In the quest for strong lightweight materials, silica aerogels would be very attractive, if they were not fragile. The strength of silica aerogel monoliths has been improved by a factor of over 100 through cross-linking the nanoparticle building blocks of preformed silica hydrogels with poly(hexamethylene diisocyanate). Composite monoliths are much less hygroscopic than native silica, and they do not collapse when in contact with liquids.

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Outer-shell s 0 /p 0 orbital mixing with d 10 orbitals and symmetry reduction upon cupriphication of cyclic trinuclear trigonal-planar gold(I) complexes are found to sensitize ground-state Cu(I)-Au(I) covalent bonds and near-unity phosphorescence quantum yields. Heterobimetallic Au 4 Cu 2 {[Au 4 (μ-C 2 ,N 3 -EtIm) 4 Cu 2 (μ-3,5-(CF 3 ) 2 Pz) 2 ], (4a)}, Au 2 Cu {[Au 2 (μ-C 2 ,N 3 -BzIm) 2 Cu(μ-3,5-(CF 3 ) 2 Pz)], (1) and [Au 2 (μ-C 2 , N 3 -MeIm) 2 Cu(μ-3,5-(CF 3 ) 2 Pz)], (3a)}, AuCu 2 {[Au(μ-C 2 ,N 3 -MeIm)Cu 2 (μ-3,5-(CF 3 ) 2 Pz) 2 ], (3b) and [Au(μ-C 2 ,N 3 -EtIm)Cu 2 (μ-3,5-(CF 3 ) 2 Pz) 2 ], (4b)} and stacked Au 3 /Cu 3 {[Au(μ-C 2 ,N 3 -BzIm)] 3 [Cu(μ-3,5-(CF 3 ) 2 Pz)] 3 , (2)} form upon reacting Au 3 {[Au(μ-C 2 ,N 3 -(N-R)Im)] 3 ((N-R)Im = imidazolate; R = benzyl/methyl/ethyl = BzIm/MeIm/EtIm)} with Cu 3 {[Cu(μ-3,5-(CF 3 ) 2 Pz)] 3 (3,5-(CF 3 ) 2 Pz = 3,5-bis(trifluoromethyl)pyrazolate)}. The crystal structures of 1 and 3a reveal stair-step infinite chains whereby adjacent dimer-of-trimer units are noncovalently packed via two Au(I)⋯Cu(I) metallophilic interactions, whereas 4a exhibits a hexanuclear cluster structure wherein two monomer-of-trimer units are linked by a genuine d 10 -d 10 polar-covalent bond with ligandunassisted Cu(I)-Au(I) distances of 2.8750(8) Å each-the shortest such an intermolecular distance ever reported between any two d 10 centers so as to deem it a "metal-metal bond" vis-à-vis "metallophilic interaction." Density-functional calculations estimate 35-43 kcal/mol binding energy, akin to typical M-M single-bond energies. Congruently, FTIR spectra of 4a show multiple far-IR bands within 65-200 cm −1 , assignable to v Cu-Au as validated by both the Harvey-Gray method of crystallographic-distance-to-force-constant correlation and dispersive density functional theory computations. Notably, the heterobimetallic complexes herein exhibit photophysical properties that are favorable to those for their homometallic congeners, due to threefold-to-twofold symmetry reduction, resulting in cuprophilic sensitization in extinction coefficient and solid-state photoluminescence quantum yields approaching unity (Φ PL = 0.90-0.97 vs. 0-0.83 for Au 3 and Cu 3 precursors), which bodes well for potential future utilization in inorganic and/or organic LED applications. Here, we show that outer 4s/p (Cu I ) and 6s/p (Au I ) orbitals can admix with the respective valence 3d and 5d orbitals to sensitize a bona fide polar-covalent metal-metal bond between two d 10 centers manifest by two rather short, 2.8750(8) Å, Cu(I)-Au(I) bonds without any ligand-bite-size assistance. The reduced symmetry in this family of complexes is also shown to impart higher extinction coefficients and phosphorescence quantum yields than those attained by the parent homometallic precursor complexes.Heterometallic complexes are remarkable molecules owing to their unique catalytic and optoelectronic properties (2, 3). Heterometallic species involving coinage metals have received immense attention owing to their fascinating structural a...

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.

2-Arylethynyl-and 2,6-and 2,7-diarylethynyl-substituted 9,10-anthraquinones were synthesized via Sonogashira coupling reactions of 2-bromo-, 2,6-dibromo-, and 2,7-dibromo-9,10-anthraquinone with para-substituted phenylacetylenes. While the redox properties of those compounds are almost insensitive to substitution, their absorption maxima are linearly related to the Hammett constants with different slopes for electron donors and electron acceptors. All compounds are photoluminescent both in solution (quantum yields of emission e6%), and as solids. The emission spectra have the characteristics of charge-transfer bands with large Stokes shifts (100-250 nm). The charge-transfer character of the emitting state is supported by large dipole moment differences between the ground and the excited state as concluded on the basis of molecular modeling and Lippert-Mataga correlations of the Stokes shifts with solvent polarity. Maximum Stokes shifts are attained by both electrondonating and -withdrawing groups. This is explained by a destabilization of the HOMO by electron donors and a stabilization of the LUMO by electron acceptors. X-ray crystallographic analysis of, for example, 2,7-bisphenylethynyl-9,10-anthraquinone reveals a monoclinic P2 1 /n space group and no indication for π-overlap that would promote quenching, thus explaining emission from the solid state. Representative reduced forms of the title compounds were isolated as stable acetates of the corresponding dihydro-9,10-anthraquinones. The emission of these compounds is blue-shifted relative to the parent oxidized forms and is attributed to internal transitions in the dihydro-9,10-anthraquinone core.