Simultaneously enhancing the planarity and electron-donating capability of donors for through-space charge transfer TADF towards deep-red emission

Abstract

Through-space charge transfer (TSCT) has been proven effective for designing thermally activated delayed fluorescence (TADF) emitters due to the separation of the frontier molecular orbitals. Although tuning of the interaction between the donor and acceptor by controlling the conformation is known to be crucial for the photophysical properties of TSCT excited states, it remains a challenge to realize efficient red and deep-red emissions. Herein, we designed two TSCT molecules, namely TPXZ-QX and TPXZ-2QX, by using oxygen-bridged triphenylamine (TPXZ) as the electron donor with enhanced planarity and electron-donating capability. With a face-to-face orientation of the donor and acceptor segments and close π–π contacts, the new emitters have strong intramolecular noncovalent donor–acceptor interactions. The emissions of TPXZ-QX and TPXZ-2QX in doped thin films lie in the red (λ_max = 632 nm) to deep-red (λ_max = 665 nm) region. The photoluminescence quantum yields are 41% and 32% for TPXZ-QX and TPXZ-2QX, respectively. Organic light-emitting diodes (OLEDs) based on TPXZ-QX and TPXZ-2QX show external quantum efficiencies (EQEs) of up to 13.8% and 11.4%, respectively. This work indicates that the modulation of TSCT excited states based on strong intramolecular cofacial π-stacking interactions is a viable choice for the development of high-efficiency long-wavelength TADF emitters.