Switching the heavy-atom effect in blue thermally activated delayed fluorescence emitters

With the development of all-organic light-emitting diode (OLED) technology, understanding the complex phenomenon of "heavy-atom effect" (HAE) is becoming increasingly important. The ability of abundant heavy atoms (HAs) to accelerate reverse intersystem crossing (rISC) is seen as a promising and cost-effective solution for overcoming the challenges of slow triplet harvesting in thermally activated delayed fluorescence (TADF) emitters and enhancing the operational stability of OLEDs that incorporate them. However, controlling the HAE and balancing it with other molecular and electronic effects during material design are challenging. In this work, our investigations on a series of blue s-triazine emitters with phenylchalcogen substituents revealed strong dependence of the internal HAE on the mechanism of rISC. When rISC involved locally excited triplet states, both the spin-orbit coupling and rISC rate significantly increased, following the order of OPh > SPh > SePh. Conversely, when the charge transfer (CT) triplet state dominated in the rISC mechanism, the order was reversed. This reversal was owing to the much higher sensitivity of the CT states to the mesomeric effect of the substituent than its atomic number. For this reason in polar medium, the external HAE was applied to achieve a moderate increase in the rISC rate, improving the efficiency roll-off in OLEDs. Thus, we concluded that the optimal molecular design should balance the contribution of the electronic density of HA in the orbital momentum change to afford effective internal HAE with the electronic and conformational influences of HA on the chromophore system to ensure a favorable rISC mechanism.