Charge distribution in arylhydrazine-centered mixed valence compounds with smaller bridges (five to nine bonds...

Charge distribution in six aromatic-bridged, aryldialkylhydrazine-centered mixed valence radical cations is discussed through consideration of their optical spectra. The compounds considered have two 2-phenyl-2,3-diazabicyclo-[2.2.2]octane-3-yl (HyPh) charge-bearing units linked by a 1,4-phenylene bridge and its p-methoxyphenyl (HyAn) analogue, as well as the (HyPh)(2)-substituted 1,4-naphthalene, 2,6-naphthalene, 9,10-anthracene, and 4,4'-biphenyl compounds in methylene chloride and acetonitrile. Consideration of band shape and position leads us to assign the 1,4-phenylene- and 2,6-naphthalene-bridged compounds as charge-delocalized (class III) in both solvents, but the 1,4-naphthalene-bridged one lies closer to the borderline, and appears to be charge-localized (class II) in acetonitrile. The 4,4'-biphenyl-bridged compound is clearly class II in acetonitrile, and possibly also in methylene chloride. The lowest energy absorption band for the 9,10-anthracene-bridged compound is assigned as a bridge-to-HyPh band, and its charge distribution is not clear. Problems with the often-used relationship that the electronic coupling is half the transition energy for the lowest energy band of class III mixed valence compounds are discussed, as is interpretation of the vertical reorganization energy near the class II, class III borderline.