Comparison of alternant and nonalternant aromatic bridge systems with respect to their ET-properties

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 pi-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 CH2O2 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. (Less)