Significant Structural Relaxations of Excited [n]Cycloparaphenylene Dications (n = 5 - 9)
Significant Structural Relaxations of Excited [n]Cycloparaphenylene Dications (n = 5 - 9)
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激发的 [n] 环对亚苯基双阳离子 (n = 5 - 9) 的显着结构松弛
DOI：
10.1039/c8cp04860f
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发表时间：
2018
期刊：
影响因子：
--
通讯作者：
T.
中科院分区：
文献类型：
--
作者：
Fujitsuka;M.; Kayahara;E.; Chao;L.; Yamago;S.; Majima;T.
环状大环如环芳苯([n]CPPs，其中n表示苯环的数目)近年来引起了人们的极大关注，因为它们具有高度应变的芳香结构和辐射取向的p-轨道的有趣性质。虽然[n]CPP的自由基阳离子和正离子状态已经被表征，但关于它们的激发态还没有可用的信息，这些激发态有望表现出增强的氧化还原性能。在本研究中，我们通过在可见光和近红外区的瞬时吸收测量研究了[n]CPP2+的S1态。瞬时吸收峰的能量与重复单元的倒数呈线性关系，表明激发态的分布随环的尺寸增大而扩大。此外，较小的CPP2+S具有较长的激发态寿命。理论计算表明，伴随着电荷分布的变化，较小的CPP2+S有较大的结构弛豫。因此，可以得出结论，较小的CPP2+S的S1态寿命较长的原因是结构变化引起的较小的Franck-Condon因子和较大的S1态能量。
Hoop-shaped macrocycles such as cycloparaphenylenes ([n]CPPs, where n denotes the number of phenylene rings) have attracted considerable attention in recent years because of their interesting properties arising from the highly strained aromatic structure and radially oriented p-orbitals. While the radical cation and dication states of [n]CPPs have been characterized, there is no information available about their excited states, which are expected to exhibit enhanced redox properties. In this study, we investigated the S1 state of [n]CPP2+ by transient absorption measurements in the visible and near-IR regions. The energy of the transient absorption peak exhibited a linear relationship with the reciprocal of the repeating unit, which indicated that the distribution of the excited state expanded with the size of the ring. In addition, smaller CPP2+s showed longer excited state lifetimes. Theoretical calculations suggested that there was a substantial structural relaxation of the smaller CPP2+s accompanying the changes in the charge distribution. Therefore, it was concluded that the smaller Franck–Condon factor resulting from the considerable structural change and larger S1 energy were responsible for the longer S1 state lifetime of smaller CPP2+s.