference in excitation energy introduces stark differences in the observed radiation and photoinduced chemistry.

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“…In the presence of coadsorbed electron donors the decay kinetics of the anthracene radical cation are again complex and do not conform to a single-exponential decay. A model that has been used by us 7 and others ,,− to describe the kinetics of reactions on surfaces, including electron transfer on silica gel, is a dispersive kinetic model that assumes a distribution of rate constants for the observed reaction . We have applied this model to kinetic data obtained for electron transfer on silica gel from an electron donor to the anthracene radical cation 7 and have shown that plots of observed mean electron-transfer rate versus coadsorbed donor concentration are linear for each of the electron donors studied up to a loading of 2 μmol g -1 .…”

Section: Resultsmentioning

confidence: 99%

Electron Transfer on Insulator Surfaces: Exciplex Emission and the Role of Electron Diffusion in Determining Radical Deactivation Rates

1998

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The kinetics of electron transfer on silica gel between the anthracene radical cation and the electron donors triphenylamine (TPA) and N,N,N‘,N‘-tetramethyl-1,4-phenylenediamine (TMPD) have been investigated using the technique of diffuse reflectance laser flash photolysis. The mean rate of decay of the anthracene radical cation, determined as the maximum of the rate constant distribution, correlates with the surface concentration of the electron donor in both cases up to a loading of 2 μmol g-1. In the case of triphenylamine as donor, using 355 nm excitation, plots of mean electron-transfer rate constant versus electron-donor concentration deviate positively from linearity at higher donor concentrations. In the case of TMPD, linearity is preserved at higher loadings with 355 nm excitation, where TMPD does not absorb appreciably and is not therefore directly photoionized, but again a positive deviation ensues with 266 nm excitation that causes direct photoionization of TMPD. In both cases the increased rate of decay of the anthracene radical cation correlates with appreciable direct photoionization of the electron donor. The enhanced rate of decay is explained as being due to donor photoionization increasing the local concentration of electrons on the surface and electron diffusion competing efficiently with donor diffusion leading to increased anthracene radical cation decay. In addition, we have demonstrated the formation between anthracene and TPA coadsorbed on silica gel of an emissive exciplex and have found no evidence that relaxation of this exciplex results in radical ion pair formation.

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Formation of Methylviologen Radical Monopositive Cations and Ensuing Reactions with Polychloroalkanes on Silica Gel Surfaces

Cited by 31 publications

References 19 publications

Intrazeolite Photochemistry. 16. Fluorescence of Methylviologen Adsorbed within Medium- and Large-Pore Zeolites

Intrazeolite Photochemistry. 16. Fluorescence of Methylviologen Adsorbed within Medium- and Large-Pore Zeolites

The Laser Two-photon Photolysis of Liquid Carbon Tetrachloride†

Electron Transfer on Insulator Surfaces: Exciplex Emission and the Role of Electron Diffusion in Determining Radical Deactivation Rates

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