Institut für Organische Chemie, Universität Würzburg and Wilhelm-Conrad-Röntgen Research Center for Complex Material Systems, Am Hubland, 97074 Würzburg, Germany.
Chemistry. 2012 Sep 17;18(38):11937-48. doi: 10.1002/chem.201104020. Epub 2012 Aug 9.
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.
对称和不对称六芳基苯（HAB），每个都被三个电子给体三芳基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，通过钴催化的环三聚反应合成，从而分别形成具有六个和四个供体-受体相互作用的化合物。通过循环伏安法（CV）、吸收光谱和荧光光谱研究了这些体系的电化学和光物理性质，并与仅含有一个相邻供体-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，其峰分离受到导电盐的极大影响，这是由于离子对和屏蔽效应。因此，不能根据生成的混合价态物种中的电子耦合来解释峰分离。瞬态吸收光谱、荧光溶剂化变色和吸收光谱表明，光激发后会从胺到硼中心生成电荷转移态。电子给体-受体相互作用较弱，因为电荷转移主要通过空间发生。此外，局域激发电荷转移态的激发能可以在这些多维发色团的芳基取代基之间在荧光寿命（约 60 ns）内重新分布。稳态荧光各向异性测量证实了这一结果，进一步表明在表面对称的 HAB 中存在对称性破坏。添加氟离子会导致硼中心因络合而失去接受能力。因此，在吸收光谱和荧光光谱中都观察到供体-受体发色团中的电荷转移特征消失。然而，硼中心作为氟离子传感器的能力受到溶剂含水量的强烈影响，这可能是由于水分子和氟离子阴离子之间形成氢键相互作用的结果。
