Zhang Kai, Zhang Yuchen, Ma Yuying, Fan Jianzhong, Wang Chuan-Kui, Lin Lili
Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics, Shandong Normal University, 250014 Jinan, China.
J Phys Chem A. 2020 Oct 15;124(41):8540-8550. doi: 10.1021/acs.jpca.0c07152. Epub 2020 Oct 2.
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 β-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 π-hydrogen bond and disordered intermolecular interactions. Moreover, in the amorphous state, the energy gap between the first singlet excited state (S) and the first triplet excited state (T) (Δ) 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 → S 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.
具有可调谐固态发光的热激活延迟荧光（TADF）分子在有机发光二极管中显示出巨大的应用潜力。然而，考虑固态效应的有机发光体发光性质的理论研究有限。在这项工作中，使用多尺度方法结合热振动相关函数理论研究了一种基于二氟硼β-二酮的分子（M1）在液体、晶体和非晶态下的光物理性质。我们的结果表明，M1在甲苯液体和晶体状态下的几何结构均为直链形式。然而，在分子内π-氢键和无序分子间相互作用的协同作用下，非晶态的M1在三苯胺单元处发生弯曲变形。此外，在非晶态下，M1的第一单线态激发态（S）和第一三线态激发态（T）之间的能隙（Δ）显著减小，与M1在甲苯液体和晶体状态下相比，自旋轨道耦合常数显著增加。结果，非晶态下T→S的上转换受到青睐，因此观察到显著的TADF。此外，与甲苯液体相比，固态的M1发出红移荧光。基于上述结果，我们进一步从理论上设计了一种新的分子M2，其在固态下发出近红外区域的荧光。我们的理论结果有助于理解固态效应诱导的发光机制，并为设计具有可调谐固态发射的新型TADF发光体提供信息。
