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

Abstract

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...

Highly Selective Chemical Sensing in a Luminescent Nanoporous Magnet

“…[ 2 ] Along our exploratory studies on molecule-based multifunctional magnetic materials, [ 8 ] which date back to the pioneering work of Kahn and co-workers on oxamato-based heterobimetallic magnets, [ 9 ] we present herein the synthesis, crystal structure, thermal and sorption behavior, optical and magnetic properties of a new manganese(II)-copper(II) 2D MOF of formula MV[Mn 2 Cu 3 (mpba) 3 (H 2 O) 3 ] · 20H 2 O ( 1 ), where MV 2 + is the methylviologen dicationic dye [ 10 ] and mpba 4 − is the N,N′ -1,3-phenylenebis(oxamate) bridging ligand ( Scheme 1 ). Compound 1 exhibits highly selective gas sorption ability, luminescence capacity, and long-range magnetic ordering, constituting thus a unique example of luminescent porous magnet with potential applications in host-guest chemical sensing of small molecules.…”

“…In fact, a fast radiationless deactivation pathway of the singlet excited state of the methylviologen cation and a subsequent non-emissive decay were earlier attributed to signifi cant rotation around the single bond connecting the two pyridinium rings. [ 10 ] The MV 2 + cations are quite isolated from each other [the shortest MV 2 + … MV 2 + distance is 7.768(11) Å for N(8)-N(8f)]. However, they establish weak intermolecular (van der Waals and/or electrostatic) interactions among their pyridinium-nitrogen and the carboxylate-oxygen atoms which are coordinated to the Cu II ions of the anionic network [N … O contacts in the range of 3.304(13)-3.368(10) Å] ( Figure S3).…”

“…In fact, although MV 2 + in solution does not fl uoresce, it shows a bright light emission at around 330 nm when encapsulated in porous solids. [ 10 ] The absorption and emission spectra of 1 dispersed in toluene are illustrated in Figure S10. Two distinct absorption bands centered at 244 and 400 nm can be observed.…”

Highly-controlled grafting of mono and dicationic 4,4′-bipyridine derivatives on SBA-15 for potential application as adsorbent of CuCl2 from ethanol solution

“…Among several different functional groups reported in the literature, there is considerable interest in the use of 4,4 0 -bipyridine derivatives to modify the surface of mesoporous silicas because of their interesting and well-known electron acceptor, charge transfer and fluorescence properties [24][25][26]. The quaternary salts of 4,4 0 -bipyridine, commonly known as viologen, are widely used as anion exchange groups in membranes [27], thermosensors [28], colorimetric sensors [29], for photocatalytic hydrogen evolution [30], for the design of optical materials [31] and for fluorescence probing [32].…”

Mono- and Di-Quaternized 4,4′-Bipyridine Derivatives as Key Building Blocks for Medium- and Environment-Responsive Compounds and Materials

“…It was observed that not only the electrochemical one-electron reduction potentials of these viologens were altered when compared to parent paraquat (1) but also the spectral characteristics (absorption maxima and molar extinction coefficients) of the formed radical cations. The situation becomes even more interesting when investigating the fluorescence of viologens (especially the ones of low molecular weight), a research topic which has been debated for a long time [1,[26][27][28]. The prevailing belief has been that viologens are non-fluorescent [1].…”