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Figure 12.4 Calculated molecular structures of (a) unfolded and silver-bound folded confor-
mation of a fluorene- p -xylene-fluorene unit and (b) representative examples of fluorene-
p -xylene oligomers in their silver-bound conformation. Reprinted with permission from [ref 34]
Copyright 2007 American Chemical Society.
Most metal-binding ligands employ heteroatoms as the electron donors. Chebny and
Rathore reported fluorene- p -xylene oligomers ( 9 ) with up to 10 fluorene groups in the
main chain [34]. The fluorene- p -xylene-fluorene unit was found to form a receptor for
Ag þ with the p electrons as the donor (Figure 12.4a). The binding was surprising
strong ( K a ¼
15 000M 1 ) in a mixture of chloroform and methanol. The longer oligomers
underwent similar conformational changes upon the binding of multiple silver cations
(Figure 12.4b).
Instead of using transition metals, Ramakrishnan and coworkers synthesized polymers
( 10 ) whose folding was triggered by alkali metal ions [35,36]. The polymers contained
alternate aromatic donors and acceptors, a feature similar to the aeda mers ( a romatic e lec-
tron d onor- a cceptor oligomers) reported by Iverson [37-39]. These aromatic units pro-
vided not only solvophobic and aromatic interactions to stabilize the stacked structure but
also the spectroscopic handle in the UV-vis and NMR spectra to characterize the folded
conformation. The intramolecular charge transfer (CT) complex formed between the elec-
tron donor and acceptor was found to increase upon the addition of methanol to a solution
of the polymer in chloroform. Alkali metal ions also promoted CT complexation. The
effect followed the order of K þ >
Li þ , consistent with the ability of the metal ions
to complex with the hexa(ethylene oxide) tethers. The concept was also applied to fold-
able polymers responsive to ammonium salts [40-42].
Na þ >
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