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

With the rise of all-organic light-emitting diode (OLED) technology, understanding the complex phenomenon known as the “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, as well as enhancing the operational stability of OLEDs that incorporate them. The challenge lies in controlling the HAE and balancing it with other molecular and electronic effects during material design. In this work, our investigations of a series of blue s-triazine emitters with phenylchalcogen substituents reveal strong dependence of the internal HAE on the mechanism of rISC. When rISC involves locally-excited triplet states, both spin-orbit coupling and the rISC rate significantly increase following the trend OPh > SPh > SePh. Conversely, when the charge transfer (CT) triplet state dominates in the rISC mechanism, the trend is reversed. This reversal is due to the much higher sensitivity of CT states to the mesomeric effect of the substituent than to its atomic number. In such cases, the external HAE can achieve a moderate increase in the rISC rate, improving efficiency roll-off in OLEDs. We conclude that 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 favorable rISC mechanism.