6 research outputs found
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<sup>IV</sup> 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><sup>0</sup>) 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><sup>0</sup>, 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
cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for tubular
assemblies of 3<sup>1</sup>-hydroxy ZnChls and up to 0.28 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for 2D stacked assemblies
of 3<sup>1</sup>-methoxy ZnChls at room temperature. The efficient
charge transporting capabilities of these organized assemblies opens
the way to supramolecular electronics based on biological systems
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
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 <sup>1</sup>H 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
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 ZnÂ(II) porphyrin struts [5,15-dipyridyl-10,20-bisÂ(pentafluorophenyl)Âporphinato]ÂzincÂ(II)
and [5,15-bisÂ[4-(pyridyl)Âethynyl]-10,20-diphenylporphinato]ÂzincÂ(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