Using the Intervalence Charge Transfer Band in Mixed Valence Mixed Protonated Metal Dithiolene Complexes to Follow Ground State Proton-coupled Electron Transfer PDF Download
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Author: Steven Kennedy
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Languages : en
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Proton-coupled electron transfer (PCET) is an important phenomenon for controlling charge mobility in chemistry and biology because it allows the simultaneous movement of a proton and electron with a lower energy barrier than otherwise possible. Much work has been done on PCET systems, particularly for excited state processes in which charge mobility can be easily followed using pump-probe methods. However, while excited state PCET is utilized for the initial step of many solar energy-driven processes, including photosynthesis, ground state PCET is critical for all subsequent processes, including regeneration of solar cells. Homogeneous ground state PCET systems are of particular interest for this regeneration, but no convenient method exists for measuring parameters governing such reactions. Our work is directed toward understanding homogeneous ground state PCET reactions as probed using solution-phase steady-state methods.In order to establish a probe for these homogeneous ground state PCET reactions, we design self-exchange model systems for PCET in analogy to classical electron transfer. With our first model system, [Ni(2,3-pyrazinedithiol)2], we demonstrate that protonation of a mixed valence species, generating a mixed valence mixed protonated (MVMP) state, results in severe reduction of the electronic coupling intimately connected with electron transfer kinetics. This ligand-based mixed valence molecule can be asymmetrically protonated, rendering the MVMP state. We characterize the structural, electronic, vibrational, and magnetic properties of this complex in five different states, including the mixed valence and MVMP states, and then analyze the intervalence charge transfer (IVCT) band to demonstrate a five-fold reduction in electronic coupling upon protonation. We conclude that the reduction in electronic coupling is a result of the asymmetry of the electronic orbitals of the redox sites that results from the asymmetric protonation. As a result, the IVCT band is established as a probe for interrogating the electronic coupling in the MVMP state, which reflects the change in the PCET potential energy landscape as a result of protonation. This conclusion suggests that many systems designed to link electron and proton transfer will also exhibit a decrease in electronic coupling upon protonation as the strength of the interaction between redox and protonation sites is increased.After having established the MVMP state as a useful model system to study homogeneous ground state PCET, we explored structural modifications to control the communication between electron transfer and protonation sites. These studies allow for a more fine-tuned response to protonation in a series of metal dithiolene complexes when moving from the mixed valence to the MVMP state. We investigate the effect of changing the bridge between ligands simply by changing the metal center. In this study, we find nearly five-fold decreases in electronic coupling for both Ni and Pt, while, for the Pd complex, the electronic coupling is reduced to the point that the IVCT band is no longer observable. We ascribe the reduction in electronic coupling to charge pinning induced by asymmetric protonation. The more severe reduction in coupling for the Pd complex is a result of greater energetic mismatch between ligand and metal orbitals, reflected in the smaller electronic coupling for the pure mixed valence state. This work demonstrates that the bridging metal center can be used to tune the electronic coupling in both the mixed valence and MVMP states, as well as the magnitude of change of the electronic coupling that accompanies changes in protonation state.In addition, we explore 2,3-quinoxalinedithiol and 2,3-pyridinedithiol ligands, which are structurally altered versions of the above dithiolene ligands in which the aromatic rings are extended and the number of ring nitrogen atoms is reduced, respectively. With these complexes, we find that these modifications cause changes in the electronic coupling both in the mixed valence and MVMP states, and the degree of response to protonation, generating the MVMP state, is controlled as well. For [Ni(2,3-quinoxalinedithiol)2], the only complex with the 2,3-quinoxalinedithiol ligand that reversibly generated its MVMP state, the IVCT band, and hence the electronic coupling, disappeared upon protonation. This disappearance of electronic coupling resulted from additional electron density being placed on the ligands and not being channeled into ligand-ligand electronic coupling through the metal center. The complex [Ni(2,3-pyridinedithiol)2] retained its IVCT band in the MVMP state, but with less electronic coupling than in the 2,3-pyrazinedithiol analogue. This lower value of electronic coupling is a result of higher energy ligand orbitals that overlap with the metal orbitals to a lesser extent.Lastly, we explore the [Au(2,3-pyrazinedithiol)2] complex, which is appealing for the non-innocent character of its ligands. We report its synthesis and characterization, along with electrochemistry and spectrophotometric response to acid titration. This molecule did not exhibit generation of its singly oxidized mixed valence species, so it does not permit direct comparison to the mixed valence species of the other metal dithiolene compounds in this study.Ultimately, our investigations of these metal dithiolene MVMP model systems allow for more informed control over PCET self-exchange transformations, as interrogated through their IVCT bands. The IVCT band is established as a probe for monitoring the effect of asymmetric protonation upon electronic coupling, seeking to extend classical electron transfer model systems into the domain of PCET. The interdependence of asymmetric protonation and electron transfer will allow for better control over PCET charge mobility through structural modifications, which will allow for more rational design of systems that undergo ground state PCET in device applications.
