Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore, 560064, India.
Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fedéralé de Lausanne (EPFL), 1015, Lausanne, Switzerland.
Chemphyschem. 2021 Nov 18;22(22):2297-2304. doi: 10.1002/cphc.202100518. Epub 2021 Sep 23.
Mechanochromic luminescent materials, exhibiting a change in luminescence behavior under external stimuli have emerged as one of the promising candidates for upcoming efficient OLEDs. Recently mechanochromic luminescence was reported in a donor-acceptor-donor (D-A-D) triad featuring two phenothiazine units separated by a dibenzo[a,j]phenazine motif. The triad follows different emissive routes ranging from phosphorescence to TADF based on the conformational switching of the D units. In this article, we investigate such conformation-dependent photophysical behavior of this triad through theoretical calculations. By analyzing the nature of ground state, excited state and factors determining the reverse ISC crossing rates associated with the relative orientation of the D and A units, we delineate the effect of the conformational changes on their photophysical properties. Our findings reveal that axial orientation of both the donor groups enhances the overlap between HOMO and LUMO leading to a large singlet-triplet gap ( ) which drives phosphorescence emission. On the contrary, the equatorial orientation of the donor groups minimizes to facilitate rISC making the conformers TADF active. The role of several geometric factors affecting the photophysical properties of the conformers is also highlighted. Finally, we show how to regulate the population difference among the conformers by functionalizing the triad to harvest the maximum TADF efficiency.
机械变色发光材料在外部刺激下表现出发光行为的变化，已成为即将出现的高效 OLED 的有前途的候选材料之一。最近，在具有两个吩噻嗪单元由二苯并[a,j]吩嗪基序隔开的给体-受体-给体 (D-A-D) 三联体中报道了机械变色发光。根据 D 单元的构象切换，三联体遵循从磷光到 TADF 的不同发射途径。在本文中，我们通过理论计算研究了这种三重态的这种构象依赖性光物理行为。通过分析基态、激发态的性质以及与 D 和 A 单元相对取向相关的反向 ISC 交叉速率的决定因素，我们描绘了构象变化对其光物理性质的影响。我们的研究结果表明，两个供体基团的轴向取向增强了 HOMO 和 LUMO 之间的重叠，导致大的单重态-三重态能隙 ( )，从而驱动磷光发射。相反，供体基团的赤道取向最小化 ，以促进 rISC 使构象 TADF 活跃。还强调了影响构象体光物理性质的几个几何因素的作用。最后，我们展示了如何通过官能化三联体来调节构象体之间的种群差异，以获得最大的 TADF 效率。
