Theoretical study of pyrazolate-bridged dinuclear platinum(II) complexes: interesting potential energy curve of the lowest energy triplet excited state and phosphorescence spectra.
Theoretical study of pyrazolate-bridged dinuclear platinum(II) complexes: interesting potential energy curve of the lowest energy triplet excited state and phosphorescence spectra.
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DOI：
10.1021/ic702367f
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发表时间：
2008-04
影响因子：
4.6
通讯作者：
Ken Saito;Y. Nakao;S. Sakaki
中科院分区：
文献类型：
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作者：
Ken Saito;Y. Nakao;S. Sakaki
用密度泛函理论（B3 PW 91）研究了4种3，5-二烷基吡唑（R2 pz）桥联双核铂（II）配合物[Pt 2（mu-R2 pz）2（dfppy）2]（dfppy=2-（2，4-二氟苯基）吡啶; R2 pz =吡唑基，1，3，5-二甲基吡唑基，2，3-甲基-5-叔丁基吡唑基，3，5-双叔丁基吡唑基）.根据最低能量三重激发态（T1）的势能曲线（PEC），讨论了它们磷光光谱的斯托克斯位移。该PEC显著依赖于pz上取代基的体积。在1和2中，在pz上带有小的取代基，除了全局最小值之外，在T1状态中存在一个局部最小值。局部最小值几何类似于S 0-平衡几何。在该局部最小值处的T1态被表征为dfppy中的pi-pi* 激发态，其中Pt的dpi轨道通过与dfppy的pi轨道的反键相互作用参与该激发态;换句话说，该三重激发态被指定为配体中心的pi-pi* 激发态和金属-配体电荷转移激发态的混合物（（3）LC/MLCT）。T1-全局极小值的几何形状与S 0-平衡极小值的几何形状有很大的不同。在全局最小值处的T1态被表征为三重金属-金属-配体电荷转移（（3）MMLCT）激发态，其通过从dfppy的dsigma-dsigma反键轨道到pi* 轨道的单电子激发形成。由于局部极小值的存在，T1态的几何变化在室温（RT）下的聚苯乙烯和在77 K下冷冻的2-甲基四氢呋喃（2-MeTHF）中被抑制。结果，磷光的能量在这些溶剂中几乎相同。另一方面，在室温下的2-MeTHF流体中，T1态的几何结构很容易达到T1全局最小值。由于T1-全局最小几何形状与S 0-平衡几何形状有很大不同，磷光发生在相当低的能量下。这就是为什么斯托克斯位移在流体2-MeTHF中非常大，而在聚苯乙烯和冷冻的2-MeTHF中很小的原因。在3和4，轴承庞大的叔丁基取代基上的pz，只有T1-全局最小值，但局部最小值是不存在的。该T1全局最小值的电子结构被指定为（3）MMLCT激发态，如1和2。虽然冷冻的2-MeTHF抑制了3和4在T1态的几何形状变化，但由于T1-局部最小值的存在，它们在聚苯乙烯中的几何形状适度变化。因此，磷光的能量在聚苯乙烯中比在冷冻的2-MeTHF中适度地低。的T1-全球最小的几何形状是非常不同的S 0-平衡3，但适度不同的4，这是解释在这些配合物的对称性和叔丁基之间的空间排斥的pz和dfppy。因此，3的磷光能量在流体2-MeTHF中比在冷冻的2-MeTHF中低得多，如1和2，但4的磷光能量适度地低;换句话说，流体2-MeTHF中的斯托克斯位移仅在4中小。
Four kinds of 3,5-dialkylpyrazolate(R2pz)-bridged dinuclear platinum(II) complexes [Pt2(mu-R2pz)2(dfppy)2] (dfppy=2-(2,4-difluorophenyl)pyridine; R2pz=pyrazolate in 1, 3,5-dimethylpyrazolate in 2, 3-methyl-5- tert-butylpyrazolate in 3, and 3,5-bis(tert-butyl)pyrazolate in 4) were theoretically investigated by the DFT(B3PW91) method. The Stokes shift of their phosphorescence spectra was discussed on the basis of the potential energy curve (PEC) of the lowest energy triplet excited state (T1). This PEC significantly depends on the bulkiness of substituents on pz. In 1 and 2, bearing small substituents on pz, one local minimum is present in the T1 state besides a global minimum. The local minimum geometry is similar to the S0-equilibrium one. The T1 state at this local minimum is characterized as the pi-pi* excited state in dfppy, where the dpi orbital of Pt participates in this excited state through an antibonding interaction with the pi orbital of dfppy; in other words, this triplet excited state is assigned as the mixture of the ligand-centered pi-pi* excited and metal-to-ligand charge transfer excited state ((3)LC/MLCT). The geometry of the T1-global minimum is considerably different from the S0-equilibrium one. The T1 state at the global minimum is characterized as the triplet metal-metal-to-ligand charge transfer ((3)MMLCT) excited state, which is formed by the one-electron excitation from the dsigma-dsigma antibonding orbital to the pi* orbital of dfppy. Because of the presence of the local minimum, the geometry change in the T1 state is suppressed in polystyrene at room temperature (RT) and frozen 2-methyltetrahydrofuran (2-MeTHF) at 77 K. As a result, the energy of phosphorescence is almost the same in these solvents. In fluid 2-MeTHF at RT, on the other hand, the geometry of the T1 state easily reaches the T1-global minimum. Because the T1-global minimum geometry is considerably different from the S0-equilibrium one, the phosphorescence occurs at considerably low energy. These are the reasons why the Stokes shift is very large in fluid 2-MeTHF but small in polystyrene and frozen 2-MeTHF. In 3 and 4, bearing bulky tert-butyl substituents on pz, only the T1-global minimum is present but the local minimum is not. The electronic structure of this T1-global minimum is assigned as the (3)MMLCT excited state like 1 and 2. Though frozen 2-MeTHF suppresses the geometry change of 3 and 4 in the T1 state, their geometries moderately change in polystyrene because of the absence of the T1-local minimum. As a result, the energy of phosphorescence is moderately lower in polystyrene than in frozen 2-MeTHF. The T1-global minimum geometry is much different from the S0-equilibrium one in 3 but moderately different in 4, which is interpreted in terms of the symmetries of these complexes and the steric repulsion between the tert-butyl group on pz and dfppy. Thus, the energy of phosphorescence of 3 is much lower in fluid 2-MeTHF than in frozen 2-MeTHF like 1 and 2 but that of 4 is moderately lower; in other words, the Stokes shift in fluid 2-MeTHF is small only in 4.