CAS: Assessing the Applicability of Marcus Theory to Excited-state Proton Transfer and Protonic Photochemical Energy Conversion

In this project, funded by the Chemical Structure, Dynamics & Mechanism B Program of the Chemistry Division, Professor Shane Ardo of the Department of Chemistry at the University of California-Irvine is developing a series of photo-acid and photo-base molecules with specifically designed proton donor and proton acceptor groups. The goal of this research is to use these molecules to evaluate the driving-force dependence for rate constants associated with excited-state proton transfer, which is important for solar energy utilization.

The project lies at the interface of organic and photophysical chemistry, and is therefore well suited to the education of scientists at all levels. This group is also well-positioned to provide the highest level of education and training for students underrepresented in science. Outreach activities with middle school students that promote interest in STEM (science, technology, engineering and mathematics) and foster trust in scientists and science will also be part of the funded project.

Photo-acids and photo-bases are molecules whose acidity changes reversibly upon absorption of light. This can result in excited-state proton transfer, with energetics described using the well-known Förster cycle analysis. Recently, Professor Shane Ardo and his group showed that many photo-acids and photo-bases that do not undergo excited state proton transfer due to kinetic limitations can be re-engineered to overcome this barrier through the introduction of proton acceptor and/or proton donor buffer species.

This allowed the research team to accurately predict excited state acidity, and thus standard state driving force, for proton transfer. In this project, they aim to validate their new method and then use it to quantify the relationship between standard-state driving force and activation free energy for proton-transfer reactions, including in the Marcus inverted region.

They also aim to leverage this fundamental understanding to design and demonstrate photochemical protonic energy conversion systems that use light to modulate the protonation state of two buffer species. They attempt to answer the following critical questions: 1) Are driving-force-dependent theories for the kinetics of electron transfer and proton transfer, e.g.

Rehm–Weller, Marcus–Cohen, Agmon–Levine, suitable to describe excited-state proton transfer reactions? 2) Can the ground-state acidity/basicity of photo-acids/photo-bases be tuned so that buffers and/or protic solvents serve as proton donors/acceptors for regeneration of the ground-state conjugate base/acid of photo-acids/photo-bases?

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.