Silica sol-gels were covalently modified with N-(3-trimethoxysilylpropyl)-2,7-diazapyrenium bromide (DAP). Luminescent aerogels are created in which none of the fluorophore leaches from the gels during either the washing or supercritical drying necessary to prepare DAP-modified aerogels. The bulk density (0.17 g/cm 3 ), N 2 -adsorption surface area (870 m 2 / g), and thermogravimetric and scanning electron micrographic characteristics of the dyemodified aerogels remain identical to those of the unmodified silica aerogels. The bulk concentration of the dye in the aerogels was e6.6 mM; at these loadings the aerogels demonstrate bulk photoluminescence. As based on the mesoporous surface area, the surface coverage of the dye is 7-8% of a monolayer. The absorption, emission, and O 2 -quenching characteristics of the diazapyrenium dye in the aerogels parallel those obtained in alcoholic (rather than aqueous) solution, which further indicates that the dopant molecules are isolated from each other and that they see an environment with a ∼OH polarity. Time-resolved emission studies indicate that all DAP moieties reside in a single type of microenvironment. Emission quenching of ∼1-cm-diameter monoliths of DAP-silica aerogel is complete in <15 s, which compares very favorably with the best response times for pyrene guests in micrometer-thick xerogel films. The apparent diffusion coefficient of O 2 or Ar in the DAPaerogel monoliths was estimated at g0.01-0.02 cm 2 /s, which is only 10× less than the unimpeded diffusion coefficient of Ar in air.

In anhydrous CH 3 CN, 4-benzoyl-N-methylpyridinium cations undergo two reversible, well-separated (∆E 1/2 ∼ 0.6 V) one-electron reductions in analogy to quinones and viologens. If the solvent contains weak protic acids, such as water or alcohols, the first cyclic voltammetric wave remains unaffected while the second wave is shifted closer to the first. Both voltammetric and spectroelectrochemical evidence suggest that the positive shift of the second wave is due to hydrogen bonding between the two-electron reduced form of the ketone and the proton donors. While the one-electron reduction product is stable both in the presence and in the absence of the weak-acid proton donors, the two-electron reduction wave is reversible only in the time scale of cyclic voltammetry. Interestingly, at longer times, the hydrogen bonded adduct reacts further giving nonquaternized 4-benzoylpyridine and 4-(R-hydroxybenzyl)pyridine as the two main terminal products. In the presence of stronger acids, such as acetic acid, the second wave merges quickly with the first, producing an irreversible two-electron reduction wave. The only terminal product in this case is the quaternized 4-(Rhydroxybenzyl)-N-methylpyridinium cation. Experimental evidence points toward a common mechanism for the formation of the nonquaternized products in the presence of weaker acids and the quaternized product in the presence of CH 3 CO 2 H.

Anisotropic composite architectures of vastly different length scales are organized upon nanogluing magnetic microparticulate guests (ca. 10−15 μm iron powder) into a guest−host network by gelation of silica sol in the presence of an externally applied magnetic field. The magnetic-field-induced structure of the guest (long needle-like alignment) is frozen in place by the silica nanoscale network created by gelation. No organization is observed in composites in which the silica sol undergoes gelation in the absence of an external magnetic field or when the field is applied to the iron-silica composite after gelation. Aged iron−silica composite gels that are dried supercritically yield highly porous aerogels that retain both their structural integrity and the anisotropic alignment of the magnetic guest, unlike gels dried to form xerogels in which the partial collapse of the porous network destroys the integrity of the monoliths, reducing them to powder. Aged iron−silica composite gels may also be treated with a solution of Na2Cr2O7/HCl to dissolve the iron guest, thereby producing nanostructured mesoporous gels with anisotropic macroporosity.

The title compounds were synthesized by Sonogashira coupling reactions of appropriate Ru(II) complexes with the electron acceptors. Characterization was conducted in solution and in frozen matrixes. Finally, the title compounds were evaluated as dopants of sol-gel materials. It was found that the intramolecular quenching efficiency of 4-benzoyl-Nmethylpyridinium cation in solution depends on the solvent: photoluminescence is quenched completely in CH 3 CN, but not in methanol or ethanol. On the other hand, intramolecular emission quenching by 4-benzyl-N-methyl viologen is complete in all solvents. The difference between the two quenchers is traced electrochemically to the solvation of the 4-benzoyl-Nmethylpyridiniums by alcohol. In frozen matrixes or adsorbed on the surfaces of silica aerogel, both Ru(II) complex/electron acceptor dyads of this study are photoluminescent, and the absence of quenching has been traced to the environmental rigidity. When doped aerogels are cooled at 77 K, the emission intensity increases by ∼4×, and the spectra shift to the blue, analogous to what is observed with Ru(II) complexes in solutions undergoing fluidto-rigid transition. However, in contrast to frozen solutions, the luminescent moieties in the bulk of aerogels kept at low temperatures are still accessible to gas-phase quenchers diffusing through the mesopores, leading to more sensitive platforms for sensors than other room-temperature configurations. Thus, the photoluminescence of our Ru(II) complex dyads adsorbed on aerogel is quenchable by O 2 both at room temperature and at 77 K. Furthermore, it was also found that O 2 modulates the photoluminescence of aerogels doped with 4-benzoyl-N-methylpyridinium -based dyads over a wider dynamic range compared with aerogels doped with either our viologen-based dyads or with Ru(II) tris(1,10-phenanthroline) itself.