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- All Subjects: Charge exchange
- Creators: Moore, Ana L
- Creators: Richert, Ranko
Description
Natural photosynthesis dedicates specific proteins to achieve the modular division of the essential roles of solar energy harvesting, charge separation and carrier transport within natural photosynthesis. The modern understanding of the fundamental photochemistry by which natural photosynthesis operates is well advanced and solution state mimics of the key photochemical processes have been reported previously. All of the early events in natural photosynthesis responsible for the conversion of solar energy to electric potential energy occur within proteins and phospholipid membranes that act as scaffolds for arranging the active chromophores. Accordingly, for creating artificial photovoltaic (PV) systems, scaffolds are required to imbue structure to the systems. An approach to incorporating modular design into solid-state organic mimics of the natural system is presented together with how conductive scaffolds can be utilized in organic PV systems. To support the chromophore arrays present within this design and to extract separated charges from within the structure, linear pyrazine-containing molecular ribbons were chosen as candidates for forming conductive linear scaffolds that could be functionalized orthogonally to the linear axis. A series of donor-wire-acceptor (D-W-A) compounds employing porphyrins as the donors and a C60 fullerene adduct as the acceptors have been synthesized for studying the ability of the pyrazine-containing hetero-aromatic wires to mediate photoinduced electron transfer between the porphyrin donor and fullerene acceptor. Appropriate substitutions were made and the necessary model compounds useful for dissecting the complex photochemistry that the series is expected to display were also synthesized. A dye was synthesized using a pyrazine-containing heteroaromatic spacer that features two porphyrin chromophores. The dye dramatically outperforms the control dye featuring the same porphyrin and a simple benzoic acid linker. A novel, highly soluble 6+kDa extended phthalocyanine was also synthesized and exhibits absorption out to 900nm. The extensive functionalization of the extended phthalocyanine core with dodecyl groups enabled purification and characterization of an otherwise insoluble entity. Finally, in the interest of incorporating modular design into plastic solar cells, a series of porphyrin-containing monomers have been synthesized that are intended to form dyadic and triadic molecular-heterojunction polymers with dedicated hole and electron transport pathways during electrochemical polymerization.
ContributorsWatson, Brian Lyndon (Author) / Gust, Devens (Thesis advisor) / Gould, Ian (Committee member) / Moore, Ana L (Committee member) / Arizona State University (Publisher)
Created2013
Description
The first chapter reviews three decades of artificial photosynthetic research conducted by the A. Moore, T. Moore, and D. Gust research group. Several carotenoid (Car) and tetrapyrrole containing molecules were synthesized and investigated for excitation energy transfer (EET), photoregulation, and photoprotective functions. These artificial photosynthetic compounds mimicked known processes and investigated proposed mechanisms in natural systems. This research leads to a greater understanding of photosynthesis and design concepts for organic based solar energy conversion devices. The second and third chapters analyze the triplet energy transfer in carotenoid containing dyads. Transient absorption, time-resolved FTIR and resonance Raman spectra revealed that in a 4-amide linked carotenophthalocyanine dyads the Car triplet state is shared across the larger conjugated system, which is similar to protein complexes in oxygenic photosynthetic organisms. In a carotenopurpurin dyad (CarPur) a methylene ester covalent bond prevents the purpurin (Pur) from influencing the Car triplet based on the transient absorption, time-resolved FTIR and resonance Raman spectra. Thus CarPur resembles the antenna proteins from anoxygenic photosynthetic bacteria. Additional examples of carotenoporphyrin dyads further demonstrates the need for orbital overlap for ultrafast triplet energy transfer and the formations of possible intramolecular charge transfer state. The fourth chapter studies a 4-amino phenyl carotenophthalocyanine and its model compounds using high temporal resolution transient absorption spectroscopy techniques. EET from the Car second excited (S2) state to the phthalocyanine (Pc) was determined to be 37% and a coupled hot ground state (S*)/Pc excited state spectrum was observed. Excitation of the tetrapyrrole portion of the dyad did not yield any kinetic differences, but there was an S* signal during the excited states of the dyad. This demonstrates the EET and photoregulating properties of this artificial photosynthetic compound are similar to those of natural photosynthesis. The last chapter covers the synthesis of silicon Pc (SiPc) dyes and the methods for attaching them to gold nanoparticles and flat gold surfaces. SiPc attached to patterned gold surfaces had unperturbed fluorescence, however the selectivity for the gold was low, so alternative materials are under investigation to improve the dye's selectivity for the gold surface.
