USING THE INTERVALENCE CHARGE TRANSFER BAND IN MIXED VALENCE MIXED PROTONATED METAL DITHIOLENE COMPLEXES TO FOLLOW GROUND STATE PROTON-COUPLED ELECTRON TRANSFER

Open Access
Author:
Kennedy, Steven Robert
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 01, 2016
Committee Members:
  • Benjamin James Lear, Dissertation Advisor
  • Benjamin James Lear, Committee Chair
  • Alexander Thomas Radosevich, Committee Member
  • Joseph M Bollinger Jr., Committee Member
  • John H Golbeck, Outside Member
Keywords:
  • mixed valence
  • proton-coupled electron transfer
  • mixed valence mixed protonated
  • metal dithiolene
  • electronic spectroscopy
  • intervalence charge transfer
  • solar energy
  • ground state electron transfer
Abstract:
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.