Symmetry‐Engineered Ultralong Phosphorescence in Double π‐Helical Nanographenes

Abstract

Abstract The development of long‐lived organic phosphorescent materials faces significant challenges in achieving precise control over triplet exciton emission processes. Herein, we present an innovative design strategy toward a novel class of double π‐helical nanographenes ( 8 – 10 ) by integrating a chiral cyclooctahexaphenylene (COTh) core with diverse polycyclic aromatic hydrocarbons (PAHs). The process underwent sequential Knoevenagel condensation, Diels–Alder [4 + 2] cycloaddition, and final Scholl dehydrocyclization, using phenanthraquinone dimer 3 as the key chiral building block. By systematically varying the symmetry of PAH subunits from C 2 to C 2v to D 6h , we achieved remarkable orange‐red phosphorescence with lifetimes ( τ p ) reaching 5.5 s and afterglow durations up to 45 s at 77 K for 10 featuring highly D 6h ‐symmetric hexabenzocoronene (HBC) moieties. This exceptional performance stems from three synergistic effects in high‐symmetry, rigid PAHs: (1) significantly suppressed S 1 →S 0 radiative rates ( k f ) due to symmetry‐forbidden transitions governed by Clar's π‐sextet rule, (2) extremely low triplet radiative decay ( k p ), and (3) minimized nonradiative dissipation ( k nr ). These double π‐helical nanographenes further demonstrate tunable chiroptical properties, with absorption dissymmetry factor (| g abs |) of 0.011 for 9 and exceptional Cotton effects (Δ ε = 457 M −1 cm −1 ) for 10 . This research offers a valuable insight into the molecular design of chiral afterglow materials.