Through‐space interaction enables simultaneous enhancements of <i>k</i> _<i>r</i> and <i>k</i> _RISC in highly efficient spiro‐acridine based thermally activated delayed fluorescence emitter with acridone acceptor

Most of acridine based thermally activated delayed fluorescence (TADF) emitters are characterized by advantageous reverse intersystem crossing (RISC) rate (<i>k</i> _RISCs) due to the perpendicular orientation of the acridine donor to the acceptor moiety, but suffer from a poor radiation rate (<i>k</i> _<i>r</i> ) typically in the order of 10⁶ s^-1. Herein, two sky blue TADF emitters 3,6-DMAC-AD-Py and 3,6-SFAC-AD-Py were developed by linking acridine (DMAC) and spiro-fluorene-acridine (SFAC) donors to 10-(pyridin-2-yl)acridin-9(10<i>H</i>)-one (AD-Py) acceptor. Larger SFAC and electron-deficient pyridyl groups are deliberately incorporated in 3,6-SFAC-AD-Py since the unique through-space interaction between them is designed to drive the rotation of inner acridine ring in SFAC for enhancing frontier molecular orbitals overlap while keeping a decent TADF behavior. Thus, the <i>k</i> _<i>r</i> of 3,6-SFAC-AD-Py is increased to 1.5 × 10⁷ s^-1. Simultaneously, SFAC donors improve spin orbital coupling strength and reduce the energy gaps, generating <i>k</i> _RISC of 1.8 × 10⁶ s^-1. This is the first acridine donor based TADF emitter realizing <i>k</i> _<i>r</i> of 10⁷ s^-1 and <i>k</i> _RISC of 10⁶ s^-1 by a through-space interaction strategy. 3,6-SFAC-AD-Py enables a highly efficient sky-blue organic light-emitting diode with a maximum external quantum efficiency (EQE) of 34.7% and Commission International de I'Eclairage coordinates of (0.19, 0.37). More importantly, the EQE still remained 27.6% and 16.9% at high brightness of 1000 and 10,000 cd m^-2.