Theoretical Investigation of Charge Transfer in Metal Organic Frameworks for Electrochemical Device Applications

For electrochemical device applications metal organic frameworks (MOFs) must exhibit suitable conduction properties. To this end, we have performed computational studies of intermolecular charge transfer in MOFs consisting of hexa-Zr^IV nodes and tetratopic carboxylate linkers. This includes an examination of the electronic structure of linkers that are derived from tetraphenyl benzene <b>1</b>, tetraphenyl pyrene <b>2</b>, and tetraphenyl porphyrin <b>3</b> molecules. These results are used to determine charge transfer propensities in MOFs, within the framework of Marcus theory, including an analysis of the key parameters (charge transfer integral <i>t</i>, reorganization energy λ, and free energy change Δ<i>G</i>⁰) and evaluation of figures of merit for charge transfer based on the chemical structures of the linkers. This qualitative analysis indicates that delocalization of the HOMO/LUMO on terminal substituents increases <i>t</i> and decreases λ, while weaker binding to counterions decreases Δ<i>G</i>⁰, leading to better charge transfer propensity. Subsequently, we study hole transfer in the linker <b>2</b> containing MOFs, <b>NU-901</b> and <b>NU-1000</b>, in detail and describe mechanisms (hopping and superexchange) that may be operative under different electrochemical conditions. Comparisons with experiment are provided where available. On the basis of the redox and catalytic activity of nodes and linkers, we propose three possible schemes for constructing electrochemical devices for catalysis. We believe that the results of this study will lay the foundation for future experimental work on this topic

Efficient Charge Transport in Semisynthetic Zinc Chlorin Dye Assemblies

We have studied the charge transport properties of self-assembled structures of semisynthetic zinc chlorins (ZnChls) in the solid state by pulsed radiolysis time-resolved microwave conductivity measurements. These materials can form either a two-dimensional (2D) brickwork-type slipped stack arrangement or a one-dimensional (1D) tubular assemblies, depending on the exact molecular structure of the ZnChls. We have observed efficient charge transport with mobilities as high as 0.07 cm² V^–1 s^–1 for tubular assemblies of 3¹-hydroxy ZnChls and up to 0.28 cm² V^–1 s^–1 for 2D stacked assemblies of 3¹-methoxy ZnChls at room temperature. The efficient charge transporting capabilities of these organized assemblies opens the way to supramolecular electronics based on biological systems

Structure and Electronic Spectra of Purine–Methyl Viologen Charge Transfer Complexes

The structure and properties of the electron donor–acceptor complexes formed between methyl viologen and purine nucleosides and nucleotides in water and the solid state have been investigated using a combination of experimental and theoretical methods. Solution studies were performed using UV–vis and ¹H NMR spectroscopy. Theoretical calculations were performed within the framework of density functional theory (DFT). Energy decomposition analysis indicates that dispersion and induction (charge-transfer) interactions dominate the total binding energy, whereas electrostatic interactions are largely repulsive. The appearance of charge transfer bands in the absorption spectra of the complexes are well-described by time-dependent DFT and are further explained in terms of the redox properties of purine monomers and solvation effects. Crystal structures are reported for complexes of methyl viologen with the purines 2′-deoxyguanosine 3′-monophosphate (DAD′DAD′ type) and 7-deazaguanosine (DAD′ADAD′ type). Comparison of the structures determined in the solid state and by theoretical methods in solution provides valuable insights into the nature of charge-transfer interactions involving purine bases as electron donors

Metal Oxide Nanoparticle Growth on Graphene via Chemical Activation with Atomic Oxygen

Chemically interfacing the inert basal plane of graphene with other materials has limited the development of graphene-based catalysts, composite materials, and devices. Here, we overcome this limitation by chemically activating epitaxial graphene on SiC(0001) using atomic oxygen. Atomic oxygen produces epoxide groups on graphene, which act as reactive nucleation sites for zinc oxide nanoparticle growth using the atomic layer deposition precursor diethyl zinc. In particular, exposure of epoxidized graphene to diethyl zinc abstracts oxygen, creating mobile species that diffuse on the surface to form metal oxide clusters. This mechanism is corroborated with a combination of scanning probe microscopy, Raman spectroscopy, and density functional theory and can likely be generalized to a wide variety of related surface reactions on graphene

Introducing Perovskite Solar Cells to Undergraduates

Introducing Perovskite Solar Cells to Undergraduate

Light-Harvesting and Ultrafast Energy Migration in Porphyrin-Based Metal–Organic Frameworks

Given that energy (exciton) migration in natural photosynthesis primarily occurs in highly ordered porphyrin-like pigments (chlorophylls), equally highly ordered porphyrin-based metal–organic frameworks (MOFs) might be expected to exhibit similar behavior, thereby facilitating antenna-like light-harvesting and positioning such materials for use in solar energy conversion schemes. Herein, we report the first example of directional, long-distance energy migration within a MOF. Two MOFs, namely <b>F-MOF</b> and <b>DA-MOF</b> that are composed of two Zn­(II) porphyrin struts [5,15-dipyridyl-10,20-bis­(pentafluorophenyl)­porphinato]­zinc­(II) and [5,15-bis­[4-(pyridyl)­ethynyl]-10,20-diphenylporphinato]­zinc­(II), respectively, were investigated. From fluorescence quenching experiments and theoretical calculations, we find that the photogenerated exciton migrates over a net distance of up to ∼45 porphyrin struts within its lifetime in <b>DA-MOF</b> (but only ∼3 in <b>F-MOF</b>), with a high anisotropy along a specific direction. The remarkably efficient exciton migration in <b>DA-MOF</b> is attributed to enhanced π-conjugation through the addition of two acetylene moieties in the porphyrin molecule, which leads to greater Q-band absorption intensity and much faster exciton-hopping (energy transfer between adjacent porphyrin struts). The long distance and directional energy migration in <b>DA-MOF</b> suggests promising applications of this compound or related compounds in solar energy conversion schemes as an efficient light-harvesting and energy-transport component