Understanding the internal heavy-atom effect on thermally activated delayed fluorescence: application of Arrhenius and Marcus theories for spin–orbit coupling analysis

Abstract

Hybrid organic emitters containing abundant heavy atoms (HAs) are of rapidly growing interest for organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF). As substitutes of expensive and toxic heavy-metals, HAs can accelerate reverse intersystem crossing (rISC), whose low rate is specifically troublesome in blue all-organic TADF materials. Unfortunately, due to complex photophysics even qualitative predictions of the HA effect on organic emitters and hence molecular design are currently very challenging. Here, we report a photoluminescence data analysis approach for investigations of the TADF mechanism based on empirical spin–orbit coupling constants. Using this approach, heavy-halogen derivatives of a popular blue TADF emitter are investigated to understand the mechanism of the HA effect and molecular design fundamentals of high-efficiency TADF materials bearing HAs. Different HA effects on direct and reverse ISC were revealed. We thus conclude that to improve TADF in organic emitters (i) HA should participate in low-energy molecular vibrations which activate rISC between the lowest excited triplet and singlet states of the charge-transfer (CT) character and (ii) ISC should be minimized through a large energy gap between the ¹CT-state and HA-affected locally-excited (³LE) triplet state.