Isomerization-Regulated Multiresonance Molecules for Achievement of Long-Wavelength Electroluminescence with Reduced Shoulder Vibronic Emission

Multiresonance (MR) emitters have attracted much attention because of their narrow-band emission. Most reported MR materials exhibit short-wavelength emissions. It is challenging to red-shift their emissions to the longer-wavelength region. Thus, we explored isomerization's impact on the emission spectra of M-B2Cz and W-B2Cz MR emitters integrating two monoboron fragments. The HOMO-LUMO energetic gap of W-B2Cz is smaller, red-shifting its emission peak by 71 nm, compared to that of M-B2Cz. This red-shift in W-B2Cz results from a lower LUMO energy level due to its bonding-like population on the bridging pyrrole. Such a feature in the HOMO and LUMO of W-B2Cz generates the bonding-like transition density in its S₁ state, reducing shoulder vibronic emission. A W-B2Cz-based OLED exhibited longer-wavelength emission, a narrow bandwidth of 90 meV, and an exceptional maximum external quantum efficiency of 39.4%. This work provides new insight into isomerization's role in MR emission spectra, promoting the development of narrow-band materials with long-wavelength emissions.