Wavelike zinc coordination polymers: anion-modulated ligand conformation and fluorescence detection of 4-aminobenzylamine

Abstract

Through a room temperature reactant diffusion strategy, two coordination polymers [Zn(bpycz)Cl₂]_n (1) and [Zn(bpycz)Br₂]_n (2) were obtained from 3,6-bis(pyridin-3-yl)-9H-carbazole (bpycz) and the corresponding zinc halides. Single-crystal structural analysis revealed that the Zn(II) center suited a tetrahedral geometry in both 1 and 2, whereas the bpycz ligand adopted an anti,anti conformation in 1 but a syn,syn conformation in 2. As a result, coordination polymer 1 exhibited a wavelike chain structure with an arch bridge manner, whereas 2 also revealed a wavelike chain structure but with a sine wave manner. The differences in the ligand coordination mode and chain structure were most likely due to the influence of halide anions (Cl⁻ in 1 and Br⁻ in 2) with different sizes. X-ray powder diffraction (XRPD) proved that 1 and 2 were highly stable in H₂O and various organic solvents, as well as over large pH ranges of 2–13 and 3–11, respectively, for 24 h. Thermogravimetric (TG) analysis confirmed that 1 and 2 have high thermal stability approaching 420 °C and 425 °C, respectively. Upon excitation, 1 and 2 emitted fluorescence at 420 and 412 nm, respectively, in the solid state and at 431 and 433 nm, respectively, in the suspension phase in water. The fluorescence of 2 dispersed in water could be obviously quenched by 4-aminobenzylamine (4-ABA) with a high quenching efficiency, excellent selectivity, and good sensitivity as well as a low limit of detection (LOD, 2.52 μM⁻¹), indicating its potential as a fluorescence turn-off sensor material. Possible sensing mechanisms were explored, suggesting that the photoinduced electron-transfer process might be responsible for the quenching response.