Magneto-optical studies of organic electroluminescent materials having fast reverse intersystem crossing

In thermally activated delayed-fluorescence (TADF) materials, the fluorescence efficiency is enhanced through reverse intersystem crossing (RISC) from triplet to singlet excitons. The performance of organic light-emitting devices (OLEDs) based on TADF materials highly depends on the thermal conversion efficiency of the lowest triplet exciton ${T}_{1}$ into the lowest singlet exciton ${S}_{1}$, which relates to the energy difference $\mathrm{\ensuremath{\Delta}}{E}_{{S}_{1}\ensuremath{-}{T}_{1}}$. Here we investigate the RISC process in two TADF compounds with vastly different $\mathrm{\ensuremath{\Delta}}{E}_{{S}_{1}\ensuremath{-}{T}_{1}}$ using magneto-optical spectroscopies that include magnetophotoinduced absorption (MPA) in films and magnetoelectroluminescence (MEL) in OLEDs. The photoinduced absorption spectrum of the fast RISC material clearly shows that both singlet and triplet excitons coexist under steady-state conditions. Since the MPA response of the singlet and triplet excitons are similar and have the same polarity, we conclude that the magnetic field does not influence the RISC process in the studied compounds. We also find that the MEL response in OLEDs based on these compounds originates from the injected polaron pair species before they decay into ${S}_{1}$ and ${T}_{1}$ excitonic states in the emissive TADF layer.