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

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 (ΔE_ST) in favor of rISC, whereas ΔE_ST 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 ΔE_ST, 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.