Folding of Oligoviologens Induced by Radical–Radical Interactions

We
report the synthesis of a series of homologous oligoviologens
in which different numbers of 4,4′-bipyridinium (BIPY2+) subunits are linked by p-xylylene bridges, as
a prelude to investigating how their radical cationic forms self-assemble
both in solution and in the solid state. The strong radical–radical
interactions between the radical cationic forms of the BIPY2+ unitsnamely, BIPY•+in these oligoviologens
induce intra- or intermolecular folding of these homologues. UV/Vis/NIR
spectroscopic studies and DFT quantum mechanics indicate that the
folding of the shorter oligoviologens is dominated by intermolecular
radical–radical interactions. In addition to intermolecular
interactions, strong intramolecular radical–radical interactions,
which give rise to an NIR absorption band at 900 nm, tend to play
a crucial role in governing the folding of the longer oligoviologens.
The solid-state superstructure of the oligoviologen with three BIPY2+ units reveals that two intertwining chains fold together
to form a dimer, stabilized by intermolecular radical–radical
interactions. These dimers continue to stack in an infinite column
through intermolecular radical–radical interactions between
them. This research features an artificial biomimetic system which
sustains delicate secondary and tertiary structures, reminiscent of
those present in nucleic acids and proteins.