ContributorsWongCarter, Katherine (Author) / Moore, Ana L (Thesis advisor) / Gust, Devens (Committee member) / Gould, Ian (Committee member) / Arizona State University (Publisher)
Created2014
Description
The work described in the thesis involves the synthesis of a molecular triad which is designed to undergo proton coupled electron transfer (PCET) upon irradiation with light. Photoinduced PCET is an important process that many organisms use and the elucidation of its mechanism will allow further understanding of this process and its potential applications. The target compound designed for PCET studies consists of a porphyrin chromophore (also a primary electron donor), covalently linked to a phenol-imidazole (secondary electron donor), and a C60 (primary electron acceptor). The phenol-imidazole moiety of this system is modeled after the TyrZ His-190 residues in the reaction center of Photosystem II (PS II). These residues participate in an intermolecular H-bond between the phenol side chain of TyrZ and the imidazole side chain of His-190. The phenol side chain of TyrZ is the electron transfer mediator between the oxygen evolving complex (OEC) and P680 (primary electron donor) in PSII. During electron transfer from TyrZ to P680*+, the phenolic proton of TyrZ becomes highly acidic (pKa~-2) and the hydrogen is preferentially transferred to the relatively basic imidazole of His-190 through a pre-existing hydrogen bond. This PCET process avoids a charged intermediate, on TyrZ, and results in a neutral phenolic radical (TyrZ*). The current research consists of building a molecular triad, which can mimic the photoinduced PCET process of PSII. The following, documents the synthetic progress in the synthesis of a molecular triad designed to investigate the mechanism of PCET as well as gain further insight on how this process can be applied in artificial photosynthetic devices.
ContributorsPatterson, Dustin (Author) / Moore, Ana L (Thesis advisor) / Gust, Devens (Committee member) / Skibo, Edward B (Committee member) / Arizona State University (Publisher)
Created2011
Description
Converting solar energy into electricity is a reasonable way to ameliorate the current untenable energy situation. One way to harness solar energy is to mimic the mechanisms already present in natural photosynthesis. A key component of many artificial photosynthetic systems is the linker connecting the dye to an electrode. Studying the associated electron transport process is important for improving linker efficiency. Similarly it is important to be able to control the electron transfer to the dye from a water oxidation catalyst, and to be able to improve the lifetime of the charge separated state. Natural photosynthesis provides a blueprint for this in the tyrosine-histidine pair in photosystem II. In this work, research on these topics is described.
ContributorsTomlin, John Jacob (Author) / Moore, Ana L (Thesis advisor) / Gust, Devens (Committee member) / Kodis, Gerdenis (Committee member) / Arizona State University (Publisher)
Created2015
Description
As sunlight is an ideal source of energy on a global scale, there are several approaches being developed to harvest it and convert it to a form that can be used. One of these is though mimicking the processes in natural photosynthesis. Artificial photosynthetic systems include dye sensitized solar cells for the conversion of sunlight to electricity, and photoelectrosynthetic cells which use sunlight to drive water oxidation and hydrogen production to convert sunlight to energy stored in fuel. Both of these approaches include the process of the conversion of light energy into chemical potential in the form of a charge-separated state via molecular compounds. Porphyrins are commonly used as sensitizers as they have well suited properties for these applications. A high potential porphyrin with four nitrile groups at the beta positions, a β-cyanoporphyrin (CyP), was investigated and found to be an excellent electron acceptor, as well as have the necessary properties to be used as a sensitizer for photoelectrosynthetic cells for water oxidation. A new synthetic method was developed which allowed for the CyP to be used in a number of studies in artificial photosynthetic systems. This dissertation reports the theories behind, and the results of four studies utilizing a CyP for the first time; as a sensitizer in a DSSC for an investigation of its use in light driven water oxidation photoelectrosynthetic cells, as an electron acceptor in a proton coupled electron transfer system, in a carotene-CyP dyad to study energy and electron transfer processes between these moieties, and in a molecular triad to study a unique electron transfer process from a C60 radical anion to the CyP. It has been found that CyPs can be used as powerful electron acceptors in molecular systems to provide a large driving force for electron transfer that can aid in the process of the conversion of light to electrochemical potential. The results from these studies have led to a better understanding of the properties of CyPs, and have provided new insight into several electron transfer reactions.