Author: Steven Kennedy
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Proton-coupled electron transfer (PCET) is an important phenomenon for controlling charge mobility in chemistry and biology because it allows the simultaneous movement of a proton and electron with a lower energy barrier than otherwise possible. Much work has been done on PCET systems, particularly for excited state processes in which charge mobility can be easily followed using pump-probe methods. However, while excited state PCET is utilized for the initial step of many solar energy-driven processes, including photosynthesis, ground state PCET is critical for all subsequent processes, including regeneration of solar cells. Homogeneous ground state PCET systems are of particular interest for this regeneration, but no convenient method exists for measuring parameters governing such reactions. Our work is directed toward understanding homogeneous ground state PCET reactions as probed using solution-phase steady-state methods.In order to establish a probe for these homogeneous ground state PCET reactions, we design self-exchange model systems for PCET in analogy to classical electron transfer. With our first model system, [Ni(2,3-pyrazinedithiol)2], we demonstrate that protonation of a mixed valence species, generating a mixed valence mixed protonated (MVMP) state, results in severe reduction of the electronic coupling intimately connected with electron transfer kinetics. This ligand-based mixed valence molecule can be asymmetrically protonated, rendering the MVMP state. We characterize the structural, electronic, vibrational, and magnetic properties of this complex in five different states, including the mixed valence and MVMP states, and then analyze the intervalence charge transfer (IVCT) band to demonstrate a five-fold reduction in electronic coupling upon protonation. We conclude that the reduction in electronic coupling is a result of the asymmetry of the electronic orbitals of the redox sites that results from the asymmetric protonation. As a result, the IVCT band is established as a probe for interrogating the electronic coupling in the MVMP state, which reflects the change in the PCET potential energy landscape as a result of protonation. This conclusion suggests that many systems designed to link electron and proton transfer will also exhibit a decrease in electronic coupling upon protonation as the strength of the interaction between redox and protonation sites is increased.After having established the MVMP state as a useful model system to study homogeneous ground state PCET, we explored structural modifications to control the communication between electron transfer and protonation sites. These studies allow for a more fine-tuned response to protonation in a series of metal dithiolene complexes when moving from the mixed valence to the MVMP state. We investigate the effect of changing the bridge between ligands simply by changing the metal center. In this study, we find nearly five-fold decreases in electronic coupling for both Ni and Pt, while, for the Pd complex, the electronic coupling is reduced to the point that the IVCT band is no longer observable. We ascribe the reduction in electronic coupling to charge pinning induced by asymmetric protonation. The more severe reduction in coupling for the Pd complex is a result of greater energetic mismatch between ligand and metal orbitals, reflected in the smaller electronic coupling for the pure mixed valence state. This work demonstrates that the bridging metal center can be used to tune the electronic coupling in both the mixed valence and MVMP states, as well as the magnitude of change of the electronic coupling that accompanies changes in protonation state.In addition, we explore 2,3-quinoxalinedithiol and 2,3-pyridinedithiol ligands, which are structurally altered versions of the above dithiolene ligands in which the aromatic rings are extended and the number of ring nitrogen atoms is reduced, respectively. With these complexes, we find that these modifications cause changes in the electronic coupling both in the mixed valence and MVMP states, and the degree of response to protonation, generating the MVMP state, is controlled as well. For [Ni(2,3-quinoxalinedithiol)2], the only complex with the 2,3-quinoxalinedithiol ligand that reversibly generated its MVMP state, the IVCT band, and hence the electronic coupling, disappeared upon protonation. This disappearance of electronic coupling resulted from additional electron density being placed on the ligands and not being channeled into ligand-ligand electronic coupling through the metal center. The complex [Ni(2,3-pyridinedithiol)2] retained its IVCT band in the MVMP state, but with less electronic coupling than in the 2,3-pyrazinedithiol analogue. This lower value of electronic coupling is a result of higher energy ligand orbitals that overlap with the metal orbitals to a lesser extent.Lastly, we explore the [Au(2,3-pyrazinedithiol)2] complex, which is appealing for the non-innocent character of its ligands. We report its synthesis and characterization, along with electrochemistry and spectrophotometric response to acid titration. This molecule did not exhibit generation of its singly oxidized mixed valence species, so it does not permit direct comparison to the mixed valence species of the other metal dithiolene compounds in this study.Ultimately, our investigations of these metal dithiolene MVMP model systems allow for more informed control over PCET self-exchange transformations, as interrogated through their IVCT bands. The IVCT band is established as a probe for monitoring the effect of asymmetric protonation upon electronic coupling, seeking to extend classical electron transfer model systems into the domain of PCET. The interdependence of asymmetric protonation and electron transfer will allow for better control over PCET charge mobility through structural modifications, which will allow for more rational design of systems that undergo ground state PCET in device applications.
