Purely Spin‐Vibronic Coupling Assisted Triplet to Singlet Up‐Conversion for Real Deep Blue Organic Light‐Emitting Diodes with Over 20% Efficiency and y Color Coordinate of 0.05

Finding narrow‐band, ultrapure blue thermally activated delayed fluorescence (TADF) materials is extremely important for developing highly efficient organic light‐emitting diodes (OLEDs). Here, spin–vibronic coupling (SVC)‐assisted ultrapure blue emitters obtained by joining two carbazole‐derived moieties at a para position of a phenyl unit and performing substitutions using several blocking groups are presented. Despite a relatively large singlet–triplet gap (∆EST) of >0.2 eV, efficient triplet‐to‐singlet crossover can be realized, with assistance from resonant SVC. To enhance the spin crossover, electronic energy levels are fine‐tuned, thereby causing ∆EST to be in resonance with a triplet–triplet gap (∆ETT). A sizable population transfer between spin multiplicities (>103 s−1) is achieved, and this result agrees well with theoretical predictions. An OLED fabricated using a multiple‐resonance‐type SVC‐TADF emitter with CIE color coordinates of (0.15, 0.05) exhibits ultrapure blue emissions, with a narrow full‐width‐at‐half‐maximum of 21 nm and a high external quantum efficiency of 23.1%.