Is a small singlet-triplet energy gap a guarantee of TADF performance in MR-TADF compounds? Impact of the triplet manifold energy splitting

Aiming at obtaining pure-color emitters for organic light-emitting diodes (OLEDs), we designed and synthesized a new organic boron-containing indolocarbazole derivative as a candidate for multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) emitters. The efficiency of the targeted MR and TADF characteristics are analyzed by steady-state and time-resolved spectroscopy at different temperatures, and by different theoretical methods. The molecule exhibits green-yellow photoluminescence and electroluminescence with a full-width at half-maximum (FWHM) of 20 nm and 23 nm for thin films observed from photoluminescence (PL) and electroluminescence spectra. The PL intensity increases by up to 100% for the degassed sonicated toluene solutions; however, no TADF behavior is observed. The increase in PL is predominantly due to the decrease of the singlet quenching by the dissolved oxygen and by the remaining microaggregates in solution, as opposed to the frequently reported quenching of triplet excitons by oxygen. The absence of TADF behavior in toluene solution is attributed to very small spin-orbit coupling between S1 and T1 (0.02 cm-1) and a large T1-T2 gap that inhibits the T1 -> T2 upconversion of triplets which likely results in a very low rate of intersystem crossing as compared to the radiative decay rate. However, a weak TADF is observed in guest-host mixtures, e.g., in thin films made with mCP, DPEPO, and Zeonex hosts. This is attributed to the reduced T1-T2 gap, resulting from the conformational distortions of the emitter in the guest-host systems. Even with 100% photoluminescence quantum yields of the toluene solution, the efficiency of the first prototypical OLEDs is limited. Different sources for the low device external quantum efficiency are discussed. Ultimately, our structure-property analysis suggests a design rule allowing tuning of the T1-T2 gap for more efficient TADF. A design strategy for efficient MR-TADF emitters needs (i) avoiding aggregation by inducing steric hindrance at peripheral strategic positions, and (ii) combining lateral groups of higher donor character as compared to the central core.