Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States.
J Phys Chem A. 2012 Mar 8;116(9):1984-92. doi: 10.1021/jp211646p. Epub 2012 Feb 22.
The metal-to-ligand-charge-transfer (MLCT) excited state of Cu(I) diimine complexes is known to undergo structural reorganization, transforming from a pseudotetrahedral D(2d) symmetry in the ground state to a flattened D(2) symmetry in the MLCT state, which allows ligation with a solvent molecule, forming an exciplex intermediate. Therefore, the structural factors that influence the coordination geometry change and the solvent accessibility to the copper center in the MLCT state could be used to control the excited state properties. In this study, we investigated an extreme case of the steric hindrance caused by attaching bulky tert-butyl groups in bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I), Cu(I)(dtbp)(2). The two bulky tert-butyl groups on the dtbp ligand lock the MLCT state into the pseudotetrahedral coordination geometry and completely block the solvent access to the copper center in the MLCT state of Cu(I)(dtbp)(2). Using ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy, we investigated the MLCT state property changes due to the steric hindrance and demonstrated that Cu(I)(dtbp)(2) exhibited a long-lived emission but no subpicosecond component that was previously assigned as the flattening of the pseudotetrahedral coordination geometry. This suggests the retention of its pseudotetrahedral D(2d) symmetry and the blockage of the solvent accessibility. We made a comparison between the excited state dynamics of Cu(I)(dtbp)(2) with its mono-tert-butyl counterpart, bis(2-tert-butyl-1,10-phenanthroline)copper(I) Cu(I)(tbp)(2). The subpicosecond component assigned to the flattening of the D(2d) coordination geometry in the MLCT excited state was again present in the latter because the absence of a tert-butyl on the phenanthroline allows flattening to the pseudotetrahedral coordination geometry. Unlike the Cu(I)(dtbp)(2), Cu(I)(tbp)(2) exhibited no detectable emission at room temperature in solution. These results provide new insights into the manipulation of various excited state properties in Cu diimine complexes by certain key structural factors, enabling optimization of these systems for solar energy conversion applications.
铜（I）二亚胺配合物的金属-配体电荷转移（MLCT）激发态已知会经历结构重排，从基态的拟四面体 D(2d)对称性转变为 MLCT 态的平面 D(2)对称性，这允许与溶剂分子配位，形成激基复合物中间体。因此，影响 MLCT 态下铜中心配位几何形状变化和溶剂可及性的结构因素可用于控制激发态性质。在这项研究中，我们研究了在双（2,9-二叔丁基-1,10-菲咯啉）铜（I）中引入庞大叔丁基基团引起的空间位阻的极端情况，Cu(I)(dtbp)(2)。dtbp 配体上的两个庞大叔丁基基团将 MLCT 态锁定在拟四面体配位几何形状中，并完全阻止溶剂进入 MLCT 态的铜中心。我们使用超快瞬态吸收光谱和时间分辨发射光谱研究了由于空间位阻引起的 MLCT 态性质变化，并证明 Cu(I)(dtbp)(2) 表现出长寿命的发射，但没有以前归因于拟四面体配位几何形状扁平化的亚皮秒成分。这表明其保留了拟四面体 D(2d)对称性并阻止了溶剂的进入。我们将 Cu(I)(dtbp)(2) 的激发态动力学与它的单叔丁基对应物，双（2-叔丁基-1,10-菲咯啉）铜（I）Cu(I)(tbp)(2)进行了比较。在后者中再次存在以前归因于 MLCT 激发态中 D(2d)配位几何扁平化的亚皮秒成分，因为菲咯啉上没有叔丁基允许扁平化到拟四面体配位几何形状。与 Cu(I)(dtbp)(2)不同，Cu(I)(tbp)(2)在溶液中室温下没有可检测到的发射。这些结果为通过某些关键结构因素操纵铜二亚胺配合物的各种激发态性质提供了新的见解，为这些系统在太阳能转换应用中的优化提供了支持。