ContributorsAntoniuk-Pablant, Antaeres' Dawn (Author) / Gust, Devens (Thesis advisor) / Moore, Ana L (Committee member) / Ghirlanda, Giovanna (Committee member) / Arizona State University (Publisher)
Created2015
Description
How water behaves at interfaces is relevant to many scientific and technological applications; however, many subtle phenomena are unknown in aqueous solutions. In this work, interfacial structural transition in hydration shells of a polarizable solute at critical polarizabilities is discovered. The transition is manifested in maximum water response, the reorientation of the water dipoles at the interface, and an increase in the density of dangling OH bonds. This work also addresses the role of polarizability of the active site of proteins in biological catalytic reactions. For proteins, the hydration shell becomes very heterogeneous and involves a relatively large number of water molecules. The molecular dynamics simulations show that the polarizability, along with the atomic charge distribution, needs to be a part of the picture describing how enzymes work. Non Gaussian dynamics in time-resolved linear and nonlinear (correlation) 2D spectra are also analyzed.
Additionally, a theoretical formalism is presented to show that when preferential orientations of water dipoles exist at the interface, electrophoretic charges can be produced without free charge carriers, i.e., neutral solutes can move in a constant electric field due to the divergence of polarization at the interface. Furthermore, the concept of interface susceptibility is introduced. It involves the fluctuations of the surface charge density caused by thermal motion and its correlation over the characteristic correlation length with the fluctuations of the solvent charge density. Solvation free energy and interface dielectric constant are formulated accordingly. Unlike previous approaches, the solvation free energy scales quite well in a broad range of ion sizes, namely in the range of 2-14 A° . Interface dielectric constant is defined such that the boundary conditions in the Laplace equation describing a micro- or mesoscopic interface are satisfied. The effective dielectric constant of interfacial water is found to be significantly lower than its bulk value. Molecular dynamics simulation results show that the interface dielectric constant for a TIP3P water model changes from nine to four when the effective solute radius is increased from 5 A° to 18 A° . The small value of the interface dielectric constant of water has potentially dramatic consequences for hydration.
Additionally, a theoretical formalism is presented to show that when preferential orientations of water dipoles exist at the interface, electrophoretic charges can be produced without free charge carriers, i.e., neutral solutes can move in a constant electric field due to the divergence of polarization at the interface. Furthermore, the concept of interface susceptibility is introduced. It involves the fluctuations of the surface charge density caused by thermal motion and its correlation over the characteristic correlation length with the fluctuations of the solvent charge density. Solvation free energy and interface dielectric constant are formulated accordingly. Unlike previous approaches, the solvation free energy scales quite well in a broad range of ion sizes, namely in the range of 2-14 A° . Interface dielectric constant is defined such that the boundary conditions in the Laplace equation describing a micro- or mesoscopic interface are satisfied. The effective dielectric constant of interfacial water is found to be significantly lower than its bulk value. Molecular dynamics simulation results show that the interface dielectric constant for a TIP3P water model changes from nine to four when the effective solute radius is increased from 5 A° to 18 A° . The small value of the interface dielectric constant of water has potentially dramatic consequences for hydration.
