Organic photoredox catalysis enables the conversion of light energy into chemical reactivity through photoinduced electron transfer processes. However, achieving highly reducing photochemistry typically requires high-energy light, which can lead to undesired side reactivity and limited light penetration. Lower-energy visible light offers advantages in selectivity, scalability, and compatibility with complex environments, but its reduced photon energy often limits access to strongly reducing excited states. Consecutive photoinduced electron transfer (conPET) strategies have emerged as a promising solution, generating highly reducing excited radical intermediates through sequential photon absorption; however, these systems are often limited by short excited-state lifetimes and reliance on blue light irradiation.

    This proposal explores helical carbenium photocatalysts as a platform for enabling highly reducing photochemistry under lower-energy visible light. In Aim 1, the green-light reactivity of N,N-di-n-propyl-1,13-dimethoxyquinacridinium tetrafluoroborate will be investigated using hydrodehalogenation of aryl halides as a benchmark transformation. Mechanistic studies employing transient absorption spectroscopy will probe potential pathways including direct single-electron transfer from excited radical species, substrate preassociation, solvated electron formation, and the generation of catalytically relevant intermediates. Aim 2 will leverage this mechanistic understanding and structure–property relationships within the helical carbenium family to design a new red-light-absorbing photocatalyst capable of accessing reducing excited species through consecutive photon absorption. Photophysical and electrochemical characterization will guide catalyst optimization and evaluate red-light-driven hydrodehalogenation reactivity. Finally, Aim 3 will expand the reactivity of these systems beyond photoreduction by exploring their potential as hydrogen-atom-transfer (HAT) catalysts, utilizing neutral radical photocatalyst species capable of forming and cleaving PC–H bonds under light irradiation.

This work aims to advance the mechanistic understanding and catalyst design principles necessary to achieve challenging photocatalysis under lower-energy light, expanding the reactivity accessible by low-energy light irradiation.