Concentration Dependent Dimensionality of Resonance Energy Transfer in a Postsynthetically Doped Morphologically Homologous Analogue of UiO-67 MOF with a Ruthenium(II) Polypyridyl Complex

A method is described here by which to dope ruthenium­(II) bis­(2,2′-bipyridine) (2,2′-bipyridyl-5,5′-dicarboxylic acid), RuDCBPY, into a UiO-67 metal–organic framework (MOF) derivative in which 2,2′-bipyridyl-5,5′-dicarboxylic acid, UiO-67-DCBPY, is used in place of 4,4′-biphenyldicarboxylic acid. Emission lifetime measurements of the RuDCBPY triplet metal-to-ligand charge transfer, ³MLCT, excited state as a function of RuDCBPY doping concentration in UiO-67-DCBPY are discussed in light of previous results for RuDCBPY-UiO-67 doped powders in which quenching of the ³MLCT was said to be due to dipole–dipole homogeneous resonance energy transfer, RET. The bulk distribution of RuDCBPY centers within MOF crystallites are also estimated with the use of confocal fluorescence microscopy. In the present case, it is assumed that the rate of RET between RuDCBPY centers has an <i>r</i>^–6 separation distance dependence characteristic of Förster RET. The results suggest (1) the dimensionality in which RET occurs is dependent on the RuDCBPY concentration ranging from one-dimensional at very low concentrations up to three-dimensional at high concentration, (2) the occupancy of RuDCBPY within UiO-67-DCBPY is not uniform throughout the crystallites such that RuDCBPY densely populates the outer layers of the MOF at low concentrations, and (3) the average separation distance between RuDCBPY centers is ∼21 Å

Rethinking Band Bending at the P3HT–TiO₂ Interface

The advancement of solar cell technology necessitates a detailed understanding of material heterojunctions and their interfacial properties. In hybrid bulk heterojunction solar cells (HBHJs), light-absorbing conjugated polymers are often interfaced with films of nanostructured TiO₂ as a cheaper alternative to conventional inorganic solar cells. The mechanism of photovoltaic action requires photoelectrons in the polymer to transfer into the TiO₂, and therefore, polymers are designed with lowest unoccupied molecular orbital (LUMO) levels higher in energy than the conduction band of TiO₂ for thermodynamically favorable electron transfer. Currently, the energy level values used to guide solar cell design are referenced from the separated materials, neglecting the fact that upon heterojunction formation material energetics are altered. With spectroelectrochemistry, we discovered that spontaneous charge transfer occurs upon heterojunction formation between poly­(3-hexylthiophene) (P3HT) and nanocrystalline TiO₂. It was determined that deep trap states (0.5 eV below the conduction band of TiO₂) accept electrons from P3HT and form hole polarons in the polymer. This equilibrium charge separation alters energetics through the formation of interfacial dipoles and results in band bending that inhibits desired photoelectron injection into TiO₂, limiting HBHJ solar cell performance. X-ray photoelectron spectroscopic studies quantified the resultant vacuum level offset to be 0.8 eV. Further spectroelectrochemical studies indicate that 0.1 eV of this offset occurs in TiO₂, whereas the balance occurs in P3HT. New guidelines for improved photocurrent are proposed by tuning the energetics of the heterojunction to reverse the direction of the interfacial dipole, enhancing photoelectron injection

Characterizing Defects in a UiO-AZB Metal–Organic Framework

Exploring defect sites in metal–organic framework materials has quickly become an interesting topic of discussion in the literature. With reports of the enhancement of material properties with increasing defect sites, we were interested in probing the defect nature of UiO-AZB (UiO = University of Oslo, AZB = 4,4′-azobenzene­dicarboxylate) nanoparticles. In this report, we investigate the use of acetic, formic, and benzoic acids as the modulators to prepare UiO-AZB. The results of ¹H NMR techniques and BET surface area analysis elucidate the extent of defects in our samples and are provided along with detailed discussions of the observed experimental trends. Interestingly, formic acid samples resulted in the most defected structure, reaching 36%. Additionally, for benzoic acid samples, with a 33% defect level, a drastic reduction in the accessible SA from 2682 m²/g to as low as 903 m²/g was observed, as the concentration of benzoic acid was increased. This was attributed to the creation of macropores in the individual crystallites and confirmed by average pore width analysis

