Metal−Organic Frameworks as Sensors: A ZIF-8 Based Fabry−Pérot Device as a Selective Sensor for Chemical Vapors and Gases

Metal−Organic Frameworks as Sensors: A ZIF-8 Based Fabry−Pérot Device as a Selective Sensor for Chemical Vapors and Gase

Interfacial Charge-Transfer Pathways: Evidence for Marcus-Type Inverted Electron Transfer in Metal Oxide Semiconductor/Inorganic Dye Systems

Interfacial Charge-Transfer Pathways:  Evidence for Marcus-Type Inverted Electron Transfer in Metal Oxide Semiconductor/Inorganic Dye System

Electroabsorption Studies of Metal-to-Ligand Charge Transfer in Ru(phenanthroline)₃²⁺: Evidence for Intrinsic Charge Localization in the Initially Formed Excited State

Electroabsorption studies of ruthenium(II) tris(phenanthroline) show that a substantial change in dipole moment (|Δμ| = 6.7 ± 1 D) accompanies ground state to “singlet” metal-to-ligand charge transfer (MLCT) excited-state formation. The change is nearly identical to that reported for the 2,2‘-bipyridine analogue (Oh; et al. J. Am. Chem. Soc. 1989, 111, 1130). Since both species lack ground-state dipole moments, the finite values for Δμ are diagnostic of intrinsic charge localization in the excited states. The nominally triplet transition for the phenanthroline complex also involves the formation of a charge localized state. The combined results are inconsistent, therefore, with an alternative “delocalized” charge transfer excited-state interpretation suggested by time-resolved resonance Raman studies of the tris(phenanthroline) complex

Surface Modification of SnO₂ Photoelectrodes in Dye-Sensitized Solar Cells: Significant Improvements in Photovoltage via Al₂O₃ Atomic Layer Deposition

We report here the exploitation of ultrathin layers of Al₂O₃ deposited via atomic layer deposition (ALD) on SnO₂ photoanodes used in dye-sensitized solar cells featuring the I₃⁻/I⁻ couple as the redox electrolyte. We find that a single ALD cycle of Al₂O₃ increases the lifetimes of injected electrons by more than 2 orders of magnitude. The modified SnO₂ photoanode yields nearly a 2-fold improvement fill factor and a greater than 2-fold increase in open-circuit photovoltage, with a slight increase in short-circuit photocurrent. The overall energy conversion efficiency increases by roughly 5-fold. The effects appear to arise primarily from passivation of reactive, low-energy tin-oxide surface states, with band-edge shifts and tunneling based blocking behavior playing only secondary roles

Tetra-Rhenium Molecular Rectangles as Organizational Motifs for the Investigation of Ligand-Centered Mixed Valency: Three Examples of Full Delocalization

Molecular rectangles having the form {[Re(CO)3]2(X)2}2-μ,μ‘-(LL)2, where X is either a bridging alkoxide or phenylthiolate group and LL is 4,4‘-bipyridine or pyrazine, are characterized by cofacial LL pairs that are in van der Waals contact across the “long” side of the rectangle. Cyclic voltammetry shows that the redox-accessible bridging ligands, LL, are reduced in sequential, one-electron reactions. The singly reduced rectangles represent an unusual type of mixed-valence compound in which the LL ligands themselves are the redox centers. Spectroelectrochemical measurements for mixed-valence forms of these rectangles reveal intense, asymmetric absorption bands in the near-infrared region, assigned as intervalence transitions. Electroabsorption (Stark spectroscopy) measurements reveal minute changes in dipole moment and therefore a lack of significant charge transfer upon intervalence excitation. Thus, the rectangles are unusual examples of class III (fully valence delocalized) molecular mixed-valence species that employ direct donor-orbital/acceptor-orbital overlap rather than covalent-bond-mediated superexchange to achieve the large electronic coupling strengths required for delocalization

Synthesis, Characterization, and Preliminary Host−Guest Binding Studies of Porphyrinic Molecular Squares Featuring <i>fac</i>-Tricarbonylrhenium(I) Chloro Corners

Synthesis, Characterization, and Preliminary Host−Guest Binding Studies of Porphyrinic Molecular Squares Featuring fac-Tricarbonylrhenium(I) Chloro Corner

Sol−Gel-Encapsulated Alcohol Dehydrogenase as a Versatile, Environmentally Stabilized Sensor for Alcohols and Aldehydes

