Urea-based polyacetylenes as an optical sensor for fluoride ions

 Novel acetylenes carrying urea groups, 1-(4-ethynylphenyl)-3-(4-nitrophenyl) urea (1), 1-(4-propargyl)-3-(4-nitrophenyl) urea (2), were synthesized and polymerized with rhodium catalyst. Polymers [poly(1) and poly(2)] with moderate molecular weights were obtained in good yields. The anion sensing ability of poly(1) and poly(2) was estimated using the tetra-n-butylammonium (TBA) salts of a series of anions in DMF. Upon the addition of F^-, the color of the DMF solution of poly(1) and poly(2) immediately turned to a different color, while the color of solution changed slightly upon addition of Cl^-, HSO₄^-, Br^-, and NO₃^-, indicating the F^- sensing ability of poly(1) and poly(2). The ¹H-NMR titrations of poly(1) revealed that the colorimetric response of poly(1) was triggered by the urea/F^- interaction through the hydrogen bonding and/or deprotonation process. The absorption spectra titration and Hill plot analysis were carried out to measure the F^- binding ability, and the Hill coefficient in the poly(1)/F^- complexation was found to be 5.8. This result clearly indicated that this binding mode between poly(1) and F^- was based on a positive homotropic allosterism.