Hyperfine versus spin-vibronic coupling: or do both drive TADF?

Thermally-activated delayed fluorescence (TADF) organic molecules undergo more efficient light emission than traditional organic fluorescent emitters, making them attractive materials for OLEDs. In TADF OLEDs, indirect emission from dark triplet states via bright singlet excitons is activated and leads to boosted electroluminescence efficiencies. Most studies of TADF use optical spectroscopies that can examine the photophysics and interconversion rates, but do not shed light on the critical spin physics. Here we use transient electron spin resonance spectroscopy to study triplet states involved in TADF and the role of hyperfine and spin-vibronic couplings on the critical singlet-triplet intersystem crossing for efficient TADF.