The bis(triarylamine) systems 1−5 were synthesized and investigated by spectroscopic and electrochemical methods. They all have an aromatic five-membered ring system in common as a central part of their π-electron bridge. The absorption spectra are presented. All compounds undergo five oxidations whereupon only the first two are reversible under semi-infinite cyclic voltammetry conditions. The spectra of the radical cations and dications of 1−5 were collected upon stepwise titration with SbCl5. All monoradical cations exhibit rather intense absorption bands in the NIR region that are assigned to optically induced charge transfer between the amine redox centers or between the amine redox center and aromatic bridge. It is suggested that with CH2Cl2 as solvent the charge in 1 + and 2 + is localized mainly at the peripheral amine redox centers whereas 3 + and 4 + are symmetrically delocalized systems with the highest charge density at the bridge. Upon increasing the solvent polarity, solvent induced symmetry breaking occurs as previously reported for the anthracene derivative 7 + . Less clear is the situation in 5 + . The nonalternant azulene derivative 4 behaves entirely different with respect to its optical and electrochemical properties if compared with the alternant naphthalene compound 6. The 1,3-azulene bridging unit turns out to mediate a strong electronic coupling combined with a low-oxidation potential.

Upon electrochemical and spectroelectrochemical investigation the radical cation of tetraanisyl-ophenylenediamine turned out to be an almost delocalized system at the borderline between class II and III with an extremely low reorganization energy.

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.