Helicenes are spatially chiral, helical polycyclic aromatic compounds (PACs) with an array of emergent applications in soft matter technology. Materials research and applications of PACs are generally limited by challenges related to increasing production scale. Helicenes are typically synthesized via the photocyclodehydrogenation of di-aryl alkene-containing molecular species that form dihydrophenanthrenes (DHPs) which are then oxidized to form phenanthrene. Low concentrations are necessary to avoid unwanted intermolecular [2+2] photocycloadditions in conventional oxidative photocyclodehyrodegation that utilizes I<sub>2 </sub>as the oxidizing agent. Optimizing this reaction should positively impact the scalability of production. This presentation will elucidate some causes of the scalability limitation when using I<sub>2</sub> and will demonstrate the potential of TEMPO as a suitable alternative oxidizing agent with stilbene serving as the model di-aryl alkene. The mechanistic insights that are gained from the photo-oxidative reaction of this simpler compound is applied to approaches for large-scale synthesis of [7]- and [9]-helicene.

Optimizing oxidative photocyclodehydrogenation reactions towards scalable production of helicenes

ACS Spring Meeting 2021

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