Symmetry-Engineered Ultralong Phosphorescence in Double π-Helical Nanographenes

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 C₂ 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 S₁→S₀ 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 (Δε = 457 M^-1 cm^-1) for 10. This research offers a valuable insight into the molecular design of chiral afterglow materials.