Efficient thermally activated delayed fluorescence materials based on the multiple resonance effect were synthesized by nucleophilic substitution and electrophilic C-H borylation, featuring a small energy gap between the singlet and triplet state, high photoluminescence quantum yield, and narrow emission band. Organic light-emitting diodes employing these materials as emitters exhibited sky-blue emission with a high external quantum efficiency of 21.4% and a narrow full width at half-maximum of 33 nm.