Climbing up the Ladder: Intermediate Triplet States Promote the Reverse Intersystem Crossing in the Efficient TADF Emitter ACRSA

It is usually assumed that the reverse intersystem crossing (rISC) in organic TADF emitters occurs at the microsecond time scale. However, the underlying dynamics can be enhanced in the presence of low-lying n pi* states as stipulated by the El-Sayed rule. Here, we report a theoretical study of the short-time quantum dynamics of the ACRSA dye in different solvents following the electrical dip. excitation. The wave packet propagation in multidimensional excited state potentials reveals the intersystem crossing (ISC) rate to be one or 2 orders of magnitude faster compared to the rISC process depending on the environment. Upon the molecular geometry distortions, the system rapidly converts the initial vibrational density to the valence excited-state potential's with subsequent of spin state owing to spin orbit interaction. We show that a local triplet pi pi* state, residing on the acceptor moiety) mediates the nonradiative passage from triplet to singlet, thus increasing the quantum yield of light harvesting in OLED. This tandem interplay of vibronic and spin-orbit interactions favors the rISC to occur at the subrianosecond, regime in ACRSA.