Singlet-Triplet Gaps for Evaluating Thermally Activated Delayed Fluorescence: Which One is the (b)E_ST ?

Abstract

The singlet–triplet energy gap (ΔEST) serves as a central screening parameter for new thermally activated delayed fluorescence (TADF) materials, and is a valuable indicator of eventual OLED performance. Surprisingly though, various measurement methodologies and reporting standards for ΔEST persist across the research community. The resulting variability undermines direct comparisons of material properties across reported works, obfuscating structure-property relationships that would otherwise guide synthetic efforts and computational validation. Here we employ 4CzPyz and 4tCzPyz as model systems, and correlate their different possible ΔEST values with their reverse intersystem crossing (rISC) kinetics in films of common and device-relevant hosts. By comparing ΔEST values with emission decay kinetics and device roll-off performance for these two materials, we propose that the steady-state room-temperature photoluminescence onset should be used to determine E(S1), in preference to either steady-state low-temperature or time-resolved singlet emission. Ultimately though, even this should only be taken as an indicator, as device performance is not always reliably predicted by comparing optically derived ΔEST gaps.