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 (k(rISC)) from 0.76 x 10(6) to 2.13 x 10(6) 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 ((CT)-C-3) and singlet ((CT)-C-1) charge transfer states. Rotational tolerance of dihedral angle, low energy gap, and low reorganization energy between the (CT)-C-3 and (CT)-C-1 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 (CT)-C-3-(CT)-C-1 transition is as much important as that of (LE)-L-3-(CT)-C-1.