Constraint-induced structural deformation of planarized triphenylboranes in the excited state
Constraint-induced structural deformation of planarized triphenylboranes in the excited state
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DOI：
10.1039/c3sc52751d
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发表时间：
2014-01-01
期刊：
影响因子：
8.4
通讯作者：
Yamaguchi, Shigehiro
中科院分区：
文献类型：
--
作者：
Kushida, Tomokatsu;Camacho, Cristopher;Yamaguchi, Shigehiro
平面化的三苯基硼烷与三个亚甲基桥表现出双荧光带约340和400 nm，尽管它们的结构限制。为了阐明起源，它们的激发态动力学进行了实验和理论研究。荧光寿命和瞬态吸收光谱的测量表明，平面化的三苯基硼烷在最低能量的单重激发态（S-1）具有两个局部极小结构. S-1态的TD-DFT势能面具有至少两个与平面和碗状分子结构相关的最小能量结构。在这些几何构型下的S-1-S-0跃迁能的理论值与实验值符合得很好。这些结果表明，在S-1态的平面到碗结构弛豫是双重荧光的起源。基于计算的硼原子上的部分原子电荷，碗状结构的结构变形导致硼中心上的电子密度增加。因此，增强的分子内电荷转移特性在这种结构变形中起作用。对于三角π扩展的平面化硼烷衍生物也观察到类似的行为。这些结果提供了一个重要的含义，结构约束在一个平面的方式不仅是一个策略，以构建一个刚性的骨架，但也是一个可行的机制，赋予灵活性的骨架。
Triphenylboranes planarized with three methylene bridges exhibited dual fluorescence bands around 340 and 400 nm despite their structural constraint. To elucidate the origin, their excited state dynamics were experimentally and theoretically studied. The measurements of fluorescence lifetimes and transient absorption spectra indicated that the planarized triphenylboranes adopt two local minimum structures in the lowest-energy excited singlet (S-1) state. The TD-DFT potential energy surface of the S-1 state possesses at least two minimum energy structures associated with a planar and a bowl-shaped molecular structure. The theoretical S-1-S-0 transition energies at these geometries were in good agreement with the experimentally observed values. These results indicated that the plane-to-bowl structural relaxation in the S-1 state is the origin of the dual fluorescence. Based on the calculated partial atomic charge on the boron atom, the structural deformation to the bowl-shaped structure results in an increase in the electron density on the boron center. Thus, the enhanced intramolecular charge-transfer character plays a role in this structural deformation. Similar behavior was also observed for trigonally pi-expanded planarized borane derivatives. These results provide an important implication that structural constraint in a planar fashion is not only a strategy to construct a rigid skeleton, but also a viable mechanism to impart flexibility to the skeleton.