Anti-Kasha fluorescence and persistent room-temperature phosphorescence from a nearly flat non-symmetric polycyclic hydrocarbon

Polycyclic aromatic hydrocarbons are molecular frameworks that enable the development of efficient emitters owing to their extensive conjugation and rigid structure. Here, we introduce monohomotruxene (MHTX), designed by reducing the symmetry of homotruxene (HTX) and truxene (TX). This partial planarization greatly enhances the complex photophysics over symmetric, triply twisted HTX, substantially increasing the photoluminescence quantum yield (PLQY), a result we attribute to its enhanced molecular rigidity and symmetry breaking. MHTX displays an unconventional violation of Kasha's rule, yielding excitation-dependent dual emission. Excitation of the S₂ state under anaerobic conditions produces intense, 455-480 nm, ultralong room-temperature phosphorescence (RTP), lasting over 12 s at room temperature. In contrast, excitation into the S₁ state yields prompt fluorescence and weak delayed fluorescence (DF) by triplet-triplet annihilation (TTA) in the neat film. Aggregate species also present a complex emissive behavior with weak orange phosphorescence and TTA-DF. These findings establish MHTX as a promising emissive scaffold, highlighting the potential for accessing complex excited-state dynamics and advanced photophysical functionalities.