Solvothermal Preparation of an Electrocatalytic Metalloporphyrin MOF Thin Film and its Redox Hopping Charge-Transfer Mechanism

A thin film of a metalloporphyrin metal–organic framework consisting of [5,10,15,20-(4-carboxyphenyl)­porphyrin]­Co­(III) (CoTCPP) struts bound by linear trinuclear Co^II-carboxylate clusters has been prepared solvothermally on conductive fluorine-doped tin oxide substrates. Characterization of this mesoporous thin film material, designated as CoPIZA/FTO, which is equipped with large cavities and access to metal active sites, reveals an electrochemically active material. Cyclic voltammetry displays a reversible peak with <i>E</i>_1/2 at −1.04 V vs ferrocyanide attributed to the (Co^III/IITCPP)­CoPIZA redox couple and a quasi-reversible peak at −1.45 V vs ferrocyanide, which corresponds to the reduction of (Co^II/ITCPP)­CoPIZA. Analysis of the spectroelectrochemical response for the (Co^II/ITCPP)­CoPIZA redox couple revealed non-Nernstian reduction with a nonideality factor of 2 and an <i>E</i>_1/2 of −1.39 V vs ferrocyanide. The film was shown to retain its structural integrity with applied potential, as was demonstrated spectroelectrochemically with maintenance of isosbestic points at 430, 458, and 544 nm corresponding to the (Co^III/IITCPP)­CoPIZA transition and at 390 and 449 nm corresponding to the (Co^II/ITCPP)­CoPIZA transition. The mechanism of charge transport through the film is proposed to be a redox hopping mechanism, which is supported by both cyclic voltammetry and spectroelectrochemistry. A fit of the time-dependent spectroelectrochemical data to a modified Cottrell equation gave an apparent diffusion coefficient of 7.55 (±0.05) × 10^–14 cm²/s for ambipolar electron and cation transport throughout the film. Upon reduction of the metalloporphyrin struts to (Co^ITCPP)­CoPIZA, the CoPIZA thin film demonstrated catalytic activity for the reduction of carbon tetrachloride

Mn^II/III Complexes as Promising Redox Mediators in Quantum-Dot-Sensitized Solar Cells

The advancement of quantum dot sensitized solar cell (QDSSC) technology depends on optimizing directional charge transfer between light absorbing quantum dots, TiO₂, and a redox mediator. The nature of the redox mediator plays a pivotal role in determining the photocurrent and photovoltage from the solar cell. Kinetically, reduction of oxidized quantum dots by the redox mediator should be rapid and faster than the back electron transfer between TiO₂ and oxidized quantum dots to maintain photocurrent. Thermodynamically, the reduction potential of the redox mediator should be sufficiently positive to provide high photovoltages. To satisfy both criteria and enhance power conversion efficiencies, we introduced charge transfer spin-crossover Mn^II/III complexes as promising redox mediator alternatives in QDSSCs. High photovoltages ∼1 V were achieved by a series of Mn poly­(pyrazolyl)­borates, with reduction potentials ∼0.51 V vs Ag/AgCl. Back electron transfer (recombination) rates were slower than Co­(bpy)₃, where bpy = 2,2′-bipyridine, evidenced by electron lifetimes up to 4 orders of magnitude longer. This is indicative of a large barrier to electron transport imposed by spin-crossover in these complexes. Low solubility prevented the redox mediators from sustaining high photocurrent due to mass transport limits. However, with high fill factors (∼0.6) and photovoltages, they demonstrate competitive efficiencies with Co­(bpy)₃ redox mediator at the same concentration. More positive reduction potentials and slower recombination rates compared to current redox mediators establish the viability of Mn poly­(pyrazolyl)­borates as promising redox mediators. By capitalizing on these characteristics, efficient Mn^II/III-based QDSSCs can be achieved with more soluble Mn-complexes

Benzene, Toluene, and Xylene Transport through UiO-66: Diffusion Rates, Energetics, and the Role of Hydrogen Bonding