Silicate-encapsulated yeast alcohol dehydrogenase (ADH) can be employed as a sensor for short-chained alcohols in standard aqueous, harsh nonaqueous, and gas-phase environments. Specifically, the implementation of sensing schemes based on encapsulated ADH/NAD+ or ADH/NADH, and utilization of changes in fluorescence from the soluble, reduced cofactor nicotinamide adenine dinucleotide (NADH) upon exposure to alcohols or aldehydes, allows for semiquantitative determination of both substrates. Additionally, by using fluorescence from NADH, we find that cycling of the enzymatic probe can be accomplished via successive exposure to alcohol and aldehyde substrates, thus converting the system into a multiple-use sensor. Finally, we find that the gel matrix provides sufficient enzyme stabilization to permit the assemblies to be used analytically in hostile and inherently denaturing sample environments, including vapor-phase and nonpolar liquid (e.g., hexane) environments

Alkali Metal Cation Effects on Hydrogen Uptake and Binding in Metal-Organic Frameworks

A 2-fold interwoven metal-organic framework has been chemically reduced and doped with Li+, Na+, and K+. At low pressures and temperatures, the reduced and doped materials exhibit enhanced H2 uptakeup to 65% higher than for the neutral framework. Notably, at similar doping levels, H2 binding is strongest with Li+ and decreases as Li+ > Na+ > K+. However, the uptake increases in the opposite order. We attribute the behavior to structural changes accompanying framework reduction

Probing the Symmetry of the Nonlinear Optic Chromophore Ru(<i>trans</i>-4,4‘-diethylaminostyryl-2,2‘-bipyridine)₃²⁺: Insight from Polarized Hyper-Rayleigh Scattering and Electroabsorption (Stark) Spectroscopy

Polarized hyper-Rayleigh scattering (HRS) has been used to interrogate a putative octupolar “super” chromophore Ru(trans-4,4‘-diethylaminostyryl-2,2‘-bipyridine)32+. While the hyperpolarizability (β) is impressive, it is apparent from polarized HRS measurements that the symmetry of the excited state, and thus the hyperpolarizability tensor, is best described as dipolar in nature. Electroabsorption (electronic Stark effect) experiments support these findings, implying that changes in dipole moment (Δμ) accompany optical excitation. The measured |Δμ| contributions from both metal-to-ligand charge transfer and intraligand transitions have been used to model the wavelength-dependent hyperpolarizability; reasonable qualitative agreement between experimental results and a simple three-state, two-level dipolar model is obtained

Thermally Activated, Inverted Interfacial Electron Transfer Kinetics: High Driving Force Reactions between Tin Oxide Nanoparticles and Electrostatically-Bound Molecular Reactants

The kinetics and mechanism of fast electron transfer (ET) between tin oxide nanoparticles and electrostatically bound Os(III) and Ru(III) complexes have been examined via transient absorbance spectroscopy. Reaction-order studies establish that, at least in the short time regime, electrons are transferred directly from the tin oxide conduction band, rather than through localized redox trap states. The reactions occur in the high driving force regime (ΔG = −1.1 to −2.3 eV) and span the Marcus normal region, barrierless region, and inverted region. (Inverted reactivity, while commonplace in homogeneous solution-phase reactions, has only rarely been observed in interfacial reactions.) Depending on the reactant, normal or inverted kinetic behavior can also be observed via pH-induced manipulation of the conduction band-edge energy and, therefore, the overall reaction driving force. The observation of kinetically resolved ET over such a wide range of driving forces permits the reorganization energy to be evaluated directly from the maximum of a log(rate constant) versus driving force plot. The value obtained, 1.4 eV, is much larger than expected based on solvent contributions alone. Further analysis of driving force effects suggests that significant, but not dominant, nonclassical contributions (high-frequency vibrational contributions) to the reorganization energy exist. Rate measurements in the barrierless region yield an estimated initial-state/final-state electronic coupling energy, Hab, of 15−30 cm-1, a value consistent with a moderately nonadiabatic ET pathway. Remarkably, even in the inverted region the reactions are thermally activated, with the activation effect evidently being amplified via an entropic driving force effect. Finally, the overall pattern of reactivity stands in remarkable contrast to the pH-independent, trapped-mediated kinetic behavior encountered for closely related metal complexes covalently bound to nanocrystalline TiO2 surfaces