Coherence in Metal-Metal-to-Ligand-Charge-Transfer Excited States of a Dimetallic Complex Investigated by Ultrafast Transient Absorption Anisotropy
Coherence in Metal-Metal-to-Ligand-Charge-Transfer Excited States of a Dimetallic Complex Investigated by Ultrafast Transient Absorption Anisotropy
复制标题
DOI：
10.1021/jp109174f
复制
发表时间：
2011-04-28
影响因子：
2.9
通讯作者：
Chen, Lin X.
中科院分区：
文献类型：
--
作者：
Cho, Sung;Mara, Michael W.;Chen, Lin X.
通过在单波长瞬态吸收（TA）各向异性信号中观察到的振荡特征及其相应的频率和偏振依赖性，研究了双铂分子[Pt(ppy)(mu-(t)Bu(2)pz)](2)金属-配体电荷转移（MMLCT）激发态的相干性。捕获了相对于激发极化方向反相关的平行和垂直TA信号，而在魔角TA信号中观察到最小振荡特征。结合实验结果与先前文献计算结果的综合分析，在势能面上绘出了一个合理的激发态轨迹，表明：(1)由于分子的对称性，两个能量接近的MMLCT激发态可能与两个苯基吡啶（ppy）配体之间来回移动的电荷密度进行电子相干耦合；(2)可以从TA信号的振荡频率中提取出(MMLCT)- m -1和(MMLCT)- m -3状态的电子耦合强度分别为160和55 cm(-1);(3)系统间交叉级联依次为(MMLCT)- m -1 -> (MMLCT)- m -3 (T-1b) -> (MMLCT)- m -3 (T-1a);(4)可能的电子相干性可以通过1皮秒内Pt-Pt σ相互作用调制，并在系统间交叉的第一步中幸存下来。未来的实验正在进行中，以进一步研究振荡特征的起源。这些实验观察结果可能对设计用于光活化反应的多金属中心配合物具有普遍意义，其中激发态的相干性可能促进分子不同部分之间沿某一方向的定向电荷或能量转移。
Coherence in the metal metal-to-ligand-charge transfer (MMLCT) excited state of diplatinum molecule [Pt(ppy)(mu-(t)Bu(2)pz)](2) has been investigated through the observed oscillatory features and their corresponding frequencies as well as polarization dependence in the single-wavelength transient absorption (TA) anisotropy signals. Anticorrelated parallel and perpendicular TA signals with respect to the excitation polarization direction were captured, while minimal oscillatory features were observed in the magic angle TA signal. The combined analysis of the experimental results coupled with those previous calculated in the literature maps out a plausible excited state trajectory on the potential energy surface, suggesting that (1) the two energetically close MMLCT excited states due to the symmetry of the molecule may be electronically and coherently coupled with the charge density shifting back and forth between the two phenylpyridine (ppy) ligands, (2) the electronic coupling strength in the (MMLCT)-M-1 and (MMLCT)-M-3 states may be extracted from the oscillation frequencies of the TA signals to be 160 and 55 cm(-1), respectively, (3) a stepwise intersystem crossing cascades follows (MMLCT)-M-1 -> (MMLCT)-M-3 (T-1b) -> (MMLCT)-M-3 (T-1a), and (4) a possible electronic coherence can be modulated via the Pt-Pt sigma-interactions over a picosecond and survive the first step of intersystem crossing. Future experiments are in progress to further investigate the origin of the oscillatory features. These experimental observations may have general implications in design of multimetal center complexes for photoactivated reactions where coherence in the excited states may facilitate directional charge or energy transfer along a certain direction between different parts of a molecule.