Theoretical Design of Heavy-Atom Embedded ICz-Based Multi-resonance TADF Emitters with High Efficiency and Narrow Emission Width

Indolo[3,2,1-<i>jk</i>] carbazole (ICz)-based multiple resonance (MR) organic emitters bearing small full width at half-maximum (fwhm) values are of interest in organic light-emitting diodes. The challenge is to boost the intrinsic delayed fluorescence by enhancing the reverse intersystem crossing (RISC) process to avoid efficiency roll-off. Here, we propose the utilization of the nonmetallic heavy-atom effects to promote the RISC process while keeping the advantage of structural rigidity for narrowband emission. The emission spectrum and quantum efficiency were investigated by using quantum chemistry calculations coupled with our thermal vibration correlation function (TVCF) formalism. For heavy-atom embedded dICz-MR emitters, the synergistic effect of spin-vibronic coupling and spin-orbit coupling promotes the potential RISC channels (T₁ → T₂/T₃ → S₁), the effective <i>k</i>_RISC, up to 10⁶ s^-1. Besides, heavy-atom embedded para-linked ICz-MR emitters have little effect on emission width due to the considerable structural rigidity and low-frequency dominated vibronic coupling.