Electron Delocalization in Reduced Forms of 2-(BMes2)pyrene and 2,7-Bis(BMes2)pyrene
Lei JiRobert M. EdkinsAndreas LorbachIvo KrummenacherCharlotte BrücknerAntonius EichhornHolger BraunschweigBernd EngelsPaul J. LowTodd B. Marder
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Abstract:
Reduction of 2-(BMes2)pyrene (B1) and 2,7-bis(BMes2)pyrene (B2) gives rise to anions with extensive delocalization over the pyrenylene bridge and between the boron centers at the 2- and 2,7-positions, the typically unconjugated sites in the pyrene framework. One-electron reduction of B2 gives a radical anion with a centrosymmetric semiquinoidal structure, while two-electron reduction produces a quinoidal singlet dianion with biradicaloid character and a relatively large S0–T1 gap. These results have been confirmed by cyclic voltammetry, X-ray crystallography, DFT/CASSCF calculations, NMR, EPR, and UV–vis–NIR spectroscopy.Keywords:
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Abstract A model is worked out for the interpretation of the magnetic properties of polyoxovanadium clusters, containing both localized and delocalized vanadium(IV). The role of exchange and delocalization is worked out parametrically for two model systems containing four localized and two and four delocalized vanadium(IV) over eight sites respectively. The effect of vibronic coupling is also taken into account. Additional informationNotes on contributorsBoris S. TsukerblatOn leave of absence from Institute of Chemistry, Moldavian Academy of Sciences, Kishinev, Moldavia.
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Abstract Synthetic details are presented for a series of more or less rigid, “parallel‐in‐plane” preoriented bis(diazenes), with N=N/N=N distances ( d ) of 3.3−2.9 Å and interorbital angles (ψ) of 142−164° (X‐ray crystal structures). DFT calculations (B3LYP/6−31G*) and one‐/two‐electron reduction experiments with the two least preoriented, most “distant” bis(diazenes) ( d N=N/N=N ca. 3.3 Å; ψ 142−146°) provide more insight into the structural prerequisites for bis(homoconjugative) in‐plane electron delocalization in 4N/5e radical anions and 4N/6e dianions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
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Abstract An efficiency of delocalization in strongly conjugated systems remains an important factor crucial for modulation of the optical properties directly correlated with its range. An ortho‐substituted phenylene derivative bearing electron donating/accepting functionality was built‐in a fully unsaturated macrocyclic system with a global delocalization of a diatropic and/or a paratropic current. A precisely located structural modification influence observed behaviour in spectroscopic parameters that are only slightly recognizable in 4n+2 systems but showing a significant influence on the reduced derivatives with a contribution of 4n π‐electrons delocalization path.
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The σ delocalization plays an important role in the stability of small boron clusters; therefore, it is important to establish under which circumstances this delocalization contributes to the aromaticity of these molecules. In this work, using electron localization function (ELF) calculations, we show that σ and π electrons follow different patterns of delocalization. For σ electrons the delocalization is mainly due to the pσ radial overlapping which decreases with ring size, thus, considerable delocalization is expected for small rings, while for the π subsystem, the Hückel rule of organic chemistry works successfully regardless of the cluster size.
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