Electronic Coupling in Tetraanisylarylenediamine Mixed-Valence Systems: The Interplay between Bridge Energy and Geometric Factors

We have investigated three organic mixed-valence systems that possess nearly identical inter-redox site distances and differ by the nature of the bridging units benzene, naphthalene, and anthracene: 
the N,N,N‘,N‘-tetra(4-methoxyphenyl)-1,4-phenylenene-diamine radical cation (1+), the 1,4-bis(N,N-di(4-methoxyphenyl)-amino)naphthalene radical cation (2+), and the 9,10-bis(N,N-di(4-methoxyphenyl)amino)anthracene radical cation (3+). The electronic interactions in these systems have been studied by means
of gas-phase ultraviolet photoelectron spectroscopy, vis/NIR spectroscopy, and electronic-structure
calculations. The experimental and theoretical results concur to indicate that the strength of electronic
interaction decreases in the following order of bridging units:  benzene > naphthalene > anthracene. This
finding contradicts the usual expectation that anthracene is superior to benzene as a driving force for
electronic communication. We explain these results in terms of a super-exchange mechanism and its strong
dependence on steric interactions.