Solid-State Fluorophore Based on π‑Extended Heteroaromatic Acceptor: Polymorphism, Mechanochromic Luminescence, and Electroluminescence

A novel light-emitting material 4-(4-diphenylaminophenyl)-benzo­[d,e]­benzo­[4,5]­imidazo­[2,1-a]­isoquinolin-7-one (4-TPA-BBI) is synthesized and characterized. 4-TPA-BBI exhibits excellent polymorphism, mechanochromic luminescence (MCL), and electroluminescence properties. It can form three different aggregate species: yellow rod-like crystal (Y-crystal), orange needle-like crystal (O-crystal), and red solid (R-solid). It is observed that the Y-crystal can be reversibly changed to R-solid upon external stimuli, while O-crystal can be irreversibly transformed to R-solid via sublimation under a vacuum. Crystallographic and photophysical studies indicate that the Y-crystal featuring loose molecular packing demonstrates a high photoluminescence quantum yield (Φ_PL ≈ 1) and obvious MCL property, while the compactly packed O-crystal shows a lower Φ_PL of 0.646 with negligible MCL. A nondoped light-emitting device based on 4-TPA-BBI as an emitter exhibits bright orange emission with a peak at 588 nm, a high external quantum efficiency of 4.4%, and a maximum luminance of 22470 cd m^–2.