Theoretical studies on spiro[acridine-fluorene]-based emitters with efficient thermally activated delayed fluorescence

Abstract

Thermally activated delayed fluorescence (TADF) emitters with through space charge transfer (TSCT) have been attracting great attention owing to their unique characteristics and potential applications in organic light-emitting diodes. In this study, five TSCT TADF emitters based on spiro[acridine-fluorene] donor have been investigated in detail using density functional theory (DFT) and time-dependent DFT calculations. The results indicate that five emitters exhibit cofacial orientations between acridine donor and acceptor units, leading to the well-separated HOMO and LUMO and small singlet-triplet energy gaps (ΔEST) (less than 0.20 eV). The lowest singlet excited state (S1) and lowest triplet excited state (T1) are characterized to be the mixed charge transfer (CT) and local excitation (LE) natures, which ensures strong oscillator strength of S1 state and triggers high radiative rates over 106s-1. Importantly, the obvious differences in the CT component between S1 and T1 states lead to significantly strong spin-orbit coupling, especially for SAF-XT, SAF-Q and SAF-Qx, reaching 3.93, 2.17, and 4.26 cm−1, respectively, resulting in high reverse intersystem crossing (RISC) rates up to 4.4 × 106s-1, which endow them with great potential to be efficient TSCT-TADF emitters. Our calculation results would be beneficial to enriching the TSCT-TADF emitters.