Aromaticity Localization Effects in Polycyclic Aromatic Hydrocarbons for Discovering Narrowband Fluorescence Materials

Achieving narrowband fluorescence in polycyclic aromatic hydrocarbons (PAHs) is crucial for ultrahigh-definition organic light-emitting diodes (UD-OLEDs), yet the underlying structure-property relationships that dictate emission bandwidth remain insufficiently understood. In this study, we introduce aromaticity localization as a predictive framework for identifying narrowband emitters. Using nucleus-independent chemical shift (NICS) analysis, we uncover a strong correlation between localized aromaticity and reduced vibrational coupling, demonstrating that restricting π-electron delocalization effectively suppresses shoulder peaks, thereby minimizing spectral broadening. To validate this concept, we designed a new class of imine-amine-type PAHs (IA-PAHs) that integrates electron-deficient imine and electron-rich amine units, generating a multiple-resonance-type electronic structure. Building on a steric-hindrance-guided C-H activation strategy, we precisely controlled the regioselectivity of pyridine fusion within the triphenylamine framework, leading to the discovery of narrowband red-emitting II-b and green-emitting III-c featuring localized aromaticity. Notably, II-b exhibited an exceptionally narrowband red emission at 660 nm with a full width at half-maximum of only 35 nm (0.10 eV). OLEDs incorporating II-b demonstrated high efficiency with minimal roll-off and fully met the stringent BT.2020 red standard, with Commission Internationale de l'Eclairage (CIE) coordinates of [0.71, 0.29]. This work not only establishes aromaticity localization as an empirical and intuitive design principle for narrowband fluorophores but also represents a significant advancement in deep-red narrowband OLED technology, setting a new benchmark for conventional fluorescent emitters.