Charge Resonance Excitations in 1,3-Bis[di(4-methoxyphenyl)amino]azulene Radical Cations

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• 作者：Gilbert N&ouml ; ll ; Stephan Amthor ; Manuele Avola ; Christoph Lambert ; J&ouml ; rg Daub
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：March 1, 2007
• 年：2007
• 卷：111
• 期：8
• 页码：3512 - 3516
• 全文大小：94K
• 年卷期：v.111,no.8(March 1, 2007)
• ISSN：1932-7455

文摘

The 1,3-bis[di(4-methoxyphenyl)amino]azulene 1 and 1,3-bis[di(4-methoxyphenyl)amino]azulene-2-carboxylicacid methyl ester 2 were synthesized and investigated by spectroscopic and electrochemical methods in orderto estimate the electronic coupling between the attached amine redox centers. We found that the electroniccoupling V which is mediated by a nonalternant 1,3-azulene bridge in the radical cation 2⁺ (V = 3900 cm^-1)is almost the same as in the isomeric alternant 1,4-naphthalene system 3⁺ (V = 4000 cm^-1). The electrochemicalstability of azulene derivatives is drastically increased by substituents at the 2-position: whereas the oxidationof compound 1 is irreversible, the methylcarboxylate derivative 2 undergoes four reversible oxidation processesin CH₂Cl₂ under semi-infinite conditions. The different redox states of the corresponding radical cations arewell separated. However, under finite diffusion conditions only the first three oxidation processes are reversible.The absorption spectra of the radical cations of 2⁺, 2²⁺, and 2³⁺ show intense absorption bands in the NIRregion. The analysis of the optical spectra as well as DFT calculations indicate that in 2⁺ the charge issymmetrically distributed similar to the naphthalene isomer 3⁺. Thus, the 1,3-azulene unit and its derivativesare useful bridging units due to their ability to mediate a strong electronic coupling similar to naphthalenebut with a less positive redox potential.