Solid-State Effect Induced Thermally Activated Delayed Fluorescence with Tunable Emission: A Multiscale Study

Thermally activated delayed fluorescence (TADF) molecules with tunable solid-state luminescence have shown great application potential in organic light-emitting diodes. However, theoretical studies on luminescence properties of organic emitters with consideration of solid-state effect are limited. In this work, the photophysical properties of a difluoroboron beta-diketonate-based molecule (M1) in liquid, crystal, and amorphous states are studied using multiscale methods combined with the thermal vibration correlation function theory. Our results indicate that the geometric structures of M1 in liquid with toluene and crystal state are all in straight-chain form. However, M1 in amorphous state is subjected to form bending deformation at the triphenylamine unit under collaboration between intramolecular pi-hydrogen bond and disordered intermolecular interactions. Moreover, in the amorphous state, the energy gap between the first singlet excited state (S-1) and the first triplet excited state (T-1) (Delta E-ST) of M1 is significantly reduced, and the spin-orbit coupling constant is remarkably increased in comparison with those of M1 in liquid with toluene and crystal state. As a result, the up-conversion of T-1 -> S-1 in the amorphous state is favored, and remarkable TADF is thus observed. Besides, M1 in the solid state gives fluorescence in red shift emission compared to that in liquid with toluene. On the basis of the results above, we further theoretically design a new molecule noted as M2 which emits fluorescence in the near-infrared region in the solid state. Our theoretical results help in understanding the light-emitting mechanism induced by the solid-state effect and provide information for designing new-type TADF emitters with tunable solid-state emission.