Author: Starla Demorest Glover
Publisher:
ISBN: 9781124703428
Category :
Languages : en
Pages : 175
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Investigations into the dynamics of picosecond electron transfer in a series of mixed valence systems of the type [Ru3([mu]3-O)(OAc)6(py)(CO)-([mu]2-BL)- Ru3([mu]3-O)(OAc)6(py)(CO)]−1, where BL = 1,4-pyrazine or 4,4'-bipyridine and py = 4-dimethylaminopyridine, pyridine, or 4-cyanopyridine are described. Solvent and temperature dependence into the rate of ground state intramolecular electron transfer is probed by infrared analysis of [nu](CO) bandshapes where simulated rate constants yield to rates ranging from 4 E 11 to 3 E 12 s-1. Correlations between rate constants and solvent properties including solvent reorganization energy, optical and static dielectric constants, microscopic solvent polarity, viscosity, principal rotational moments of inertia, and solvent dipolar relaxation times, have been examined. Correlations revealed a marked lack of dependence on electron transfer rates with respect to solvent thermodynamic parameters, and a strong dependence on solvent dynamic parameters. This is consistent with electron transfers having very low activation barriers that approach zero. Temperature dependent studies revealed electron transfer rates accelerated as the freezing points of solvent solutions were approached with a sharp increase in the rate of electron transfer upon freezing. This has been attributed to a localized-to-delocalized transition in these mixed valence ions at the solvent phase transition. This non-Arrhenius behavior is explained in terms of decoupling the slower solvent motions involved in the frequency factor, [nu]N, which weights faster vibrational promoter modes that increase the value of [nu]N. Solvent and temperature dependence of optically induced intramolecular electron transfer is probed by analysis of intervalence charge transfer bands in NIR spectra. The application of a semi-classical three-state model for mixed valency best describes the electronic spectra wherein is the appearance of two intervalence bands; a band which has metal-to-metal-charge-transfer character and another having metal-to-ligand-charge-transfer character. This three-state model fully captures the observed spectroscopic behavior where the MBCT transition increases in energy and the MMCT band decreases in energy as electronic communication increases through the series of mixed valence ions. The solvent and temperature dependence of the MBCT and MMCT electronic transitions is found to persist as coalescence of infrared vibrational spectra suggest ground state delocalization on the vibrational timescale. The solvent and temperature dependence of the MBCT and MMCT electronic transitions defines the mixed valence complexes as lying at the borderline of delocalization. Fine tuning the electronic coupling in the series of dimers has allowed for the resolution of a full Class II, early Class II/III, late Class II/III to Class III systems and the influence of solvent dynamics in each regime. These investigations have prompted the redefinition of borderline Class II/III mixed valency to account for outer sphere (solvent) contributions to electron transfer; in nearly delocalized systems, solvent dynamics localized otherwise delocalized electronic ground states. Further, studies explore the origins and dynamics behind spectral coalescence of vibrational [nu] (CO) bandshapes in [Ru3([mu]3-O)(OAc)6(py)(CO)-([mu]2-BL)- Ru3([mu]3-O)(OAc)6(py)(CO)]−1 systems and a picosecond isomerization in square pyramidal Ru(S2C4F6)(P(C6H5)3)2(CO) system.
