On the Photocyclization of Tetraphenyl-Substituted Anthraquinodimethanes: Mechanistic Insights, Substitution Effects,...

Mallory reactions taking place on tetraphenyl-substituted anthraquinodimethanes (Ph₄-AQDs) were investigated through a joint theoretical and experimental approach, offering fundamental understanding of the critical factors that govern regioselectivity and stepwise reactivity. Our density functional theory (DFT) modeling demonstrates that the reaction proceeds via a twisted C═C bond in the first excited state, which facilitates efficient crossing from the first excited state to the ground state via a conical intersection that resembles the thermal transition state both geometrically and energetically. For this reason, Mallory reactions on Ph₄-AQD are inherently limited to two photocyclization events, with the second step showing a particular regioselectivity. Experimentally, we synthesized and characterized a series of Ph₄-AQD derivatives with alkoxy groups attached to various positions of the phenyl moieties. Monitoring of their Mallory reactions indicates that steric effects from substituents exert kinetic control over the reaction pathway. While the first photocyclization is rate-limiting in minimally hindered systems, increased steric encumbrance dramatically slows down the second photocyclization step, enabling the isolation and characterization of the monocyclized intermediate. Beyond the mechanistic knowledge established, we also investigated Mallory products resulting from various Ph₄-AQDs, which exhibit significantly enhanced fluorescence efficiency with quantum yields up to 0.44 and diverse fluorescence decay parameters. Overall, this study provides fundamental mechanistic insights and synthetic guidelines for synthesizing relevant functional polycyclic aromatic structures using the Mallory reaction as a key cyclization strategy.