Two difunctional Co(II) coordination polymers for natural sunlight photocatalysis of methylene blue and selective fluorescence sensing of Cr(VI) ion in water media

Highlights: • Two new Co(II)-based CPs were synthesized and characterized. • 1–2 could serve as energy-saving photocatalysts for degrading MB under natural sunlight irradiation. • 1 exhibited highly sensitive and selective sensing for Cr(VI) anion. • The fluorescence quenching mechanisms were discussed. -- Abstract: Two new difunctional Co(II) coordination polymers (CPs), namely, [Co(TBTA)(L)_1.5]_n (1) and {[Co(NPHT)(L)]·H₂O}_n (2) were hydrothermally synthesized from 4,4′-bis(imidazole-1-yl)-biphenyl and different aromatic carboxylic acid ligands (H₂TBTA = tetrabromoterephthalic acid, and H₂NPHT = 3-nitrophthalic acid). Two CPs feature 2D (3,6)-connected hcb, and (4,4)-connected sql topologies for 1, and 2, respectively. 1 further extends into a three dimensional supramolecular architecture through π···π stacking interactions. Both of CPs exhibit high photocatalytic activities for degradation of methylene blue (MB) with remarkable efficiencies (85.83% for 1, and 93.76% for 2) under natural sunlight irradiation. 1 can sensitively and selectively detect Cr(VI) anions in water media with the Stern-Volmer quenching constant (K_sv) of 4.79 × 10⁴ M⁻¹ for Cr₂O₇²⁻ (pH = 3) or 2.36 × 10⁴ M⁻¹ for CrO₄²⁻ (pH = 12). The linear detection ranges are 0–350 μM, and 0–200 μM for Cr₂O₇²⁻, and for CrO₄²⁻, severally.