Charge-transfer interactions in tris-donor-tris-acceptor hexaarylbenzene redox chromophores |
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Authors: | Markus Steeger Christoph Lambert |
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Institution: | Institut für Organische Chemie, Universit?t Würzburg and Wilhelm-Conrad-R?ntgen Research Center for Complex Material Systems, Am Hubland, 97074 Würzburg (Germany), Fax: (+49)?931-31-87218. |
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Abstract: | 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. |
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