Donor- and/or Acceptor-Substituted Expanded Radialenes: Theory, Synthesis, and Properties

The synthesis of donor- (D) and/or
acceptor (A)-expanded [4]­radialenes
has been developed on the basis of readily available dibromoolefin
(7), tetraethynylethene (10 and 20), and vinyl triflate (12) building blocks. The successful
formation of D/A radialenes relies especially on (1) effective use
of a series alkynyl protecting groups, (2) Sonogashira cross-coupling
reactions, and (3) the development of ring closing reactions to form
the desired macrocyclic products. The expanded [4]­radialene products
have been investigated by spectroscopic (UV–vis absorption
and emission) and quantum chemical computational methods (density
functional theory and time dependent DFT). The combined use of theory
and experiment provides a basis to evaluate the extent of D/A interactions
via the cross-conjugated radialene framework as well as an interpretation
of the origin of D/A interactions at an orbital level.