Zhang Bo-Hua, Li Jun-An, Wang Min, Ren Ai-Min, He Teng-Fei, Lin Pan-Pan, Zhang Yun-Li, Xi Xiao-Yue, Zou Lu-Yi
Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, P. R. China.
Gastroenterology and Center of Digestive Endoscopy, The Second Hospital of Jilin University, 218 Ziqiang Street, Changchun 130041, P. R. China.
Phys Chem Chem Phys. 2021 Mar 11;23(9):5652-5664. doi: 10.1039/d0cp06269c.
To develop solid-state light-emitting materials with high luminescence efficiency, determining the potential photophysics and luminescence mechanisms of the aggregation state remains a challenge and a priority. Here, we apply density functional theory to study the photophysical properties of a series of square planar Pt(ii) complexes in both monomeric and dimeric forms. We reveal that four monomeric Pt(ii) complexes are dominated by triplet ligand-to-ligand charge-transfer, and the lack of the triplet metal-to-ligand charge-transfer feature results in weak spin-orbit coupling (SOC), which leads to limited radiative rates; moreover, calculated nonradiative transition rates are one or two orders of magnitude higher than those radiative rates because a large amount of reorganization energy caused by the vibration of the bipyrazolate (bipz) ligand cannot be readily suppressed in the monomeric form. Therefore, four monomers exhibit photoluminescence quenching in CH2Cl2 solution in both theoretical calculations and experiments. However, in the solid state, the intense luminescence phenomenon indicates obviously distinct properties between the monomer and aggregation. We carried out a dimer model to interpret that the interaction of PtPt induces a metal-metal-to-ligand charge-transfer excimeric state, which leads more metal components to participate in the charge transfer and enhance the SOC effect. At the same time, the ligand vibration can be significantly reduced by the shortened distance, and there is a strong π-π packing interaction in the dimer; thus, an excellent quantum yield can be achieved in aggregation. In addition, we disclose that introducing bulky substituents bearing electron-donating groups at R' and R'' positions have little effect on the properties of the monomers; however, there is a benefit of restricting the internal reorganization energy through the intermolecular interaction when packing in the solid state. Therefore, substitutions can be tuned to improve the properties of monomers (such as emission energy and reorganization energy). We hope that our work will shine some light on Pt(ii) emitters in the fabrication of efficient OLEDs.
为了开发具有高发光效率的固态发光材料，确定聚集态的潜在光物理性质和发光机制仍然是一个挑战和首要任务。在此，我们应用密度泛函理论研究了一系列正方形平面Pt(ii)配合物单体和二聚体形式的光物理性质。我们发现，四种单体Pt(ii)配合物以三重态配体到配体电荷转移为主，缺乏三重态金属到配体电荷转移特征导致自旋轨道耦合(SOC)较弱，从而导致辐射速率有限；此外，计算得到的非辐射跃迁速率比辐射速率高一个或两个数量级，因为联吡唑(bipz)配体振动引起的大量重组能在单体形式中不易被抑制。因此，在理论计算和实验中，四种单体在CH2Cl2溶液中均表现出光致发光猝灭。然而，在固态中，强烈的发光现象表明单体和聚集体之间的性质明显不同。我们建立了一个二聚体模型来解释Pt-Pt相互作用诱导了金属-金属到配体电荷转移准分子态，这使得更多的金属成分参与电荷转移并增强了SOC效应。同时，配体振动可因距离缩短而显著降低，且二聚体中存在强π-π堆积相互作用；因此，在聚集体中可实现优异的量子产率。此外，我们还发现，在R'和R''位置引入带有供电子基团的大体积取代基对单体的性质影响不大；然而，在固态堆积时，通过分子间相互作用限制内部重组能是有益的。因此，可以调整取代基来改善单体的性质(如发射能量和重组能)。我们希望我们的工作能为高效OLEDs制造中的Pt(ii)发光体提供一些启示。
