Molecular designing for Intensely emissive transition metal complexes

具有蓝色磷光的过渡金属化合物的分子设计是近年来磷光OLED器件发展的重要问题之一。虽然吡唑配体经常被用来增加磷光Ir(III)配合物的发射能量，但这些配体的引入被认为显著地提高了非辐射跃迁速率。在这项工作中，非辐射过程的动力学参数已经由两个吡唑-Ir键的Ir络合物的发射寿命在较宽的温度范围内进行了测量。得到的激活能为1800 cm-1，表明发光磷光态是通过dd态猝灭的，为了深入了解猝灭过程，用无约束密度泛函理论计算了该络合物最低三重态的势能面，并研究了通过dd态猝灭的几何变化。结果表明，在Ir-吡唑键比磷光态延长0.6 nm的几何构型下，三重态激发态和基态的能量都发生了简并，因此发生了非常快的失活。计算得到的发射能和活化能与观测结果吻合较好，这极大地验证了密度泛函理论的计算结果。

Molecular designing of transient-metal compounds with blue phosphorescence is one of the most important problems in recent development of phosphorescent OLED devices. While pyrazole ligands have been often used to increase emission energy of phosphorescent iridium(III) complexes, introduction of these ligands is known to increase nonradiative transition rates remarkably. In this work, kinetic parameters for the nonradiative process have been determined from emission lifetimes of iridium complexes with two pyrazole-iridium bonds were measured over a wide temperature range. The obtained activation energy of 1800 cm-1 indicates that the emissive phosphorescent state is quenched via closely lying dd state.To get insights into the quenching process, the potential energy surface of the lowest triplet state of this complex was calculated using unrestricted DFT calculation and the geometrical changes associated with the quenching via the dd state were studied. It was revealed that at the geometry where the iridium-pyrazole bonds are elongated by 0.6nm compare to that for the phosphorescent state the energies of both the triplet excited state and the ground state become degenerating and therefore very fast deactivation occurs. Both the calculated emission energy and activation energy were in good agreement with those observed, which greatly validates the DFT calculations.