Through-space interaction enables simultaneous enhancements of k _r and k _RISC in highly efficient spiro-acridine...

Most of acridine based thermally activated delayed fluorescence (TADF) emitters are characterized by advantageous reverse intersystem crossing (RISC) rate (k _RISCs) due to the perpendicular orientation of the acridine donor to the acceptor moiety, but suffer from a poor radiation rate (k _r ) 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(10H)-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 k _r 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 k _RISC of 1.8 × 10⁶ s^-1. This is the first acridine donor based TADF emitter realizing k _r of 10⁷ s^-1 and k _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.