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 (BIPY^(2+) 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 BIPY^(2+) 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 BIPY^(2+) 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.
© 2014 American Chemical Society. Received: October 29, 2014; Published: December 10, 2014. This research is part (Project 34-945) of the Joint Center of Excellence in Integrated Nano-Systems (JCIN) at King Abdul-Aziz City for Science and Technology (KACST) and Northwestern University (NU). The authors thank both KACST and NU for their continued support of this research. The computational studies (W.-G.L., W.A.G.) were supported by NSF (EFRI-ODISSEI 1332411)
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