Photocatalysis offers unparalleled opportunities for greener synthesis conditions as well as allows for the exploitation of chemical reactions that are thermodynamically or kinetically unfeasible in the ground state. Photobases, molecules whose basicity strength significantly increases upon electronic excitation, have not yet been widely used in photocatalysis despite their potential. In particular, experiments on 5-substituted quinolines performed by Dawlaty and coworkers demonstrated a strong dependence of the photobasicity strength on the identity of the substituent. We have expanded upon this by developing structure-function relationships in <i>N</i>-heterocycles that include the effects of substituent identity and location, the number of substituent groups, the number of fused aromatic rings, and the presence of multiple ring nitrogen atoms.<br/> <br/>In this talk, we will describe two new directions in our research group on understanding the photochemistry of photobases. In the first, we explore how the propensity for intersystem crossing into the triplet manifold is affected by changes to the molecular structure. This is motivated by previous experimental studies by the Dawlaty group which demonstrate a complex relationship between the identity of the substituent group on the 5-substituted quinoline and the role of triplet states in the photochemistry. We describe our efforts to use TDDFT to develop a mechanistic explanation for the experimental observations. In the second, we extend our photobasicity structure-function relationships to transition metal complexes in which a photobase is incorporated as a pendant ligand. We demonstrate that our computational approach agrees with previous photochemical studies of the two photobasic transition metal complexes that have been experimentally studied. We then explore how changing the structure of the photobase, the transition metal, and the other ligands affect the thermodynamic driving force for photobasicity. Overall, our work gives new insights into the photochemistry of photobases that will hopefully inspire the development of photocatalysts that exploit photobasicity.

New directions in the photochemical structure-function relationships of photobases: The role of intersystem crossing and the incorporation of photobases in transition metal complexes

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ACS Fall 2022

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