High‐Temperature‐Induced Fused Polycyclic Aromatic Multiple Resonance Emitters Exhibiting Narrowband and Pronounced Red‐Shifted Emission

Synthetic methodology is a fundamental framework for preparing functional materials, significantly advancing their development. Herein, a novel 6π electrocyclization reaction is unexpectedly discovered that promotes further ring closure in materials derived from multi-resonance thermally activated delayed fluorescence (MR-TADF) compounds, known for their narrow emission. By simply raising the reaction temperature, this process significantly red-shifts the emission peak of the target material while effectively narrowing its emissive width and greatly enhancing its optoelectronic performance. Utilizing this method, the newly synthesized MR-TADF substrate material GCz-4B2 is successfully converted into the target compound GCz-4B1. Compared to GCz-4B2, the emission peak of GCz-4B1 exhibited a redshift of 26 nm while concurrently achieving a significant reduction in its full width at half-maximum (FWHM) value and corresponding shoulder intensity. Notably, the photoluminescence quantum yield (PLQY) of GCz-4B1 reached 95.1%, compared to only 85.6% for GCz-4B2. This enhancement can be attributed to the increased rigidity from the further ring closure reaction, which reduced unfavorable vibrational relaxation processes and improved PLQY values. Furthermore, OLEDs based on GCz-4B1 attained a maximum external quantum efficiency (EQEmax) of 28.0%, with a small FWHM value of 19.4 nm, significantly surpassing that of devices derived from GCz-4B2.