Colorimetric Ion Sensors Based on Polystyrenes Bearing Side Chain Triazole and Donor-Acceptor Chromophores

Side chain clicked polystyrene derivatives formed by the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction showed colorimetric ion sensing behaviors when donor-acceptor chromophores, prepared by a [2+2] cylcoladdition-retroelectrocyclization between electron-rich alkynes and tetracyanoethylene (TCNE)/7,7,8,8-tetracyanoquinodimethane (TCNQ), were attached to the triazole rings. The metal ion sensing behaviors could be explained according to the theory of hard and soft acids and bases (HSAB). Hard acidic metal ions were mainly recognized by the hard basic anilino-nitrogen moieties, resulting in a decrease in the charge-transfer (CT) bands. In contrast, soft acidic metal ions led to a bathochromic shift in the CT bands due to the selective interactions with the soft basic cyano-nitrogen atoms. With the triazole spacers, more soft (and/or borderline) metal ions were recognized by the donor-acceptor chromophores probably due to more space for the various sized metal ions. The chemodocimetric anion sensing behaviors of the clicked polystyrenes were almost the same as those of the counter polystyrenes without the triazole spacers. Overall, the triazoles in this study do not serve as colorimetric sensor units towards both metal ions and anions, but they are effective spacers between the polymer main chain and ion sensing donor-acceptor side chain chromophores.