Samsung Advanced Institute of Technology

The recently developed narrow-band blue-emitting organoboron chromophores have
now become one of the most important components for constructing efficient organic
light emitting diodes (OLEDs). While they basically emit through fluorescence, they are
also known for showing substantial thermally activated delayed fluorescence (TADF)
even with a relatively large singlet？triplet gap (DEST). Indeed, understanding the
reverse intersystem crossing (RISC) dynamics behind this peculiar TADF will allow
judicious molecular designs toward achieving better performing OLEDs. Explaining
the underlying nonadiabatic spin-flip mechanism, however, has often been equivocal
and how the sufficiently fast RISC takes place even with the sizeable DEST and vanishingly
small spin？orbit coupling is not well understood. Here, we show that a vibronic
resonance that orchestrates three electronic states together is playing a major role in
enhancing RISC in a typical organoboron emitter. Interestingly, the mediating upper
electronic state is quite high in energy to an extent that its thermal population is
vanishingly small. Through semiclassical quantum dynamics simulations, we further
show that the geometry dependent non-Condon coupling to the upper triplet state that
oscillates with the frequency DEST/h is the main driving force behind the peculiar
resonance enhancement. Our investigation may provide a new guide for future blue
emitting molecule developments.