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

Abstract

The 1,3-bis[di(4-methoxyphenyl)amino]azulene 1 and 1,3-bis[di(4-methoxyphenyl)amino]azulene-2-carboxylic acid methyl ester 2 were synthesized and investigated by spectroscopic and electrochemical methods in order to estimate the electronic coupling between the attached amine redox centers. We found that the electronic coupling 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 electrochemical stability of azulene derivatives is drastically increased by substituents at the 2-position: whereas the oxidation of compound 1 is irreversible, the methylcarboxylate derivative 2 undergoes four reversible oxidation processes in CH2Cl2 under semi-infinite conditions. The different redox states of the corresponding radical cations are well separated. However, under finite diffusion conditions only the first three oxidation processes are reversible. The absorption spectra of the radical cations of 2 + , 2 2+ , and 2 3+ show intense absorption bands in the NIR region. The analysis of the optical spectra as well as DFT calculations indicate that in 2 + the charge is symmetrically distributed similar to the naphthalene isomer 3 + . Thus, the 1,3-azulene unit and its derivatives are useful bridging units due to their ability to mediate a strong electronic coupling similar to naphthalene but with a less positive redox potential.

Organische gemischtvalente Verbindungen: ein Spielplatz für Elektronen und Löcher

“…Weitere Studien konzentrierten sich auf die Art der Brücke in Bis(triarylamin)-MV-Verbindungen, bei welchen der N-N-Abstand konstant gehalten wurde: [122] Diese Ergebnisse gehen mit dem allgemein beobachteten Trend einher, dass in kationischen MV-Systemen mit der Erniedrigung des Brücken-HOMO auch die elektronische Kopplung V abnimmt. [112,194] [196] Alle oben diskutierten Beispiele zeigen, dass die Art der Brücke großen Einfluss auf das ET-Verhalten in organischen MV-Verbindungen hat. Deshalb müssen speziell die elektronischen Eigenschaften der Brücke und die sterischen Wechselwirkungen zwischen der Brücke und den Redoxzentren berücksichtigt werden.…”

Trends in Electron-Vibration and Electronic Interactions in Bis(dimethylamino) Mixed-Valence Systems: A Joint Experimental and Theoretical Investigation

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In such systems, the inherent ET properties and patterns of charge (de)localization strongly depend on the interplay and strengths of electronic and vibrational interactions [i.e., transfer integrals (electronic coupling) and intermolecular reorganization energies, respectively]. A characteristic feature of MV compounds is the presence of an absorption band in the visible or near-infrared (NIR).…”

Electronic Structure and Properties of Poly- and Oligoazulenes

“…Most of the oligomers and polymers which are used in OPVs today contain thiophene or phenylenevinylene subunits. − In order to cover most of the solar irradiation spectrum, semiconductor materials with an appropriately low band gap are highly sought for. In contrast to the transparent polythiophene or polyphenylenevinylene systems, the broad low-energy absorption band in azulene and azulene derivatives (HOMO−LUMO transition) , covers the range between 400 and 850 nm where the solar irradiation spectrum has its highest intensity. Unsubstituted azulene itself is a blue nonalternating hydrocarbon with a dipole moment of about 1 D that shows emission from the S 2 state. − The molar absorptivity of the low-energy absorption band of azulene is rather low (ε ≈ 700 M -1 cm -1 ).…”