Towards Designer Photocatalysts: Structure-Property Relationships in 2,6-Diaryl-pyryliums

Fully organic photocatalyst systems are highly attractive, not merely because they are transition-metal free, but more importantly due to their unique and often potent reactivity. A detailed understanding of the various redox states, both ground and excited state, and specifically what structural parameters control them is therefore crucial for harnessing the full potential of these system in organic synthesis. However, unlike their organometallic counterparts, detailed structure-property relationships for organic photocatalysts are largely absent from the literature. In this study, we demonstrate linear free-energy relationships across a range of key photophysical and electrochemical properties of 2,6-diarylpyryliums. Electronic absorption and emission maxima can be carefully tuned over the ranges of 83 nm and 102 nm respectively. Intramolecular charge transfer (ICT) interactions were revealed in cases of substitution with polarizable heavy-atoms. A strong linear dependence of ground state reduction potentials on substituent electronics was observed. Notably, the excited state reduction potential, E*red, could be controlled over a range of nearly 1000 mV. Systematic errors in computational modeling of ground and excited state redox potentials were identified and corrected. We believe the quantitative structure-property relationships identified here provide foundational tools for rational and predictive organic photocatalyst design.