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

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.