Charge resonance excitations in 1,3-bis[di(4-methoxyphenyl)aminolazulene radical cations

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 methylearboxylate 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.