An Alternative Copolymer of Carbazole and Thieno[3,4b]-Pyrazine: Synthesis and Mercury Detection

Graphical Abstract

A donor-π-acceptor alternative copolymer of carbazole and a thieno[3,4b]-pyrazine unit [P(CZ-TPZ)] is synthesized by the Wittig–Horner reaction. P(CZ-TPZ) can be used as Hg²⁺ chemosensor by dual-channel assays of colorimetry and fluorometry with high selectivity and high sensitivity. This is the first report on the pyrazine-containing polymers that show an interesting Hg²⁺ sensing ability.

Abstract

A donor-π-acceptor (D-π-A) alternative copolymer of carbazole and thieno[3,4b]-pyrazine [P(CZ-TPZ)] is synthesized through a Wittig–Horner reaction. In dilute THF solution, the absorption spectrum of P(CZ-TPZ) shows two absorption peaks at 306 and 452 nm, respectively, and the PL spectrum of the polymer solution displays a PL peak maximum at 543 nm. The polymer possesses relatively high sensitivity and selectivity for Hg²⁺ detection. Upon addition of Hg²⁺ into its THF solution (containing 0.3% CH₃CN), P(CZ-TPZ) exhibits a new absorption peak at 560∼600 nm and its emission was quenched dramatically. The Hg²⁺ detection shows high selectivity in comparison with the other cations of Na⁺, K⁺, Mg²⁺, Ba²⁺, Al³⁺, Cu²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mn²⁺, and Co²⁺. The Hg²⁺ detection limit of the polymer solution by emission quenching is found to be 1 × 10⁻⁷ mol L⁻¹. P(CZ-TPZ) also shows a selective chromogenic behavior toward Hg²⁺ with color change of the solution from yellow to blue dark which can be detected with the naked eye, the detection limit reaches 1 × 10⁻⁶ mol L⁻¹ with a 1 × 10⁻⁴ mol L⁻¹ polymer solution. The absorption and PL spectral change can be resumed after adding thiourea, therefore the sensing ability of the polymer is re-usable with the treatment of thiourea. The results indicate that P(CZ-TPZ) is a promising chemosensor for the Hg²⁺ detection.