Institute for Physical and Theoretical Chemistry, University of Bonn , Wegelerstr. 12, 53115 Bonn, Germany.
Max Planck Institute for MPI for Chemical Energy Conversion , Stiftstr. 34-36, 45470 Mülheim an der Ruhr, Germany.
J Am Chem Soc. 2017 Sep 20;139(37):12968-12975. doi: 10.1021/jacs.7b04561. Epub 2017 Sep 6.
Utilization of triplets is important for preparing organic light-emitting diodes with high efficiency. Very recently, both electrophosphorescence and electrofluorescence could be observed at room temperature for thienyl-substituted phenazines without any heavy metals ( Ratzke et al. J. Phys. Chem. Lett. , 2016 , 7 , 4802 ). It was found that the phosphorescence efficiency depends on the orientation of fused thiophenes. In this work, the thienyl-substituted phenazines are investigated in more detail by time-resolved electron paramagnetic resonance (EPR) and quantum chemical calculations. Spin dynamics, zero-field splitting constants, and electron-spin structures of the excited triplet states for the metal-free room-temperature triplet emitters are correlated with phosphorescence efficiency. Complete active space self-consistent field (CASSCF) calculations clearly show that the electron spin density distributions of the first excited triplet states are strongly affected by the molecular geometry. For the phosphorescent molecules, the electron spins are localized on the phenazine unit, in which the sulfur atom of the fused thiophene points upward. The electron spins are delocalized onto the thiophene unit just by changing the orientation of the fused thiophenes from upward to downward, resulting in the suppression of phosphorescence. Time-resolved EPR measurements and time-dependent density functional theory (TD-DFT) calculations demonstrate that the electron spins delocalized onto the thiophene unit lead to the acceleration of nonradiative decays, in conjunction with the narrowing of the singlet-triplet energy gap.
三重态的利用对于制备高效率的有机发光二极管非常重要。最近，在没有任何重金属的情况下，噻吩取代的吩嗪既可以观察到电磷光，也可以观察到电荧光（Ratzke 等人，J. Phys. Chem. Lett.，2016，7，4802）。研究发现，磷光效率取决于稠合噻吩的取向。在这项工作中，通过时间分辨电子顺磁共振（EPR）和量子化学计算更详细地研究了噻吩取代的吩嗪。无金属室温三重态发光体的激发三重态的自旋动力学、零场分裂常数和电子自旋结构与磷光效率相关。完全活性空间自洽场（CASSCF）计算清楚地表明，第一激发三重态的电子自旋密度分布强烈受到分子几何形状的影响。对于磷光分子，电子自旋局域在吩嗪单元上，其中稠合噻吩的硫原子向上指向。通过将稠合噻吩的取向从向上改为向下，仅仅改变稠合噻吩的取向，电子自旋就会离域到噻吩单元上，从而抑制磷光。时间分辨 EPR 测量和含时密度泛函理论（TD-DFT）计算表明，离域到噻吩单元上的电子自旋导致非辐射衰减加速，同时单重态-三重态能隙变窄。