Author: Gabriele Canzi
Publisher:
ISBN: 9781321011050
Category :
Languages : en
Pages : 204
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Understanding the intricacies of inner sphere electron transfer has been a challenge for nearly 50 years. Since the preparation of the Creutz-Taube ion extensive research in inorganic mixed valence systems has been performed. We employ coalescence of [nu](CO) bandshapes observed in the 1-D infrared (IR) spectra of mixed valence complexes to determine rate constants of electron transfer (ET). Herein we report synthesis, characterization, and spectroscopy of Ru3O clusters bound to metallic nanoparticles, and report ET rates in the "ultrafast" regime. We observe that ET rates are faster when there is favorable electronic alignment between the Ru clusters and the Au nanoparticle. In addition, results show that ground state ET rate constants that are in the "ultrafast" regime depend on the pre-exponential term within the frequency factor, [nu]N not the activation energy as expected in a system undergoing ergodic electron transfer. We extended our knowledge of these complexes by studying ET at a semiconducting nanoparticle interface. Working in collaboration with Prof. Emily Weiss at Northwestern University, a complementary view of the parameters that govern ET in such systems has been developed by investigating ET rates between the triruthenium clusters and QDs. The photoinduced electron transfer rate from photoexcited CdSe QDs to triruthenium clusters having either a pyridine-4-carboxylic acid or a 4-mercaptopyridine linkage are reported. Results show that the intrinsic charge separation rate constant (kCS,int), is approximately seven times faster for a thiol linked cluster compared to a nicotinic acid bound cluster. Thus the charge transfer rates between colloidal quantum dots and redox-active ligands adsorbed to their surfaces can be tuned through the choice of the coordinating headgroup of the ligand. We report that exchange of electrons across hydrogen bonds can increase the strength of typically weak interactions. A thermodynamically stable mixed valence dimer is obtained upon the one electron reduction of a Ru3O cluster with a isonicotinic acid ancillary ligand. Observed intervalence charge transfer bands (IVCT) indicate significant coupling between the two Ru centers through linked by a hydrogen bonding interaction. The IVCT bands are found to be best explained by a semi-classical 3-state model, further highlighting the importance of the bridging interaction in these systems. Additionally, we report that the electronic coupling between two metal centers can be modulated by simple ancillary ligand substitution. The wavefunction overlap of two metal centers bridged by a hydrogen bond is found to be non-zero. We report a series of new Ru3O clusters with ancillary ligands capable of pi-stacking in solution upon a single electron reduction. Large splittings are observed berween the reductions in the electrochemical responses of these newly synthesized systems. The effects on the electrochemical splitting of the reduction waves by donating and withdrawing ligands on the "bridge" are compared. A crystal structure of the ground state shows no significant evidence of pi-pi interaction between clusters in solution. The major themes of this thesis are the role of electronic coupling, Hab, on long range ET in supramolecular mixed valence systems, and the importance of the bridging interaction in modulating Hab in these systems.
Author: Jane Susan Henderson
Publisher:
ISBN: 9781321848304
Category :
Languages : en
Pages : 10
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Mixed-valence dimers of the type Ru3(O)(OAc)6(CO)L-BL-Ru3(O)(OAc)6(CO)L (BL = bridging ligand and L = a pyridyl ligand) form strongly coupled systems in their [Ru3III,III,II-BL- Ru3III,II,II]- state, observed by intervalence charge transfer (IVCT) bands in the near-IR. Ancillary ligand substitution has been shown to control bimolecular electron transfer rates from electronically excited zinc tetraphenylporphyrin (ZnTPP); quenching constants, kq, for 3ZnTPP* are 3.0 × 109, 1.5 × 109, and 1.1 × 109 M−1 s−1 for BL = pyrazine (pz), L = 4-cyanopyridine (cpy), pyridine (py), or 4-dimethylaminopyridine (dmap), respectively. The preparation, electrochemistry, and spectroscopic characterization of three new species, Ru3(O)(OAc)6(CO)(ZnTPPpy)-pz-Ru3(O)(OAc)6(CO)L, where ZnTPPpy = zinc(II) 5-(4-pyridyl)- 10,15,20-triphenylporphyin and L = dmap, py or cpy, are reported. Observation of IVCT band growth under continual photolysis ([lambda]exc = 568 nm) confirms a phototriggered intramolecular electron transfer from Zn porphyrin to the Ru3O donor-bridge-acceptor dimer, resulting in a strongly coupled mixed-valence species. Femtosecond transient absorption spectroscopy was implemented to follow photoinduced electron transfer reactions in the series of asymmetric porphyrin-coordinated dyads. Excitation of the porphyrin subunit resulted in electron transfer to the Ru3O dimer with a time constant [tau] ≈ 0.6 ps. The intramolecular electron transfer was confirmed by excitation of the Ru3O MLCT, which resulted in the formation of a vibrationally unrelaxed porphyrin ground state. Under both excitation experiments, the back electron transfer was extremely fast ([tau]CR
Author: Angela Bischof
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ISBN:
Category :
Languages : en
Pages :
