Organische gemischtvalente Verbindungen: ein Spielplatz für Elektronen und Löcher

The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox-active Schiff-base ligands connected via a 1,2-phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2. Oxidation of 1 provides [1˙](+), which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm(-1). Oxidation of 2 to the bis-oxidized form affords a bis-ligand radical species [2˙˙](2+). Variable temperature EPR spectroscopy of [2˙˙](2+) shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. [2˙˙](2+) is thus best described as incorporating two non-interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand-radical [1˙](+) exhibits exciton splitting in [2˙˙](2+), due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis-ligand radical systems. Addition of excess pyridine to [2˙˙](2+) results in a shift in the oxidation locus from a bis-ligand radical species to the Ni(III) /Ni(III) derivative [2(py)4](2+), demonstrating that the ligand system can incorporate significant bulk in the axial positions.

Section: Discussionsupporting

confidence: 60%

Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

Dunn

Chiang

Ramogida

et al. 2013

“…Furthermore, we performed fluoride-titration experiments with HABs 1-3. Because both of them contain reducible-and oxidizable redox centers, they may form different mixed-valence compounds, [20] that is, species that contain both oxidized-and neutral (or reduced and neutral) redox centers of the same chemical nature. [16b] Typically, the binding constants of triorganoboranes depend on the Lewis acidity of the boron center and on the steric hindrance of the substituents.…”

Section: Introductionmentioning

confidence: 99%

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

Steeger¹,

Lambert²

2012

Self Cite

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.

“…Such chargel ocalization and delocalization in am olecule is fundamental for understandingt he stabilization and transportation of charges and is related to the reactivity of the molecule. [1,2] In this regard, aromatich ydrocarbons that can easily undergo one-electrono xidation/reduction have been widely recognized as good model systems for studying chargel ocalization/delocalization in their radicali ons as well as charge stabilization through charge-resonancedimerization. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] Experimentally,c harged elocalization over multiple units has been identified with characteristic absorption bands in the near-IR region due to the spatialo verlap of p-orbitals.…”

Section: Introductionmentioning

confidence: 99%

Formation of the Charge‐Localized Dimer Radical Cation of 2‐Ethyl‐9,10‐dimethoxyanthracene in Solution Phase

Choi

Ahn

Fujitsuka

et al. 2019

Although dimer radicali ons of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in as ingle monomer unit insteado f being shared between two monomer units,i ss till elusive. In this study,t he formation of ac harge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene( DMA), (DMA) 2 C + is investigated by transient absorption (TA) and time-resolved resonance Raman( TR 3 )s pectroscopic methodsc ombined with ap ulse radiolysis technique. Visible-and near-IR TA signals in highly concentrated DMA solutionss upported the formation of non-covalent( DMA) 2 C + by association of DMA and DMAC + .T R 3 spectra obtainedf rom 30 ns to 300 ms time delays showedt hat the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay,s uggesting that the positivec hargeo fnoncovalent( DMA) 2 C + is localized in as inglem onomer unit. From DFT calculationsf or (DMA) 2 C + ,o ur TR 3 spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-faceT -shaped (DMA) 2 C + ,t ermed DTC + ,a lthought he charge-delocalizeda symmetric p-stacked face-to-face (DMA) 2 C + ,t ermedD F3C + ,i st he most stable structure of (DMA) 2 C + according to the energetics from DFT calculations. The calculated potentiale nergy curves for the association between DMAC + and DMA showed that DTC + is likely to be efficientlyf ormed and contributes ignificantly to the TR 3 spectra as ar esult of the permanent charge-induced Coulombic interactions and ad ynamic equilibrium between chargel ocalized and delocalized structures.