The high-energy demand of benzene, toluene, and xylene (BTX) separation highlights the need for improved nonthermal separation techniques and materials. Because of their high surface areas, tunable structures, and chemical stabilities, metal–organic frameworks (MOFs) are a promising class of materials for use in more energy efficient, adsorption-based separations. In this work, BTX compounds in the pore environment of UiO-66 were systematically examined using in situ infrared (IR) spectroscopy to understand the fundamental interactions that influence molecular transport through the MOF. Isothermal diffusion experiments revealed BTX diffusivities between 10^–8 and 10^–12 cm² s^–1, where the rate follows the trend: <i>o</i>-xylene < <i>m</i>-xylene < <i>p</i>-xylene. Corresponding activation energies of diffusion (<i>E</i>_diff) were determined to be 44 kJ mol^–1 for the xylene isomers and 34 kJ mol^–1 for both benzene and toluene, with the diffusion-limiting barrier identified to be molecular passage through the small triangular pore apertures of UiO-66. Furthermore, IR spectroscopy and computational methods showed the formation of two types of hydrogen bonds between BTX molecules and the μ₃-OH groups located in the tetrahedral cavities of UiO-66, which indicates that BTX molecules are capable of fully accessing the inner pore environment of the MOF. The molecular-level insight into the diffusion mechanism and energetics of BTX transport through UiO-66 presented in this work provides rich insight for the design of next-generation MOFs for cost-effective separation processes

Solvothermal Growth and Photophysical Characterization of a Ruthenium(II) Tris(2,2′-Bipyridine)-Doped Zirconium UiO-67 Metal Organic Framework Thin Film

A thin film of a Ru^II(bpy)₂(dcbpy)­Cl₂, RuDCBPY, doped metal–organic framework of Zr₆O₄(OH)₄(bpdc)₆, RuDCBPY-UiO67 (where bpy is 2,2′-bipyridine, dcbpy is 5,5′-dicarboxyphenyl-2,2′-bipyridine, and bpdc is 4,4′-biphenyldicarboxylic acid), has been prepared on fluorine-doped tin oxide and glass slides solvothermally. The film is shown to be isostructural with UiO-67 and similarly doped RuDCBPY-UiO67 powders. The photophysical properties of the film show significant line broadening of the diffuse reflectance spectra, a successive red shift of the emission maxima, and biphasic kinetics with increased RuDCBPY doping of UiO-67 films above 10 m<i>m</i>. The two lifetime components are consistent with a dual population of RuDCBPY within the UiO-67 material: a population of RuDCBPY incorporated into the framework of UiO-67 replacing a bpdc ligand and a second population of RuDCBPY encapsulated within the octahedral cavities. The RuDCBPY dopant within the UiO-67 films interact with each other and undergo self-quenching via a resonance energy transfer mechanism. It was determined that the average distance between RuDCBPY is decreased in the film relative to similarly doped powders. This is attributed to an electrostatic effect upon formation of the framework due to increased charge at the bpdc self-assembled monolayer at the surface of the substrate

Improving the Efficiency of the Mn^2+/3+ Couple in Quantum Dot Solar Cells: The Role of Spin Crossover

In this study, we present the synthesis of a family of first-row transition metal redox mediators based on the bis­[hydrotris­(pyrazolyl)]­borate manganese­(II/III) (MnTp₂) redox couple. Using cyclic voltammetry, the electrochemical properties and characteristic spin crossover inherent in this class of metal complexes were analyzed. From the electrochemical analysis the standard heterogeneous rate constant (<i>k</i>_s,h) was estimated. These constants were 1–2 orders of magnitude lower than other outer-sphere redox couples, such as Co­(bpy)₃^2+/3+ with <i>k</i>_s,h values decreasing from 6.39 (± 0.7) × 10^–3 cm/s in Co­(bpy)₃ to 1.60 (± 0.3) × 10^–5 cm/s in bis­[hydrotris­(4-ethylpyrazolyl)]­borate manganese­(II/III). It was theorized that the drastic reduction in the rate of electron transfer could be used to increase the lifetimes of the injected electrons in quantum dot sensitized solar cells (QDSSCs). Indeed, this was found to be the case with the slope of open-circuit voltage decay measurements being an order of magnitude lower in the Mn-based redox couples, compared to Co­(bpy)₃ when using cells prepared under the same conditions. This increase led us to then focus on optimizing the electrolyte solvent to assess the current–voltage characteristics of cells prepared using the MnTp₂ family of redox mediators. These cells displayed enhanced power conversion efficiencies when compared to Co­(bpy)₃ despite poor diffusion throughout the nanostructured TiO₂ film. Analysis of the quenching rate constant via Stern–Volmer quenching analysis suggested that the MnTp₂ family of redox mediators possesses an adequate ability to regenerate the quantum dot sensitizer, with values of <i>k</i>_q being on similar orders of magnitude as other Co- and Cu-based redox couples employed in dye-sensitized solar cells. Ultimately, it was concluded that the increase in the lifetime of the injected electron, working in concert with increased open-circuit voltage potentials, was the source of the significantly improved power conversion efficiencies

