Systematic Substituent Control in Blue Thermally Activated Delayed Fluorescence (TADF) Emitters: Unraveling the Role of Direct Intersystem Crossing between the Same Charge‐Transfer States

A molecular structural approach is applied by introducing substituent groups (X) to explore the structure–property correlation of thermally activated delayed fluorescence (TADF) mechanism and develop blue TADF materials. D–A–X emitters show blue emissions from 446 to 487 nm and exhibit high rate constants of reverse intersystem crossing (krISC) from 0.76 × 106 to 2.13 × 106 s−1. Organic light emitting diodes (OLEDs) based on D–A–X emitters exhibit efficient external quantum efficiency from 17.2% to 23.9%. Furthermore, the theoretical analysis of spin–flip transitions between states of various nature reveals that the highest rISC rates can be achieved by the increase of charge-transfer (CT) strength and enhancement of direct transition between triplet (3CT) and singlet (1CT) charge transfer states. Rotational tolerance of dihedral angle, low energy gap, and low reorganization energy between the 3CT and 1CT states provides fast rISC even when triplet states of different (LE) nature have much higher energy not to enable the three-level interaction. By both experimental and theoretical methods, the investigations reveal that for the design of efficient TADF-OLED emitters, the enhancement of the 3CT–1CT transition is as much important as that of 3LE–1CT.