Excited-state properties and deactivation pathways of multi-resonance TADF: The case of DABNA-1 and DiKTa

The design of new thermally activated delayed fluorescence (TADF) emitter materials to be employed in the context of organic light-emitting diodes (OLED) has received considerable attention from experimentalists and theorists in the last decade. TADF emitters can reach reach up to 100% internal quantum efficiency thanks to the possibility to convert dark triplet states to singlet emissive states via a reverse intersystem crossing process (RISC). In this contribution, we are interested in those compounds where the minimization of the singlet-triplet gap (ΔEST) is achieved through the promotion of multiple resonance effects 1, especially on DABNA-1 and DiKTa. Although these two molecules have shown very high device efficiencies their photophysics are not yet fully understood. Recent transient absorption and EPR measurements show the signature of one triplet state for DABNA-1, and two triplet states for DiKTa. With the aim to shed light about new insights on how the triplet states are populated upon irradiation of light, we optimized the minimal energy crossing points (MECP) for the relevant excited states involved in the ISC process at the ADC(2)/def2-TZVP level of theory and geodesic scans are employed to connect these optimized structures in order to help us to better understand the feasible deactivation pathways . Time constants related to the primary photophysical events are also estimated by using the thermal vibrational correlation function (TVCF) formalism.