ContributorsDinpajooh, Mohammadhasan (Author) / Matyushov, Dmitry V (Thesis advisor) / Richert, Ranko (Committee member) / Beckstein, Oliver (Committee member) / Arizona State University (Publisher)
Created2016
Temperature and polarizability effects on electron transfer in biology and artificial photosynthesis
Description
This study aims to address the deficiencies of the Marcus model of electron transfer
(ET) and then provide modifications to the model. A confirmation of the inverted energy
gap law, which is the cleanest verification so far, is presented for donor-acceptor complexes.
In addition to the macroscopic properties of the solvent, the physical properties of the solvent
are incorporated in the model via the microscopic solvation model. For the molecules
studied in this dissertation, the rate constant first increases with cooling, in contrast to the
prediction of the Arrhenius law, and then decreases at lower temperatures. Additionally,
the polarizability of solute, which was not considered in the original Marcus theory, is included
by the Q-model of ET. Through accounting for the polarizability of the reactants, the
Q-model offers an important design principle for achieving high performance solar energy
conversion materials. By means of the analytical Q-model of ET, it is shown that including
molecular polarizability of C60 affects the reorganization energy and the activation barrier
of ET reaction.
The theory and Electrochemistry of Ferredoxin and Cytochrome c are also investigated.
By providing a new formulation for reaction reorganization energy, a long-standing disconnect
between the results of atomistic simulations and cyclic voltametery experiments is
resolved. The significant role of polarizability of enzymes in reducing the activation energy
of ET is discussed. The binding/unbinding of waters to the active site of Ferredoxin leads
to non-Gaussian statistics of energy gap and result in a smaller activation energy of ET.
Furthermore, the dielectric constant of water at the interface of neutral and charged
C60 is studied. The dielectric constant is found to be in the range of 10 to 22 which is
remarkably smaller compared to bulk water( 80). Moreover, the interfacial structural
crossover and hydration thermodynamic of charged C60 in water is studied. Increasing the
charge of the C60 molecule result in a dramatic structural transition in the hydration shell,
which lead to increase in the population of dangling O-H bonds at the interface.
(ET) and then provide modifications to the model. A confirmation of the inverted energy
gap law, which is the cleanest verification so far, is presented for donor-acceptor complexes.
In addition to the macroscopic properties of the solvent, the physical properties of the solvent
are incorporated in the model via the microscopic solvation model. For the molecules
studied in this dissertation, the rate constant first increases with cooling, in contrast to the
prediction of the Arrhenius law, and then decreases at lower temperatures. Additionally,
the polarizability of solute, which was not considered in the original Marcus theory, is included
by the Q-model of ET. Through accounting for the polarizability of the reactants, the
Q-model offers an important design principle for achieving high performance solar energy
conversion materials. By means of the analytical Q-model of ET, it is shown that including
molecular polarizability of C60 affects the reorganization energy and the activation barrier
of ET reaction.
The theory and Electrochemistry of Ferredoxin and Cytochrome c are also investigated.
By providing a new formulation for reaction reorganization energy, a long-standing disconnect
between the results of atomistic simulations and cyclic voltametery experiments is
resolved. The significant role of polarizability of enzymes in reducing the activation energy
of ET is discussed. The binding/unbinding of waters to the active site of Ferredoxin leads
to non-Gaussian statistics of energy gap and result in a smaller activation energy of ET.
Furthermore, the dielectric constant of water at the interface of neutral and charged
C60 is studied. The dielectric constant is found to be in the range of 10 to 22 which is
remarkably smaller compared to bulk water( 80). Moreover, the interfacial structural
crossover and hydration thermodynamic of charged C60 in water is studied. Increasing the
charge of the C60 molecule result in a dramatic structural transition in the hydration shell,
which lead to increase in the population of dangling O-H bonds at the interface.
