Charge Transfer Pathways in Three Isomers of Naphthalene-Bridged Organic Mixed Valence Compounds

Naphthalene was substituted at different
positions with two identical
triarylamine moieties to result in species which are mixed-valence
compounds in their one-electron oxidized forms. They were investigated
by cyclic voltammetry, optical absorption, EPR spectroscopy, X-ray
crystallography, and DFT calculations. When the two redox-active triarylamine
moieties are connected to the 2- and 6-positions of the naphthalene
bridge, their electronic communication is significantly stronger than
when they are linked to the 1- and 5-positions, and this can be understood
on the basis of a simple through-bond charge transfer pathway model.
However, this model fails to explain why electronic communication
between triarylamine moieties in the 1,5- and 1,8-isomers is similarly
strong, indicating that through-space charge transfer pathways play
an important role in the latter. In particular, charge transfer in
the 1,8-isomer is likely to occur between the triarylamino C atoms
in α-position to the naphthalene linker because the respective
atoms are only about 3 Å apart from each other, and because they
carry significant spin density in the one-electron oxidized forms
of triarylamines. This particular through-space charge transfer pathway
might be generally important in molecular structures based on the
1,8-disubstituted naphthalene pillaring motif.