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The 1,3-bisdi(4-methoxyphenyl)aminoazulene 1and 1,3-bisdi(4-methoxyphenyl)aminoazulene-2-carboxylic acid methyl ester 2were 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 Vwhich is mediated by a nonalternant 1,3-azulene bridge in the radical cation 2(V3900 cm-1) is almost the same as in the isomeric alternant 1,4-naphthalene system 3(V4000 cm-1). The electrochemical stability of azulene derivatives is drastically increased by substituents at the 2-position:  whereas the oxidation of compound 1is irreversible, the methylcarboxylate derivative 2undergoes four reversible oxidation processes in CH2Cl2under 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, 22, and 23show intense absorption bands in the NIR region. The analysis of the optical spectra as well as DFT calculations indicate that in 2the 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.

An organic mixed valence compound with a spacer length of 25 unsaturated bonds separating two amine redox centres was synthesised and the electron transfer behaviour was investigated in the context of a Mulliken–Hush analysis in order to estimate the longest redox centre separation for which an intervalence charge transfer band can be observed.