Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications
Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications
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DOI：
10.1021/acs.jpcc.8b01636
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发表时间：
2018-06-28
影响因子：
3.7
通讯作者：
Chen, Lin X.
中科院分区：
文献类型：
--
作者：
Kim, Pyosang;Kelley, Matthew S.;Chen, Lin X.
研究了具有赝C-2对称性的环化铂二聚体的金属-金属-配体-电荷转移（MMLCT）激发态的振动相干性，其中可以同时诱导两个从最高占据分子轨道（金属-金属σ * 轨道）到高能配体π * 轨道的近简并跃迁.我们在配合物[（ppy）Pt（mu-（t）Bu（2）pz）]（2）（1）和反[ppy）Pt（mu-pyt）]（2）（2）（2）中观察到飞秒简并瞬态吸收（TA）信号的振荡特征，这归因于相干核运动调制HOMO（反键σ * 能级，从而调制MMLCT跃迁的能量。这种相干核运动的特征，如振荡频率和退相时间，在1和2之间不同，主要由两个可能影响振动相干性的结构因素解释：Pt-Pt距离（1为2.97埃，2为2.85埃）和分子形状（1在“A”框架中，2在“H”框架中）。由于两个Pt原子之间的电子耦合决定了成键sigma和反键sigma* 轨道的能量分裂，因此与MMLCT跃迁耦合的Pt-Pt伸缩模改变了Pt与基态之间的距离。有趣的是，虽然1在MMLCT状态下显示出120 cm（-1）的单个Pt-Pt伸缩频率，但2在MMLCT状态下显示出多个下移的频率（80和105 cm（-1）），其中沿着的振动失相时间比1短。基于基态优化结构和拉曼计算，发现2中振动波包动力学的明显变化与2中的“H”框架几何结构相比于1中的“A”框架几何结构密切相关，导致ppy配体之间的π-π相互作用急剧增加。虽然TA实验没有直接揭示超快系间穿越（ISC），因为在早期的时间尺度上有很强的相干尖峰，但振动波包动力学对分子几何形状的依赖性可以基于先前提出的作为Pt-Pt距离函数的势能面来理解，表明环化配体之间的相互作用可能是决定Pt（II）二聚体中Pt-Pt缩短和相关能量弛豫动力学的关键因素。进一步的实验，使用飞秒宽带TA和X射线散射光谱，计划直接调查ISC和Pt-Pt收缩，以支持基态分子几何形状和光诱导的结构变化之间的关系在Pt（II）二聚体。
Vibrational coherence in the metal-metal-toligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C-2 symmetry was investigated where two nearly degenerate transitions from the highest occupied molecular orbital (metal-metal sigma* orbital) to higher energy ligand pi* orbitals could be simultaneously induced. We observed oscillatory features in femtosecond degenerate transient absorption (TA) signals from complexes [(ppy)Pt(mu-(t)Bu(2)pz)](2) (1) and anti-[ppy)Pt(mu-pyt)](2) (2), which are attributed to coherent nuclear motions that modulate the HOMO (antibonding sigma* energy level, and hence, the energy for the MMLCT transition. The characteristics of such coherent nuclear motions, such as the oscillatory frequency and the dephasing time, differ between 1 and 2 and are explained by mainly two structural factors that could influence the vibrational coherence: the Pt-Pt distance (2.97 angstrom for 1 vs 2.85 angstrom for 2) and molecular shape (1 in an "A" frame vs 2 in an "H" frame). Because the electronic coupling between the two Pt atoms determines the energy splitting of the bonding sigma and antibonding sigma* orbital, the Pt-Pt stretching mode coupled to the MMLCT transition changes the inter Pt distance from that of the ground state. Interestingly, while 1 shows a single Pt-Pt stretching frequency of 120 cm(-1) in the MMLCT state, 2 exhibits multiple downshifted frequencies (80 and 105 cm(-1)) in the MMLCT state along with a shorter vibrational dephasing time than 1. Based on the ground state optimized structures and Raman calculations, the changes evident in the vibrational wavepacket dynamics in 2 are closely correlated with the "H" framed geometry in 2 compared to the "A" frame in 1, leading to the sharp increase in pi-pi interaction between ppy ligands. Although the TA experiments do not directly reveal the ultrafast intersystem crossing (ISC) because of a strong coherent spike at early time scales, the dependence of the vibrational wavepacket dynamics on molecular geometry can be understood based on previously proposed potential energy surfaces as a function of Pt-Pt distance, suggesting that the interaction between the cyclometalating ligands can be a key factor in determining the Pt-Pt shortening and the related energy relaxation dynamics in the Pt(II) dimers. Further experiments using femtosecond broadband TA and X-ray scattering spectroscopy are planned to investigate directly the ISC and Pt-Pt contraction to support the relationship between ground state molecular geometry and photoinduced structural changes in the Pt(II) dimers.