Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland.
Instituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Valencia, Spain.
Dalton Trans. 2019 Jan 2;48(2):446-460. doi: 10.1039/c8dt03827a.
The synthesis and characterization of five [Cu(P^P)(N^N)][PF6] complexes in which P^P = 2,7-bis(tert-butyl)-4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (tBu2xantphos) or the chiral 4,5-bis(mesitylphenylphosphino)-9,9-dimethylxanthene (xantphosMes2) and N^N = 2,2'-bipyridine (bpy), 6-methyl-2,2'-bipyridine (6-Mebpy) or 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me2bpy) are reported. Single crystal structures of four of the compounds confirm that the copper(i) centre is in a distorted tetrahedral environment. In [Cu(xantphosMes2)(6-Mebpy)][PF6], the 6-Mebpy unit is disordered over two equally populated orientations and this disorder parallels a combination of two dynamic processes which we propose for [Cu(xantphosMes2)(N^N)]+ cations in solution. Density functional theory (DFT) calculations reveal that the energy difference between the two conformers observed in the solid-state structure of [Cu(xantphosMes2)(6-Mebpy)][PF6] differ in energy by only 0.28 kcal mol-1. Upon excitation into the MLCT region (λexc = 365 nm), the [Cu(P^P)(N^N)][PF6] compounds are yellow to orange emitters. Increasing the number of Me groups in the bpy unit shifts the emission to higher energies, and moves the Cu+/Cu2+ oxidation to higher potentials. Photoluminescence quantum yields (PLQYs) of the compounds are low in solution, but in the solid state PLQYs of up to 59% (for [Cu(tBu2xantphos)(6,6'-Me2bpy)]+) are observed. Increased excited-state lifetimes at low temperature are consistent with the complexes exhibiting thermally activated delayed fluorescence (TADF). This is supported by the small energy difference calculated between the lowest-energy singlet and triplet excited states (0.17-0.25 eV). The compounds were tested in simple bilayer light-emitting electrochemical cells (LECs). The optoelectronic performances of complexes containing xantphosMes2 were generally lower with respect to those with tBu2xantphos, which led to bright and efficient devices. The best performing LECs were obtained for the complex [Cu(tBu2xantphos)(6,6'-Me2bpy)][PF6] due to the increased steric hindrance at the N^N ligand, resulting in higher PLQY.
报告了五个 [Cu(P^P)(N^N)][PF6] 配合物的合成和表征，其中 P^P = 2,7-双(叔丁基)-4,5-双(二苯基膦基)-9,9-二甲基呫吨（tBu2xantphos）或手性 4,5-双(均三甲苯基膦基)-9,9-二甲基呫吨（xantphosMes2）和 N^N = 2,2'-联吡啶（bpy）、6-甲基-2,2'-联吡啶（6-Mebpy）或 6,6'-二甲基-2,2'-联吡啶（6,6'-Me2bpy）。四种化合物的单晶结构证实，铜（i）中心处于扭曲的四面体型环境中。在 [Cu(xantphosMes2)(6-Mebpy)][PF6] 中，6-Mebpy 单元在两个等占据取向之间无序，这种无序与我们提出的溶液中 [Cu(xantphosMes2)(N^N)]+阳离子的两种动态过程相结合。密度泛函理论（DFT）计算表明，在 [Cu(xantphosMes2)(6-Mebpy)][PF6] 的固态结构中观察到的两种构象之间的能量差仅相差 0.28 kcal mol-1。在激发到 MLCT 区域（λexc = 365 nm）时，[Cu(P^P)(N^N)][PF6] 配合物为黄色到橙色发射体。在 bpy 单元中增加 Me 基团的数量会将发射移至更高的能量，并将 Cu+/Cu2+氧化移至更高的电位。化合物在溶液中的荧光量子产率（PLQY）较低，但在固态中观察到高达 59%（对于 [Cu(tBu2xantphos)(6,6'-Me2bpy)]+）的 PLQY。低温下激发态寿命的增加与配合物表现出热活化延迟荧光（TADF）一致。这得到了在最低能量单重态和三重态激发态之间计算出的小能量差（0.17-0.25 eV）的支持。在简单的双层发光电化学电池（LEC）中测试了这些化合物。与含有 tBu2xantphos 的化合物相比，含有 xantphosMes2 的化合物的光电性能通常较低，这导致了更亮和更有效的器件。由于 N^N 配体的空间位阻增加，[Cu(tBu2xantphos)(6,6'-Me2bpy)][PF6] 获得了性能最佳的 LEC，从而导致更高的 PLQY。
