58: Photophysics, linear free-energy relationships, and improved design of pyrylium photocatalysts

Abstract: The field of synthetic photocatalysis has witnessed a major renaissance in the last 15 years. Much of the focus for photocatalysts has been given to organometallic complexes of ruthenium and iridium, the photophysical and electrochemical properties of which are widely tunable via judicious choice of ligands. However, a single unified-yet-tunable fully organic photocatalyst platform has yet to emerge. Much of this is due to the fact that tuning the photophysical or electrochemical properties of organic photocatalysts is usually nontrivial, sometimes requiring a completely different chemical synthesis in order to produce a relatively minor change in substitution, and in many cases not well studied. Pyrylium salts represent an easily tunable class of photocatalysts. Previous studies have shown clear relationships between substitution on the external aromatic rings and the resulting photophysical properties. However, there are limited studies on the effects of substituents on the electrochemical properties of pyryliums and no quantitative correlations have been made for substituent effects on the excited state reduction potentials, one of the key properties for oxidizing photocatalysts. As part of our ongoing interest in the development of new pyrylium photocatalysts, we have studied the photophysical and electrochemical properties of a series of substituted 2,6-diarylpyrylium structures. Using Hammett analysis and computational calculations, we provide some quantitative relationships with predictive use for future photocatalyst design. We also briefly detail our work towards a final library of structurally improved pyrylium photocatalysts for broad synthetic utility.