o-Carborane decorated diboron-embedded multi-resonance TADF compounds featuring narrowband emission

Simultaneously achieving high luminescence quantum efficiency and narrowband emission in o-carboranyl luminophores remains a significant challenge for enhancing color purity and extending their applicability in optoelectronic materials. Herein, we report two multi-resonance induced thermally activated delayed fluorescence (MR-TADF) compounds, CB-diBNO (1) and CB-v-DABNA (2), which incorporate methyl-o-carborane units at the periphery of diboron MR cores. Both compounds exhibit characteristic narrowband MR-TADF emission with high quantum efficiency in toluene and rigid states, while displaying weak emission in THF. Compound 1 exhibits more narrowed emission spectra with bathochromic shifts compared to its phenyl-substituted counterpart, Ph-diBNO (3). Notably, its full width at half-maximum of only 11 nm ranks among the narrowest reported for boron-based MR-TADF compounds. Electrochemical analysis reveals that the incorporation of o-carborane moieties significantly stabilizes the frontier molecular orbitals of the diboron MR core compared to phenyl substitution. Theoretical studies suggest that emission quenching in THF arises from the presence of a dark lowest-energy charge transfer state, while o-carborane modification effectively suppresses low-frequency vibrations in the diboron MR-core skeleton, thereby minimizing spectral broadening. These findings highlight that diboron MR-TADF cores can serve as a promising π-skeleton for the design of narrowband emissive o-carboranyl luminophores.