The Full Photoscience of Radicals

The Physical Chemistry

Most of the field’s photophysical intuition comes from decades of study on closed-shell chromophores, composing for the most part singlet and triplet excited-state spin manifolds. Radicals are comparatively understudied chromophores because of their inherent instability and reactivity, and as a result their excited-state behavior which takes place in the doublet and quartet spin manifolds are yet to be fully explored. In preliminary work, we’ve found that radical excited-states, even just one electron different from some of best-understood chromophores on the planet, have bizarre and unintuitive photophysical dynamics. Our suspicion is that this behavior arises from mixing of vibrational and electronic degrees of freedom in these radical excited states.

The Kudisch Lab will approach these mysterious photophysical properties using an ultrafast impulsive Raman spectroscopy, which is capable of interrogating these “vibronic” interactions while the molecule exhibits its strange excited-state dynamics. By applying these cutting-edge spectroscopic methods to this unique class of chromophores, we think we can carve out a more general understanding of molecular excited-states across spin manifolds and derive new predictive rules for excited-state properties.

Relevance to Synthetic Photochemistry

Radical ion excited-states have been implicated as extremely redox-active photoreagents in a number of photoredox transformations. While this can be thermodynamically rationalized based on their excited-state redox potentials, which can span values as oxidizing as fluorine gas or as reducing as potassium metal, our recent work complicates this picture due to the ultrafast excited-state relaxation of radicals and the appearance of other highly redox-active and photoactive species under reaction conditions. Understanding the photophysics of radical excited-states will allow synthetic chemists to better deduce the mechanisms of their photochemical reactions, and we hope will eventually lead to the design of highly reactive, long-lived radical excited states for photocatalysis and more.

Relevant Publications

Rieth, A.J., Gonzalez, M.I., Kudisch, B., Nava, M. J., Nocera, D.G. How Radical Are ‘Radical’ Photocatalysts? A Closed-Shell Meisenheimer Complex Is Identified as a Super-Reducing Photoreagent. 2021, J. Am. Chem. Soc., 143, 14352.

Rafiq, S., Fu, B., Kudisch, B., Scholes, G.D. Interplay of Vibrational Wavepackets during an Ultrafast Electron Transfer Reaction. 2021, Nat. Chem., 13, 70.