Colorimetric sensing of cations and anions by clicked polystyrenes bearing side chain donor–acceptor chromophores

Abstract

Donor–acceptor chromophores were introduced into the side chains of a polystyrene derivative through the efficient click postfunctional addition reaction between dialkylaniline-activated alkynes and small acceptors, such as tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The resulting polystyrenes were fully characterized by GPC, 1H NMR, IR spectroscopy, and elemental analysis. After the postfunctionalization, the polymers were highly colored due to the formed donor–acceptor chromophores in the side chains and these chromophores showed a colorimetric sensing behavior for some metal ions and anions. Some hard-to-borderline metal ions were always recognized by the dialkylanilino nitrogen atom, resulting in a decrease in the charge-transfer (CT) band intensity of the donor–acceptor chromophores. However, the recognition site of the soft Ag+ ion was the cyano nitrogen atoms due to readily formed multivalent coordination, leading to a bathochromic shift in the CT bands for both TCNE- and TCNQ-adducted polymers. This remarkable dual colorimetric detection of two different types of metal ions was achieved by the elegant design of polymeric chemosensors. On the other hand, it was found that the dicyanoethene moieties of the side chain chromophores serve as a chemodosimeter for some anions. For example, the CN− ion underwent a nucleophilic addition reaction to the electron-deficient ethene moieties, resulting in a dramatic decrease in the CT bands. All types of sensing behavior of both cations and anions were visually recognized. The use of small molecular model compounds enabled the estimation of the kinetic parameters and the product structures.