Charge‐Transfer Interactions in Tris‐Donor–Tris‐Acceptor Hexaarylbenzene Redox Chromophores

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.

Pyrene Molecular Orbital Shuffle—Controlling Excited State and Redox Properties by Changing the Nature of the Frontier Orbitals

“…For example, ion pairing between the mono‐ and dication of interest and the counterion from the electrolyte plays an important role, as it influences the ease of adding/removing electrons to/from the system. Moreover, Δ E reflects properties of M vs. M + and M + vs. M 2+ , while the electronic coupling is only attributed to M + . The Δ E value is only a qualitative, but not a quantitative measure of the ground‐state delocalization of a mixed‐valent system .…”

Recent developments in and perspectives on three-coordinate boron materials: a bright future

“…37 The Lambert group reported a hexaarylbenzene with three triarylamine donors and three triarylborane acceptors with weak donor–acceptor interactions due to through-space charge transfer. 38 Additionally, the excitation energy can be redistributed between the aryl substituents within the fluorescence lifetime. Highly fluorescent N -borylated 2,5-diarylpyrroles with dimesitylborane as the acceptor moiety were reported by Yamaguchi in 2013.…”

Hydrostatic Pressure on Toroidal Interaction and Propeller Chirality of Hexaarylbenzenes: Explicit Solvent Effects on Differential Volumes in Methylcyclohexane and Hexane

“…Among the various propeller‐shaped molecules with distinctive features, a unique intramolecular interaction between the peripheral aromatic blades in hexaarylbenzenes (HABs) has attracted considerable attention, as they demonstrate unusually effective electron and/or exciton delocalization and hopping between the aromatic blades, which is known as toroidal interaction . Although such interactions have been (semi)quantitatively discussed with various HAB derivatives, it is imperative to take their dynamic behaviors into consideration, to comprehend the exact nature of the toroidal interaction. In fact, X‐ray crystal analysis of hexaphenylbenzene showed a propeller‐like structure with the blade tilt angles of about 65°, which apparently contradicts with an electron diffraction study in the gas phase, which implied a structure with all the blades perpendicular to the central benzene ring .…”