Redox-Active Star Molecules Incorporating the 4-Benzolypyridinium Cation: Implications for the Charge Transfer Efficiency Along Branches versus Across the Perimeter in Dendrimers

We report the redox properties of four star systems incorporating the 4-benzoyl-N-alkylpyridinium cation; the redox potential varies along the branches, but remains constant at fixed radii. Voltammetric analysis (cyclic voltammetry and differential pulse voltammetry) shows that only two of the three redox-active centers in the perimeter are electrochemically accessible during potential sweeps as slow as 20 mV/s and as fast as 10 V/s. On the contrary, both redox centers of a branch are accessible electrochemically within the same time frame. These results are discussed in terms of slow through-space charge transfer and the globular 3-D folding of the molecules

Cupriphication of gold to sensitize d10–d10 metal–metal bonds and near-unity phosphorescence quantum yields

Outer-shell s0/p0 orbital mixing with d10 orbitals and symmetry reductionuponcupriphicationofcyclic trinucleartrigonal-planargold(I) complexes are found to sensitize ground-state Cu(I)–Au(I) covalent bonds and near-unity phosphorescence quantum yields. Heterobimetallic Au4Cu2 {[Au4(μ-C2,N3-EtIm)4Cu2(μ-3,5-(CF3)2Pz)2], (4a)}, Au2Cu {[Au2(μ-C2,N3-BzIm)2Cu(μ-3,5-(CF3)2Pz)], (1) and [Au2(μ-C2, N3-MeIm)2Cu(μ-3,5-(CF3)2Pz)], (3a)}, AuCu2 {[Au(μ-C2,N3-MeIm)Cu2(μ3,5-(CF3)2Pz)2], (3b) and [Au(μ-C2,N3-EtIm)Cu2(μ-3,5-(CF3)2Pz)2], (4b)} and stacked Au3/Cu3 {[Au(μ-C2,N3-BzIm)]3[Cu(μ-3,5-(CF3)2Pz)]3, (2)} formuponreactingAu3 {[Au(μ-C2,N3-(N-R)Im)]3 ((N-R)Im = imidazolate; R =benzyl/methyl/ethyl =BzIm/MeIm/EtIm)} with Cu3 {[Cu(μ-3,5(CF3)2Pz)]3 (3,5-(CF3)2Pz = 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 twoAu(I)⋯Cu(I)metallophilicinteractions,whereas 4a exhibitsa hexanuclear cluster structure wherein two monomer-of-trimer units are linked by a genuine d10–d10 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 d10 centers so as to deem it a “metal–metal bond” vis-à-vis “metallophilic interaction.” Density-functional calculations estimate 35– 43kcal/molbindingenergy,akintotypicalM–Msingle-bondenergies. Congruently, FTIR spectra of4a showmultiple far-IR bands within 65– 200 cm−1, assignable to vCu-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 cuprophilicsensitizationinextinctioncoefficientandsolid-state photoluminescence quantum yields approaching unity (ΦPL = 0.90–0.97 vs. 0–0.83 for Au3 and Cu3 precursors), which bodes well for potential future utilization in inorganic and/or organic LED applications

Synthesis of selected electron donors and acceptors and study of inter/intramolecular electron transfer from ruthenium (II) complexes in solution, frozen matrix and silica aerogels as the basis for optical devices

Harvesting energy from the sun is something that nature does well through photosynthesis: green plants convert carbon dioxide to glucose with energy provied by the sun in a complex set of reactions. Mimicking photosynthesis through artificial systems designed to convert optical energy into chemical or electrical energy, has been the goal and challenge for many years...the photochemical and photophysical properties of transition metal complexes, especially [Ru(bpy)₃]²⁺, have been studied comprehensively in terms of the Photoinduced Electron Transfer (PET) properties in donor-acceptor systems (D/A). --Introduction, page 1

A Convenient Synthesis and Spectroscopic Characterization of N,N\u27-Bis(2-propenyl)-2,7-diazapyrenium Quaternary Salts

