Low-density nanoscale mesoporous composites may be readily synthesized by adding a colloidal or dispersed solid to an about-to-gel silica sol. The silica sol can "glue" a range of chemically and physically diverse particles into the three-dimensional silica network formed upon gelation. If the composite gel is supercritically dried so as to maintain the high porosity of the wet gel, a composite aerogel is formed in which the nanoscopic surface and bulk properties of each component are retained in the solid composite. The volume fraction of the second solid can be varied above or below a percolation threshold to tune the transport properties of the composite aerogel and thereby design nanoscale materials for chemical, electronic, and optical applications.

Two novel lumophores based on aluminum and zinc metallo-8-hydroxyquinolates have been prepared as electroluminescent materials, and their absorbance, photoluminescence, and electroluminescence properties compared with unsubstituted versions of these same complexes. 8-Hydroxy-5-piperidinylquinolinesulfonamide (1) was synthesized in order to add an electron-withdrawing substituent at the 5-position in 8-hydroxyquinoline, increasing the solubility of the corresponding metal quinolate complexes in nonpolar solvents, and producing a blue-shift in the emission wavelength maximum, relative to complexes formed from the unsubstituted compound. The aluminum complex (Al(QS)3) and the zinc complex (Zn(QS)2) of 1 were compared with the aluminum and zinc complexes of unsubstituted 8-hydroxyquinoline (AlQ3 and ZnQ2), both as solutions and as pure thin films, or as poly(N-vinylcarbazole) (PVK) thin films doped with the metal quinolates. Ultraviolet photoelectron spectroscopy data are presented to assist in estimating the energies of the highest occupied molecular orbitals (HOMO) of AlQ3, ZnQ2, Al(QS)3, and Zn(QS)2. Electroluminescence data shows that ITO/Al(QS)3−PVK/aluminum and ITO/Zn(QS)2−PVK/aluminum devices exhibit good diode-like electrical behavior. Electroluminescence spectra mimic the photoluminescence spectra for all complexes.

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.

Highly porous materials such as mesoporous oxides are of technological interest for catalytic, sensing and remediation applications: the mesopores (of size 2-50 nm) permit ingress by molecules and guests that are physically excluded from microporous materials. Connecting the interior of porous materials with a nanoscale or 'molecular' wire would allow the direct electronic control (and monitoring) of chemical reactions and the creation of nanostructures for high-density electronic materials. The challenge is to create an electronic pathway (that is, a wire) within a mesoporous platform without greatly occluding its free volume and reactive surface area. Here we report the synthesis of an electronically conductive mesoporous composite--by the cryogenic decomposition of RuO4--on the nanoscale network of a partially densified silica aerogel. The composite consists of a three-dimensional web of interconnected (approximately 4-nm in diameter) crystallites of RuO2, supported conformally on the nanoscopic silica network. The resulting monolithic (RuO2//SiO2) composite retains the free volume of the aerogel and exhibits pure electronic conductivity. In addition to acting as a wired mesoporous platform, the RuO2-wired silica aerogel behaves as a porous catalytic electrode for the oxidation of chloride to molecular chlorine.

Colloidal metal aerogels are composite nanoscale materials that combine the high surface area and porosity of aerogels with the unique optical and physical properties of metal colloids. As such, they are being developed as advanced sensor, catalytic, and electrocatalytic materials. We have prepared colloidal gold−silica aerogels containing gold colloids ranging in size from 5 to 100 nm. The results presented herein focus on 5- and 28-nm Au-containing silica aerogels for the initial characterization of the interaction between the metal colloid and the silica matrix. A blue-shift of the Au plasmon resonance for silica-immobilized Au colloids (relative to the same colloids in a native Au sol) indicates an interaction between the Au colloid and the nanoscale silica network. Transmission electron microscopy measurements have been used to determine the average size and distribution of the colloidal Au particles, as well as to image the nanoscale silica environment supporting an immobilized Au colloid. Small-angle neutron scattering measurements show no significant changes in the three-dimensional structures of either the base- or acid-catalyzed silica aerogels upon incorporation of small amounts (<0.1 vol %) of colloidal Au. However, for base-catalyzed aerogels, nitrogen physisorption measurements reveal that the average pore size (relative to the pure silica aerogel) decreases as the size of the Au colloid is increased above the ca. 10-nm domain size of the silica (which implies that the Au colloid occludes pore space) while it increases for 5-nm colloidal Au−silica aerogel. The accessibility of the Au surface in colloidal Au−silica aerogels to species introduced from solution is demonstrated by direct adsorption of the dye methyl orange to the Au surface.

