CAS: Photocatalysis Without Metals: Design Rules for Organic Photoredox Chemistry

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Shaama M. Sharada and her group at the University of Southern California (USC) are using computational methods to study organic photoredox catalysts. There is growing need for strategies to mitigate the adverse impacts of climate change.

Carbon capture and utilization offers means to trap and transform anthropogenic carbon dioxide (CO2) into useful fuels and chemicals. Harnessing sunlight to carry out CO2 conversion is essential because breaking the bonds in CO2 requires high energy input. Using organic light-activated molecules, or chromophores, as catalysts to facilitate this transformation lowers cost and toxicity concerns compared to traditional, heavy metal thermal catalysts.

Developing a fundamental mechanistic picture of the photoredox catalytic cycle is an essential first step toward unlocking the potential of these materials. Dr. Sharada and her research team aim to use quantum chemistry methods and machine learning to generate mechanistic insights and catalyst design rules for CO2 conversion.

The design rules are expected to have broader impact beyond carbon dioxide utilization as these catalysts find applications in organic synthesis, water-splitting, drug delivery, and biocides. Open-source modeling software used by the Sharada group for research can also serve as powerful visual learning aids for high school students. Since lasting impact can be achieved through partnership with teachers, summer externships are planned for teachers at the Hawthorne Math and Science Academy, to train them in the use of modeling methods and support the design of lesson plans for environmental sciences, chemistry, and biology classes.

Photoredox catalytic cycles with organic chromophores are difficult to study using experiments alone owing to the generation of several radical intermediates, complex solvation effects, varied product distributions, and low catalyst turnover numbers. Despite methodological advances in the treatment of charge transfer processes, organic chromophore studies largely focus on optoelectronic applications while mechanistic studies of photoredox cycles remain limited.

This work aims to pioneer the adaptation of a foundational principle in heterogeneous catalysis – the Sabatier principle – to photoredox systems and identify innate trade-offs that govern catalytic activity and turnover number (or resistance to degradation). The goal is to characterize exciplexes formed between the excited-state chromophore and electron donor prior to quenching and determine factors that favor complete quenching and charge separation vis-à-vis chromophore degradation via Birch reduction.

To this end, multiple quantum chemistry methods are to be employed, including density functional theory (DFT), constrained DFT, time-dependent DFT, and energy decomposition analysis for ground, excited-state, and solvated systems. Factors that lower the likelihood of degradation will be juxtaposed with those that also enhance the rates of electron transfer to CO2 to identify trade-offs underlying these performance metrics.

Characteristics of the photoredox system – solvent, electron donor, and chromophore properties – that lead to the desired balance between activity and degradation are expected to emerge. The computational work will be supported by experimental transient absorption spectroscopy studies by a collaborator at USC. Finally, these design rules will feed into machine learning methods to accelerate discovery, by rapidly searching the vast chemical space for chromophores that possess desired characteristics.

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.