Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China.
Phys Chem Chem Phys. 2018 Oct 3;20(38):24955-24967. doi: 10.1039/c8cp03657h.
Herein, we have employed B3LYP and TD-B3LYP methods with the QM/MM approach to study the thermally activated delayed fluorescence (TADF) phenomenon of two Cu(i) complexes bearing 5-(2-pyridyl)-tetrazolate (PyrTet) and phosphine (POP) ligands in the gas phase, solution, and crystal form. On the basis of spectroscopic properties, ground- and excited-state geometric and electronic structures, and related radiative and nonradiative rates, we have found that (1) the S1 and T1 excited states have clear metal-to-ligand charge transfer character from the Cu(i) atom to the PyrTet group; (2) the S1 and T1 states have a very small energy gap ΔES1-T1, less than 0.18 eV, which makes the forward and reverse intersystem crossing ISC and rISC processes between the S1 and T1 states very efficient; and (3) the low-frequency vibrational modes related to the torsional motion of the POP and PyrTet groups are found to have significant Huang-Rhys factors and are responsible for the efficient ISC and rISC rates. However, the corresponding Huang-Rhys factors are remarkably suppressed in the crystal compared with those in the gas phase and in solution due to the rigidity of the crystal surroundings; as a result, the ISC and rISC rates are accordingly reduced slightly in the crystal. This comparison also demonstrates that the surrounding effects are very important for modulating the photophysical properties of the Cu(i) complexes. Finally, our work gives helpful insights into the TADF mechanism of the Cu(i) compounds, which could assist in rationally designing TADF materials with excellent performance.
在此，我们采用了 B3LYP 和 TD-B3LYP 方法与 QM/MM 方法，研究了两种含有 5-(2-吡啶基)-四唑（PyrTet）和膦（POP）配体的 Cu(i)配合物在气相、溶液和晶体形式中热激活延迟荧光（TADF）现象。基于光谱性质、基态和激发态几何和电子结构以及相关的辐射和非辐射速率，我们发现：（1）S1 和 T1 激发态具有明显的金属-配体电荷转移特征，从 Cu(i)原子到 PyrTet 基团；（2）S1 和 T1 态的能隙ΔES1-T1 非常小，小于 0.18 eV，这使得 S1 和 T1 态之间的正向和反向系间窜越 ISC 和 rISC 过程非常有效；（3）与 POP 和 PyrTet 基团扭转运动相关的低频振动模式具有显著的 Huang-Rhys 因子，负责有效的 ISC 和 rISC 速率。然而，由于晶体周围环境的刚性，与晶体相比，在气相和溶液中，相应的 Huang-Rhys 因子显著降低；因此，ISC 和 rISC 速率在晶体中略有降低。这种比较还表明，周围环境对调节 Cu(i)配合物的光物理性质非常重要。最后，我们的工作深入了解了 Cu(i)化合物的 TADF 机制，这有助于合理设计具有优异性能的 TADF 材料。
