S,C,C- and O,C,C-Bridged Triarylamines and Their Persistent Radical Cations

This work reports the synthesis,
crystal structures, and electronic
properties of structurally constrained S,C,C- and O,C,C-bridged triarylamine
derivatives and their persistent radical cations. O,C,C-Bridged triphenylamines
and a dinaphthylphenylamine were obtained through a straightforward
synthetic protocol. Similar to a previously reported S,C,C-bridged
triphenylamine, the O,C,C-bridged triarylamines were easily oxidized
to afford the corresponding radical cations, which were obtained as
hexachloroantimonate salts. X-ray crystallographic analyses showed
almost planar structures for these O,C,C-bridged triarylamine radical
cations, which represent new members of the family of planar triarylamine
radical cations without substituents on the aryl rings. Detailed investigations
of the electronic properties of the S,C,C- and O,C,C-bridged triarylamine
radical cations demonstrated that the spin and positive charge are
sufficiently delocalized over the planar triarylamine scaffolds. The
results provide the following insights into the effects of the bridging
unit (sulfur vs oxygen) and the dibenzo-annulation on the spin delocalization
in the bridged triarylamine radical cations: (1) An effective decrease
of the spin density on the nitrogen atom is observed for the sulfur
bridge relative to the oxygen bridge; and (2) a moderate decrease
of the spin density on the oxygen atom rather than the nitrogen atom
is induced by the dibenzo-annulation.