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

摘要

A molecular structural approach is applied by introducing substituent groups(X) to explore the structure–property correlation of thermally activateddelayed fluorescence (TADF) mechanism and develop blue TADF materials.D–A–X emitters show blue emissions from 446 to 487 nm and exhibithigh rate constants of reverse intersystem crossing (k_(rISC)) from 0.76 × 10~6 to 2.13 × 10~6 s~(?1). Organic light emitting diodes (OLEDs) based on D–A–Xemitters exhibit efficient external quantum efficiency from 17.2% to 23.9%.Furthermore, the theoretical analysis of spin–flip transitions between statesof various nature reveals that the highest rISC rates can be achieved by theincrease of charge-transfer (CT) strength and enhancement of direct transitionbetween triplet (~3CT) and singlet (~1CT) charge transfer states. Rotationaltolerance of dihedral angle, low energy gap, and low reorganization energybetween the 3CT and 1CT states provides fast rISC even when triplet states ofdifferent (LE) nature have much higher energy not to enable the three-levelinteraction. By both experimental and theoretical methods, the investigationsreveal that for the design of efficient TADF-OLED emitters, the enhancementof the ~3CT–~1CT transition is as much important as that of ~3LE–~1CT.