Sensitivity of the hidden TADF to the linking topology of di-tert-butyl-carbazolyl and benzonitrile moieties in the molecules of emitters or hosts intended for efficient blue OLEDs

With the aim of detecting hidden thermally activated delayed fluorescence (TADF), six organic compounds were synthesized and characterized as emitters and hosts of blue organic light-emitting diodes (OLEDs). The effect of the linking topology of di-tert-butyl-carbazolyl and benzonitrile moieties in donor-acceptor-donor type molecules on device efficiencies was investigated. Just by altering the linking position of di-tert-butyl-carbazolyl moieties at benzonitrile fragment of the emitters and hosts of the blue TADF OLEDs, their maximum external quantum efficiencies (EQEs) can be changed from 1.13 to 5.17 % as well as from 13.8 to the record-high 22.8%. After the comprehensive investigations of the properties including the charge mobility measurements by time-of-flight technique, steady-state and time -resolved photoluminescence and electroluminescence studies, good correlation between EQE of OLEDs and the properties of the isomers with the differently linked donor and acceptor moieties were observed.