Proximal Steric Shielding Unlocks High Solid‐State Luminescence in Quinoxaline‐Fused Pyrene Heteroacenes

Achieving high quantum yields in neat films of extended heteroacenes remains challenging because their large aromatic surfaces promote aggregation-caused quenching (ACQ) and non-radiative decay, while often limiting solubility and solution processability. Here we introduce proximal steric shielding to control solid-state emission in quinoxaline-fused pyrene heteroacenes. By systematically varying tri-/tetraphenylethene and alkynylsilane substituents around the heteroacene core, we establish a clear structure-property relationship showing that solid-state emission depends not simply on the amount of steric bulk, but on the spatial positioning of these substituents relative to the emissive π-system. A modular synthetic route gives four soluble azaacenes with low solution PLQYs (6%-14%) but strongly enhanced emission in neat films. The optimised triPE-TIPS derivative exhibits a PLQY of 82%, among the highest values reported for linearly fused azaacenes. Single-crystal x-ray analysis, Hirshfeld surface analysis, fluorescence lifetime measurements, aggregation studies, and DFT calculations collectively support a mechanism in which the matched proximal arrangement of triPE and TIPS groups suppresses close π-π contacts and reduces non-radiative decay. Importantly, the enhanced neat-film emission is distinct from conventional aggregation-induced emission (AIE) behavior, instead arising from substituent-position-controlled molecular packing. These findings establish proximal steric shielding as a practical design strategy for suppressing solid-state quenching in extended π-systems.