N,N\u27-Bis(2-propenyl)-2,7-diazapyrenium salts are synthesized in good yield from the reaction of 1,4,5,8-naphthalenetetracarboxylic dianhydride with allylamine, followed by LiAlH4 reduction and subsequent oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The nature of the counteranion depends on the solvent system used for recrystallization of the crude product from the final DDQ-oxidation step. X-ray analysis shows that if recrystallization is carried out in boiling CH3OH/H2O (1:1, v/v), the counteranion in the resulting deep-red crystals is always the alkoxy anion of 2-cyano-5,6-dichloro-3-hydroxy-1,4-benzoquinone, whether the final DDQ oxidation ends with addition of HClO4 or HCl; on the other hand, if recrystallization is carried out with anhydrous acetonitrile, the product is N,N\u27-bis(2-propenyl)-2,7-diazapyrenium diperchlorate or dichloride depending on whether the DDQ oxidation is followed by addition of concd HClO4 or concd HCl, respectively. Importantly, if the DDQ oxidation is quenched with HBr, Br- is oxidized to Br2 by unreacted DDQ, and the resulting product is N,N\u27-bis(2,3-dibromopropyl)-2,7-diazapyrenium dibromide. Comparative absorption and time-resolved emission studies provide evidence for possible dimerization of N,N\u27-bis(2-propenyl)-2,7-diazapyrenium diperchlorate in CH3CN

Synthesis and Hydrolytic Stability of Tert-Butoxydimethylsilyl Enol Ethers

tert-Butoxydimethylsilyl enol ethers derived from aldehydes and ketones were synthesized in good yields with high regioand stereoselectivities, under thermodynamically and kinetically controlled conditions. The hydrolytic stability of tert-butoxydimethylsilyl enol ether of cyclohexanone was studied under acidic and basic conditions and compared to that of trimethylsilyl, tert-butyldimethylsilyl and (2,4,6-tri-tert-butylphenoxy) dimethylsilyl enol ethers

Nonadditive Voltammetric Currents from Two Redox-active Substances and Electroanalytical Implications

At the potential range where both decamethylferrocene (dMeFc) and ferrocene (Fc) are oxidized with rates controlled by linear diffusion, electrogenerated Fc.+ radicals diffusing outward from the electrode react quantitatively (K23°C = 5.8 × 108) with dMeFc diffusing toward the electrode and produce Fc and dMeFc.+. That reaction replaces dMeFc with Fc, whose diffusion coefficient is higher than that of dMeFc, and the total mass-transfer limited current from the mixture is increased by ∼10%. Analogous observations are made when mass transfer is controlled by convective diffusion as in RDE voltammetry. Similar results have been obtained with another, and for all practical purposes randomly selected pair of redoxactive substances, [Co(bipy)3] 2+ and N-methylphenothiazine (MePTZ); reaction of MePTZ.+ with [Co(bipy)3]2+ replaces the latter with MePTZ, which diffuses faster, and the total current increases by ∼20%. The experimental voltammograms have been simulated numerically and the role of (a) the rate constant of the homogeneous reaction, (b) the relative concentrations, and (c) the diffusion coefficients of all species involved have been studied in detail. Importantly, it was also identified that within any given redox system the dependence of the mass-transfer limited current on the bulk concentrations of the redoxactive species is expected to be nonlinear. These findings are discussed in terms of their electroanalytical implications

Host-guest Interactions of Cucurbit[7]uril with N-quaternized-4-(p-substituted Benzoyl)pyridinium Cations and Control of the Ketone to Gem-diol Equilibrium

N-Methyl-4-(p-substituted benzoyl) pyridinium cations show affinity towards cucurbit[7]uril (CB[7]) and they form either endo or exo complexes. The mode of complexation is controlled by solvent polarity and also by the chemical identity of the group that is attached on the positively charged nitrogen atom. For example, the N-methyl derivative prefers the endo form in water and the exo form in DMSO. The N-benzyl analogue prefers the exo form in both water and DMSO. Interestingly, in water, the N-methyl-4-(p-substituted benzoyl) pyridinium cations exist in equilibrium of their keto and gem-diol forms, whereas the ratio of the gem-diol versus the keto form is controlled by the p-substituent: with electron withdrawing substituents (e.g., nitro, formyl) the gem-diol form dominates. However, in the presence of CB[7], the stabilization realized by having the keto group inside the hydrophobic interior of CB[7] is greater than the stabilization realized via hydrogen bonding of the two hydroxyl groups of the gem-diol with water, shifting the equilibrium towards the keto for

Control of the Ketone to Gem-diol Equilibrium by Host-guest Interactions

In water, N-methyl-4-(p-substituted benzoyl)pyridinium cations, BP-X, exist in equilibrium with their hydrated forms (gem-diols), whose concentrations depend on the para substituent (-X). In the presence of cucurbit[7]uril (CB[7]), the benzoyl group shows a preference for the CB[7] cavity, and the ketone to gem-diol equilibrium is shifted toward the keto form, meaning that the stabilization realized through hydrophobic interactions of the benzoyl group in the CB[7] cavity exceeds the hydrogen-bonding stabilization of the gem-diols in the aqueous environment