Efficient Colorimetric Anion Detection Based on Positive Allosteric System of Urea-Functionalized Poly(phenylacetylene) Receptor

Poly(phenylacetylene) with rationally designed urea groups was demonstrated to be a superior anion receptor possessing both a colorimetric response ability and a positive homotropic allosteric binding mode. The target polymer, poly(phenylacetylene) with [bis(trifluoromethyl)phenyl]urea pendants (poly- 1 ), was synthesized by the stereoselective polymerization with Rh(nbd)BPh 4 (nbd = norbornadiene) in high yield. The anion sensing ability of poly- 1 was evaluated using the tetra- n -butylammonium (TBA) salts of a series of anions in THF. Upon the addition of anions such as CH 3 CO 2 −, C 6 H 5 CO 2 −, F −, Cl −, Br −, NO 3 −, N 3 −, and HSO 4 −, the yellow color of the THF solution of poly- 1 immediately turned to a different color depending on the types of anions, indicating the anion recognition capability of poly- 1 . The 1 H NMR titrations of poly- 1 by using increasing amounts of CH 3 CO 2 − revealed that the colorimetric response of poly- 1 was considered to be the direct consequence of the hydrogen-bonding complex formation between the urea functionality and the anions. Interestingly, during the course of the determination of the anion-binding affinity, we encountered highly cooperative binding ability of poly- 1 . The Hill plot analysis provided an exceptional positive cooperativity, e.g., 8.4 of Hill coefficient in the complexation between poly- 1 and C 6 H 5 CO 2 − . This result clearly indicated that this binding mode was based on a positive homotropic allosterism, in which the partially formed urea/anion complex units in the polymer chain should produce a change in the whole main chain conformation, facilitating further anion binding. Additionally, by comparing the anion binding of poly- 1 with that of previously prepared urea-functionalized poly(phenylacetylene) (poly- 2 ), an enhanced anion binding ability based on two electron-withdrawing −CF 3 groups was found to be essential to realize such cooperativity.