Charge-transfer interactions in tris-donor-tris-acceptor hexaarylbenzene redox chromophores.
Charge-transfer interactions in tris-donor-tris-acceptor hexaarylbenzene redox chromophores.
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DOI：
10.1002/chem.201104020
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发表时间：
2012-09
期刊：
影响因子：
--
通讯作者：
M. Steeger;C. Lambert
中科院分区：
文献类型：
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作者：
M. Steeger;C. Lambert
通过钴催化环三聚反应合成了对称和不对称六芳基苯（HAB），每个HAB分别被三个电子给体三芳胺氧化还原中心和三个电子受体三芳基硼烷氧化还原中心取代，从而形成分别具有六和四个供体-受体相互作用的化合物。通过循环伏安法（CV）以及吸收光谱和荧光光谱研究了这些系统的电化学和光物理性质，并与仅包含一对相邻供体-受体对的HAB进行了比较。不对称HAB的CV测量显示出三个氧化峰和三个还原峰，由于离子对和屏蔽效应，其峰分离受导电盐的影响很大。因此，峰分离不能用生成的混合价物质中的电子耦合来解释。瞬态吸收光谱、荧光溶剂化显色和吸收光谱表明，光激发后会产生从胺中心到硼中心的电荷转移态。电子供体-受体相互作用很弱，因为电荷转移必须主要通过空间发生。此外，局域激发电荷转移态的激发能可以在荧光寿命（约60 ns）内在这些多维发色团的芳基取代基之间重新分配。这一结果得到了稳态荧光各向异性测量的证实，进一步表明表面对称 HAB 中的对称性破缺。添加氟离子会导致硼中心因络合而失去其接受能力。因此，正如在吸收光谱和荧光光谱中观察到的那样，供体-受体发色团中的电荷转移特征消失。然而，硼中心作为氟化物传感器的能力受到溶剂的水分含量的强烈影响，这可能是由于水分子和氟化物阴离子之间形成氢键相互作用。
Symmetric- and asymmetric hexaarylbenzenes (HABs), each substituted with three electron-donor triarylamine redox centers and three electron-acceptor triarylborane redox centers, were synthesized by cobalt-catalyzed cyclotrimerization, thereby forming compounds with six- and four donor-acceptor interactions, respectively. The electrochemical- and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption- and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor-acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak-separation is greatly influenced by the conducting salt, owing to ion-pairing and shielding effects. Consequently, the peak-separations cannot be interpreted in terms of the electronic couplings in the generated mixed-valence species. Transient-absorption spectra, fluorescence-solvatochromism, and absorption spectra show that charge-transfer states from the amine- to the boron centers are generated after optical excitation. The electronic donor-acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge-transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60 ns). This result was confirmed by steady-state fluorescence-anisotropy measurements, which further indicated symmetry-breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge-transfer character in the donor-acceptor chromophores vanishes, as observed in both the absorption- and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen-bonding interactions between water molecules and the fluoride anions.