Electron Density-Dependent Mallory Photocyclization Constructs Rigid Saddle-Shaped Polycyclic Aromatic Hydrocarbons with Enhanced Aromaticity

Saddle-Shaped Polycyclic Aromatic Hydrocarbons have been synthesized via highly efficient Mallory reaction, and exhibited extended aromatic skeleton with high quantum yields in solutions and aggregation casued quenching in solid states.

This study reports the synthesis of three saddle-shaped polycyclic aromatic hydrocarbons (PAHs)—bFT-C, bFP-C, and bFP2-C—via Mallory photocyclization of bisfluorenylidene-dihydroacene precursors. The reaction achieved high yields (77–88%) under UV irradiation, with structural characterization confirming enhanced aromaticity and rigid frameworks in the cyclized products. Kinetic studies revealed that anthracene-containing bFP2 exhibited the fastest reaction rate, which we attribute to its elevated electron density. Photophysical studies showed strong solution fluorescence (PLQY: 45–48%) but aggregation-caused quenching (ACQ) in solids due to enhanced π–π stacking. Electrochemical analyses revealed reversible redox processes, while low-temperature spectroscopy identified deep-red phosphorescence (lifetimes: 72.6–270.0 ms). This work demonstrates how precursor architecture governs reaction kinetics and photophysical properties, providing a design strategy for nonplanar PAHs with tailored luminescence characteristics.