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.