Solution electrochemical studies have been conducted of the principle lumophores, dopants, and hole-transport agents of aluminum-quinolate(Alq3)-based organic light-emitting diodes (OLEDs) along with the characterization of their electrogenerated chemiluminescence (ECL). In acetonitrile/benzene solvent mixtures, Alq3 shows single one-electron reduction and oxidation processes, with a separation between the first oxidation and first reduction potentials, ΔE electrochemical = 3.03 V, close to the estimates of energy difference between HOMO and LUMO levels obtained from absorbance spectra of thin films of Alq3, ΔE optical = 3.17 eV. A new sulfonamide derivative of Alq3, (Al(qs)3), showed a positive shift (ca. 0.32 V) in the first reduction potential versus the parent molecule, and resolution of the overall reduction process into three successive, chemically reversible, one-electron reductions. Two successive one-electron oxidations are seen for 4,4‘-bis(m-tolyphenylamino)biphenyl (TPD), a hole-transporting material in many bilayer OLEDs, and for TPDF2, a fluorinated version of TPD, with TPDF2 oxidation occurring 0.1 V positive of that for TPD. Electrogenerated chemiluminescence reactions (Alq3 -•/TPD+• (or TPDF2 +•) and Al(qs)3 -•/TPD+• (or TPDF2 +•)) were found to produce emission spectra from Alq3*s or Al(qs)3*s states which were nearly identical to those seen from OLEDs based upon these molecules. Emission intensities increased with the increasing potential difference between the relevant redox couples. The diisoamyl derivative of quinacridone (DIQA), a quinacridone dopant for certain Alq3-based OLEDs, undergoes two successive one-electron reductions and two successive one-electron oxidations. The ECL reactions DIQA-•/DIQA+•, DIQA+•/Alq3 -•, DIQA+•/Al(qs)3 -•, DIQA-•/TPD+• and DIQA-•/TPDF2 +• all produce the same singlet emissive state, DIQA*s, and the same emission spectral response seen in quinacridone and DIQA-doped OLEDs.

We have developed highly active electrocatalytic nanostructured architectures constructed from carbon−silica composite aerogels in which the carbon is modified with nanoscopic electrocatalysts. Our procedure is general and can be applied to any preformed catalyst-modified carbon powder. The aerogel architecture locks in an electronic path through the carbon guest as well as a continuous, three-dimensional mesoporous transport path for fuel molecules, solvent, and ions. The electrocatalytic activity for methanol oxidation at colloidal-Pt-modified carbon−silica composite aerogels increases by 4 orders of magnitude per gram of Pt over that at native Pt-modified carbon powder. Supported catalysts derived from direct adsorption of colloidal Pt too large to enter the micropores of the carbon are more active per gram of Pt than those prepared using impregnation techniques. A further improvement in mass-normalized activity is achieved by appropriately annealing (temperature and atmosphere) Pt-modified carbon−silica composite aerogels to increase the size of the supported Pt to >3 nm but <4 nm.

The electronic states of vapor-deposited materials used in electroluminescent devices were measured by ultraviolet and x-ray photoelectron spectroscopy, UV-visible absorbance, and photoluminescence spectroscopy. The combination of these measurements on ultrathin films of these materials allows (1) the determination of the energy (with respect to vacuum) of the highest occupied molecular orbital (HO) and the ionization potential (IP), and (2) the estimation of the lowest unoccupied molecular orbital (LU) energy and an approximation of the electron affinity, (EA). The knowledge of the binding energies of these states is important for the understanding of light-emitting diode properties and the potential optimization of such devices. The luminescent material tris(8-hydroxy-quinoline) aluminum has an IP of 5.9 eV and an apparent EA smaller than 3.5 eV. The IP of both hole transport agents, tri-p-tolylamine and 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane, is 5.4 eV and their EA is estimated to be smaller than 1.8 eV. The electron transport agents 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, 2-t-butyl-9,10-n,n’-dicyano-anthraquinonediimine and dicyano-diphenylsulfone differ in IP from 7.1 to 7.6 eV and the EA for these materials are estimated to be smaller than 3.5, 4.9 and 5.5 eV, respectively.