Dynamics of vibrationally coupled intersystem crossing in state-of-the-art organic optoelectronic materials

This work investigates intersystem crossing (ISC) induced by spin-orbit coupling (SOC) in state-of-the-art non-fullerene acceptors (NFAs). A quantum chemistry study analyzed SOC in 10 NFAs using the optimized geometry of the ground state (OGGS), revealing the importance of excited-state character (local or charge transfer) in determining SOC. However, ISC rates calculated with Marcus formalism were significantly lower than experimental values, showing that the three-state model (S₁, T₁, and T₂) is insufficient. A simplified method to calculate coupled probabilities was proposed, leveraging a quantum walk on a one-dimensional graph. This approach aligned ISC rates with experimental data and explained Y6's higher triplet state efficiency compared to ITIC-like NFAs. Further, the dihedral angle (ϕ) in IT-4Cl and Y6 was analyzed. Y6's unique excited-state potential energy curve (PEC) showed a minimum at ϕ ≈ 90^o. Using PECs, ISC rates were refined, showing coupling via ϕ vibrations. Finally, the Wentzel-Kramers-Brillouin (WKB) approximation explained Y6's photoluminescence at low temperatures, highlighting non-adiabatic phenomena crucial for understanding the photophysics of organic semiconductors. Triplet states act as channels that enhance recombination, reducing the optoelectronic efficiency of semiconductor devices. Therefore, understanding and controlling these states can contribute to improving the efficiency of organic solar cells (OSCs) and organic light-emitting diodes (OLEDs).