Tailoring Indolocarbazole‐Bridged Multiple Resonance Emitter through Steric Pre‐Substitution for Narrowband Electroluminescence with EQE over 40%

Multiple resonance (MR) emitters bridged by multi‐nitrogen indolocarbazoles to extend molecule skeletons could modify emission maximums without scarifying narrow full width at half maximum (FWHM), which, however, face synthesis challenges of uncontrollable borylation regioselectivity and spectral broadening issue from intermolecular aggregation. Here, a steric pre‐substitution strategy is devised using tert‐butylphenyl‐functionalized indolo[3,2‐b]carbazole as a bridge to extend MR skeletons, not only steering regioselective Bora‐Friedel‐Crafts borylation but also suppressing intermolecular interactions in films. The targeted greenish emitter therefore achieves a small electroluminescence FWHM of only 22 nm in device, matching the intrinsic photoluminescence one of 21 nm in dilute toluene. The extended skeleton also enhances the horizontal orientation of emitting dipole moment with a ratio of 90%, leading to a remarkable high maximum external quantum efficiency (EQE) of 41.0% and power efficiency of 106.5 lm W‐1. The efficiency roll‐off remains remarkably low with EQEs sustaining at 35.0% and 30.6% at luminance of 1,000 cd m‐2 and 5,000 cd m‐2, respectively. This work establishes a pre‐substitution paradigm to concurrently optimize synthesis control and solid‐state emission for high‐performance MR emitters.