Conformational Relaxation and Thermally Activated Delayed Fluorescence in Anthraquinone-Based Intramolecular Charge-Transfer Compound

Abstract

A novel donor-π-acceptor-π-donor-type (D-π-A-π-D-type) chromophore, 2,6-bis[4-(diphenylamino)phenyl]-9,10-anthraquinone (AQ(PhDPA)2), has been reported as an efficient red thermally activated delayed fluorescence (TADF) emitter. Molecular structure and conformation, which directly determine the nature of excited states of a TADF emitter, are critical for obtaining efficient reverse intersystem crossing (rISC) and TADF. In this work, a series of excited-state deactivation processes of AQ(PhDPA)2, from the optical excitation to fluorescence and TADF emitting, have been investigated by theoretical calculations and ultrafast transient absorption (TA) spectroscopy. Theoretical calculations and steady-state spectra suggest that the TADF emitter appears to have conformational twisting in the excited state. Both the relaxed S0 and S1 conformations have a small energy difference between the lowest singlet and triplet excited states (ΔEST) in favor of rISC, whereas ΔEST increases at the relaxed T1 conformation. Ultrafast TA spectra reveal that the intramolecular charge transfer (ICT) state of AQ(PhDPA)2 emits efficient fluorescence after a solvation-stabilization process in nonpolar toluene, while the fluorescence from the solvation-induced conformational relaxed ICT state is quenched in polar tetrahydrofuran. Additionally, we further reveal that the suppression of the conformational relaxation in long-lived triplet states contributes to maintaining a small ΔEST, which is critical for efficient rISC and TADF. These results provide a guidance for understanding the relationship between TADF and conformational relaxation dynamics, as well as for designing and synthesizing advanced TADF emitters.