Proton-Coupled Electron Transport in Anthraquinone-Based Zirconium Metal–Organic Frameworks

The ditopic ligands 2,6-dicarboxy-9,10-anthraquinone and 1,4-dicarboxy-9,10-anthraquinone were used to synthesize two new UiO-type metal–organic frameworks (MOFs; namely, 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF, respectively). The Pourbaix diagrams (<i>E</i> vs pH) of the MOFs and their ligands were constructed using cyclic voltammetry in aqueous buffered media. The MOFs exhibit chemical stability and undergo diverse electrochemical processes, where the number of electrons and protons transferred was tailored in a Nernstian manner by the pH of the media. Both the 2,6-Zr-AQ-MOF and its ligand reveal a similar electrochemical p<i>K</i>_a value (7.56 and 7.35, respectively) for the transition between a two-electron, two-proton transfer (at pH < p<i>K</i>_a) and a two-electron, one-proton transfer (at pH > p<i>K</i>_a). In contrast, the position of the quinone moiety with respect to the zirconium node, the effect of hydrogen bonding, and the amount of defects in 1,4-Zr-AQ-MOF lead to the transition from a two-electron, three-proton transfer to a two-electron, one-proton transfer. The p<i>K</i>_a of this framework (5.18) is analogous to one of the three electrochemical p<i>K</i>_a values displayed by its ligand (3.91, 5.46, and 8.80), which also showed intramolecular hydrogen bonding. The ability of the MOFs to tailor discrete numbers of protons and electrons suggests their application as charge carriers in electronic devices

Thermodynamic Study of CO₂ Sorption by Polymorphic Microporous MOFs with Open Zn(II) Coordination Sites

Two Zn-based metal organic frameworks have been prepared solvothermally, and their selectivity for CO₂ adsorption was investigated. In both frameworks, the inorganic structural building unit is composed of Zn­(II) bridged by the 2-carboxylate or 5-carboxylate pendants of 2,5-pyridine dicarboxylate (pydc) to form a 1D zigzag chain. The zigzag chains are linked by the bridging 2,5-carboxylates across the Zn ions to form 3D networks with formulas of Zn₄(pydc)₄­(DMF)₂·3DMF (<b>1</b>) and Zn₂(pydc)₂­(DEF) (<b>2</b>). The framework (<b>1</b>) contains coordinated DMF as well as DMF solvates (DMF = <i>N</i>,<i>N</i>-dimethylformamide), while (<b>2</b>) contains coordinated DEF (DEF = <i>N</i>,<i>N</i>-diethylformamide). (<b>1</b>) displays a reversible type-I sorption isotherm for CO₂ and N₂ with BET surface areas of 196 and 319 m²/g, respectively. At low pressures, CO₂ and N₂ isotherms for (<b>2</b>) were not able to reach saturation, indicative of pore sizes too small for the gas molecules to penetrate. A solvent exchange to give (<b>2</b>)-<b>MeOH</b> allowed for increased CO₂ and N₂ adsorption onto the MOF surface with BET surface areas of 41 and 39 m²/g, respectively. The binding of CO₂ into the framework of (<b>1</b>) was found to be exothermic with a zero coverage heat of adsorption, <i>Q</i>_<i>st</i>⁰, of −27.7 kJ/mol. The <i>Q</i>_<i>st</i>⁰ of (<b>2</b>) and (<b>2</b>)-<b>MeOH</b> were found to be −3 and −41 kJ/mol, respectively. The CO₂/N₂ selectivity for (<b>1</b>), calculated from the estimated <i>K_H</i> at 296 K, was found to be 42. At pressures relevant to postcombustion capture, the selectivity was 14. The thermodynamic data are consistent with a mechanism of adsorption that involves CO₂ binding to the unsaturated Zn­(II) metal centers present in the crystal structures