ContributorsWaskasi, Morteza M (Author) / Matyushov, Dmitry (Thesis advisor) / Richert, Ranko (Committee member) / Heyden, Matthias (Committee member) / Beckstein, Oliver (Committee member) / Arizona State University (Publisher)
Created2019
Description
We studied the relationship between the polarizability and the molecular conductance
that arises in the response of a molecule to an external electric field. To illustrate
the plausibility of the idea, we used Simmons' tunneling model, which describes image
charge and dielectric effects on electron transport through a barrier. In such a
model, the barrier height depends on the dielectric constant of the electrode-molecule-electrode junction, which in turn can be approximately expressed in terms of the
molecular polarizability via the classical Clausius-Mossotti relation. In addition to
using the tunneling model, the validity of the relationships between the molecular
polarizability and the molecular conductance was tested by comparing calculated
and experimentally measured conductance of different chemical structures ranging
from covalent bonded to non-covalent bonded systems. We found that either using
the tunneling model or the first-principle calculated quantities or experimental data,
the conductance decreases as the molecular polarizability increases. In contrast to
this strong correlation, our results showed that in some cases there was a weaker or
none correlation between the conductance and other molecular electronic properties
including HOMO-LUMO gap, chemical geometries, and interactions energies. All
these results together suggest that using the molecular polarizability as a molecular
descriptor for conductance can offer some advantages compared to using other
molecular electronic properties and can give additional insight about the electronic
transport property of a junction.
These results also show the validity of the physically intuitive picture that to a first
approximation a molecule in a junction behaves as a dielectric that is polarized in the
opposite sense of the applied bias, thereby creating an interfacial barrier that hampers
tunneling. The use of the polarizability as a descriptor of molecular conductance offers
signicant conceptual and practical advantages over a picture based in molecular
orbitals. Despite the simplicity of our model, it sheds light on a hitherto neglected
connection between molecular polarizability and conductance and paves the way for
further conceptual and theoretical developments.
The results of this work was sent to two publications. One of them was accepted
in the International Journal of Nanotechnology (IJNT) and the other is still under
review in the Journal of Physical Chemistry C.
that arises in the response of a molecule to an external electric field. To illustrate
the plausibility of the idea, we used Simmons' tunneling model, which describes image
charge and dielectric effects on electron transport through a barrier. In such a
model, the barrier height depends on the dielectric constant of the electrode-molecule-electrode junction, which in turn can be approximately expressed in terms of the
molecular polarizability via the classical Clausius-Mossotti relation. In addition to
using the tunneling model, the validity of the relationships between the molecular
polarizability and the molecular conductance was tested by comparing calculated
and experimentally measured conductance of different chemical structures ranging
from covalent bonded to non-covalent bonded systems. We found that either using
the tunneling model or the first-principle calculated quantities or experimental data,
the conductance decreases as the molecular polarizability increases. In contrast to
this strong correlation, our results showed that in some cases there was a weaker or
none correlation between the conductance and other molecular electronic properties
including HOMO-LUMO gap, chemical geometries, and interactions energies. All
these results together suggest that using the molecular polarizability as a molecular
descriptor for conductance can offer some advantages compared to using other
molecular electronic properties and can give additional insight about the electronic
transport property of a junction.
These results also show the validity of the physically intuitive picture that to a first
approximation a molecule in a junction behaves as a dielectric that is polarized in the
opposite sense of the applied bias, thereby creating an interfacial barrier that hampers
tunneling. The use of the polarizability as a descriptor of molecular conductance offers
signicant conceptual and practical advantages over a picture based in molecular
orbitals. Despite the simplicity of our model, it sheds light on a hitherto neglected
connection between molecular polarizability and conductance and paves the way for
further conceptual and theoretical developments.
The results of this work was sent to two publications. One of them was accepted
in the International Journal of Nanotechnology (IJNT) and the other is still under
review in the Journal of Physical Chemistry C.
ContributorsVatan Meidanshahi, Reza (Author) / Mujica, Vladimiro (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Richert, Ranko (Committee member) / Arizona State University (Publisher)
Created2014