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Electron transfer processes are widespread in chemistry, and form the basis of photosynthetic systems and molecular electronic devices. However, studying electron transfer processes in these systems directly can be difficult. In order to understand the fundamental processes in these systems, charge transfer (CT) and mixed-valence (MV) compounds have been used extensively as model systems to study the basics of electron transfer reactions under various conditions. This dissertation uses steady-state spectroscopic analyses to determine how structure affects electronic coupling and charge delocalization in several MV and CT systems that are of interest for understanding the fundamental processes that occur in organic devices. In Chapter 2, we find that the introduction of an ethylene bridging ligand between two bis(alkoxy)benzene redox sites increases the intramolecular electronic coupling when compared to the system in which the redox active sites are directly connected, despite an increase in distance. We quantify the increase in coupling as a function of both distance and steric effects. Chapter 3 focuses on the systematic investigation of how van der Waals forces control the electronic coupling in non-covalently bound organic mixed-valence naphthalene diimide dimers, and we find that, in general, as the van der Waals forces in the intermolecular complex increase so does the electronic coupling. Chapter 4 focuses on the analysis of the CT band in donor-acceptor charge-transfer liquid crystals (DACLCs) and uses resonance Raman spectroscopy to investigate the phase-dependence of the charge transfer transition and the vibrational modes that are associated with this transition. Overall, this dissertation contributes to the fundamental understanding of how structure impacts charge transfer in organic MV and CT systems.
Author: Casey Hughes Londergan
Publisher:
ISBN:
Category :
Languages : en
Pages : 320
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Author: Christina J. Hanson
Publisher:
ISBN:
Category :
Languages : en
Pages : 111
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Proton-Coupled Electron Transfer (PCET) is an important mechanistic motif in chemistry, which allows for efficient charge transport in many biological systems. We seek to understand how the proton and electron motions are coupled in a bidirectional system allowing for individual turning of the kinetics and thermodynamics. The target of interest is a biomimedic heme system allowing for a detailed mechanistic study of the formation of the oxidation states of heme, of particular interest the highly reactive Fe(IV)=O species. The bidirectional model is prepared using a hangman porphyrin with an axially coordinated to the metal center, and the electron transfer event is triggered by excitation of the porphyrin. The synthesis of this motif is discussed as well as initial studies into the binding of a coordinated electron acceptor to the metal center. In the future, the excited state of the acceptor will be used to trigger the electron transfer portion of the PCET event. To understand the signatures of different electron acceptors and binding to the metal center, a redox inactive zinc porphyrin is used as a model to allow for longer excited state lifetimes and well known transient signatures. Three diimide acceptors have been coordinated through a pyridine ring to the metal center of the porphyrin, and electron transfer was triggered both by excitation of the porphyrin and the acceptor. Lifetimes of the charge separated state were determined using picoseconds and nanosecond transient absorption. The acceptors are then coordinated to a symmetrical iron porphyrin in an attempt to understand the behavior of charge separation in the more complicated open d shell system. Spectroscopic data of both systems is shown.
Author: T. J. Meyer
Publisher:
ISBN:
Category :
Languages : en
Pages : 18
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This report covers work done by the author in both mixed-valence dimers and metallopolymers. Concerning the dimers, crystal structures of related monomers were obtained to judge the adequacy of classical approximations in calculating inner-sphere vibrational trapping. A series of homometallic and heterometallic dimers were prepared and a relationship between redox asymmetry and intervalence transfer absorption band energy was found. Electronic structure was observed in the intervalence transfer absorption band of an OS(II)-Os(III) mixed-valence dimer, owing to the presence of well-separated spin orbit states in Os(III). Other dimers were prepared to study the transitions between various states within the dimers. Both metal to metal and ligand to ligand electron transfer was seen in optically prepared mixed-valence dimers. Concerning the polymers, redox-site incorporation into poly-vinylpyridine, P-chlorosulfonated polystyrene and oxidatively electro-polymerized films were studied. Characterization and stability of these films as well as comparison to monomeric analogues was accomplished. Practical application of the metallopolymers include their use as a chromophore in a photoelectrochemical cell and as a catalyst for the oxidation of C1- to C12.
Author: Michael Jay Powers
Publisher:
ISBN:
Category : Iron
Languages : en
Pages : 398
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Author: Bruce Conrad Bunker
Publisher:
ISBN:
Category :
Languages : en
Pages : 416
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