Reports: UNI452165-UNI4: Proton-Coupled Electron Transfer in Ground-State Charge Transfer Reactions: Bi-molecular PCET and Intervalent Charge Transfer
Elizabeth R. Young, PhD, Amherst College
IntroductionManagement of charge transfer is a fundamental problem that the work of this grant aims to address. Charge balance must be maintained as certain chemical reactions progress, meaning that movement of an electron to and away from a reaction site may be coupled to shorter proton motion.
We are interested in teasing out the influence of proton motion on various electron transfer mechanism in model systems. Coupling of electron and proton motion has far-reaching implications for the numerous chemical systems that undergo multi-proton, multi-electron transformations. Coupled proton/electron movement has been revealed to be a critical component in the chemical activation of substrate bonds at catalytically active sites as well as in amino acid radical generation and transport through protein matrices.
Model System and Control CompoundWork in this proposal focuses on the model compound, Fc-amH (Scheme 1), which has been designed for the mechanistic study of bidirectional proton-coupled charge transfer. The structure of Fc-amH accentuates the effect of proton motion, marked by the protonation state of the amidinium functionality, on movement of electrons in the small-molecule ferrocene unit. The amidinium proton handle and the ferrocene redox center are very highly coupled supporting the intended goals for this model system. We have additionally developed a control compound Fc-mam. The spectroscopic and electrochemical properties of Fc-mam do not depend on proton motion as the moiety contains no protonic center.
Scheme 1: Ferrocenyl-amidinium and Ferrocenyl-methylammonium.
Results from Year 2This year, we turned our attention from photo-induced proton-coupled electron transfer (ET) to a different proton-coupled electron transfer mechanism, that of Dexter Energy transfer (EnT) (Scheme 2).
Scheme 2: Bimolecular, bi-directional proton-coupled EnT mechanism
Dexter Energy transfer is a concerted, two electron transfer between an excited state donor moiety and a ground-state acceptor. Scheme 3 contrasts the movement of electrons in photo-induced ET and Dexter EnT. In a Dexter EnT mechanism, two electrons transfer simultaneously resulting in a ground-state donor and an excited-state acceptor. The net result of Dexter EnT is the acceptor in its excited state, in contrast to ET, which results in a charge separated state after the transfer of only one electron. We reasoned that because Dexter EnT involves the movement of electrons, the kinetics of Dexter EnT should, akin to ET, be influenced by the movement protons.
Scheme 3: Schematic representation of electron movement in ET and Dexter EnT.
Ruthenium(II) tris-2,2’-bipyridine (Ru(bpy)3) was identified as a suitable choice for the energy transfer donor. Ru(bpy)3 is known for its excellent photostability and has been shown to undergo Dexter energy transfer with ferrocene derivatives. The spectroscopy and electrochemistry of Ru(bpy)3 itself show no pH-dependence. Therefore, any pH-dependence observed in the EnT experiments may be attributed solely to the contribution of the Fc-amH moiety.
A series of pH-dependent Stern-Volmer quenching experiments have been conducted to reveal that the EnT rates between Ru(bpy)3 and Fc-amH vary as a function of the protonation state of Fc-amH. A clear jump in the rate is observed at a pH corresponding to the pKa of the amidinium functionality (Figure 1). This result indicates that when the Fc-amH is deprotonated (at higher pH conditions), the EnT rate increases, and indeed Dexter EnT is influenced by the presence or absence of the acidic proton.
A control compound, Fc-mam, was introduced to establish that in the absence of an acid group, the quenching rate remains unperturbed as a function of pH. The data reveal a nearly constant Dexter EnT rate between Ru(bpy)3 and Fc-mam across the pH range of interest.
These results clearly show that bimolecular Dexter EnT may be influenced by proton motion associated with the acceptor moiety. We are currently in the process of verifying this behavior with a second ruthenium(II) polypyridyl complex to demonstrate its generality.
Figure 1: pH-dependent Dexter EnT rates.
Impact on Faculty and StudentsThree undergraduate students have been supported during the past year. Claire Drolen ’15, Stephen Hetterich ’15 and Sam Hendel ‘15 have worked on this project during their junior year (AY 2013-2014). Claire and Sam are continuing to work in the PI’s lab for their senior thesis work in Chemistry. Claire’s work remains directly grounded in this project. A fourth student, Nico Pascual-Leone ’16 has joined this project for his junior year (AY 2014-2015) and will work closely with Claire to support this project.
These undergraduate researchers have been exposed to fundamental aspects of charge transfer mechanisms and various experimental techniques including UV-visible absorption and fluorescence spectroscopy, electrochemistry, pKa titrations and Stern-Volmer analysis.
Stephen traveled to M.I.T. in September 2013 to carry out time-resolved kinetics measurements necessary for this project. Claire is currently working with researchers at UNC-Chapel Hill to coordinate a set of transient absorption experiments that will directly lend insight to the mechanisms at play in her research project.
Stephen, Sam and the PI attended the local (Northeast Regional) ACS meeting in October 2013 where both Sam and Stephen gave oral presentations in the undergraduate session.
Integrating Research and Teaching Sam’s project involved developing a physical chemistry teaching laboratory based on PCET, catalysis and electrochemistry, which are key components of the research under this grant. The laboratory was implemented in the Spring 2014 in Chemistry-361 at Amherst College. The lab was also implemented at University of Texas El Paso in Fall 2013 under the direction of Prof. Dino Villagran. Student surveys and assessments from this first year have been collected and analyzed. Revisions to the experimental procedure are underway and the experiment will again be run in Chemistry-361 at Amherst College (Spring 2015). After running the laboratory several times, the laboratory protocol will be submitted for publication to the ACS Journal of Chemical Education.
This teaching laboratory will also serve as a tutorial for students wishing to join the PI’s research lab as a formative aspect of their training in electrochemistry. Thus, Sam’s work will impact students in the teaching and research environment at Amherst. Aditi Krishnamurthy ’18 who joined the lab this fall will be the first research student to use this training module.
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