Water-Stable Zn(II) Coordination Polymers Regulated by Polysubstituted Benzenes and Their Photocatalytic Performance toward Methylene Blue Degradation Dominated by Ligand-Field Effects

Abstract

Photodegradation of organic dyes over coordination polymer (CP) photocatalysts has become one of the effective strategies for the prevention and control of water pollution, in which the energy band structures manipulated by the ligand field of the catalytically active metal play extremely vital roles in enhancing the photocatalytic activity. To better rationalize the ligand-field function on the band structure, a series of six 2,4,6-tris(4-pyridyl)-1,3,5-triazine (tpt)-based Zn(II)-CPs have been hydrothermally prepared by transforming the types, numbers, and locations of the coordinative substitutes attached on benzene. All of these Zn(II)-CPs have been fully structurally and photophysically characterized and used as photocatalysts for methylene blue (MB) decontamination by UV light illumination. Single-crystal structural analyses reveal that isomeric 1,2,4 (H3L1)- and 1,3,5-benzenetricarboxylates (H3L2) have assembled the ternary system into the gridlike framework {[Zn(tpt)(HL1)]·H2O}n (1), ribbon-to-connector aligned sheet [Zn3(H2O)(tpt)2(L1)2]n (2), and hexagonal dimer based double layer [Zn3(H2O)2(tpt)(L2)2]n (3). 1,2,4,5-Tetrasubstituted phenyl and diphenyl coligands have assisted in honeycomb-shaped, pillared-layer, and cuboid-like frameworks via synergetic binding of carboxylate, sulfonate and hydroxyl moieties: {[Zn2(tpt)(H2L3)(L3)0.5]·0.5H2O}n (4, H4L3 = 2,5-dihydroxyltelephthalic acid), {[Zn2.5(tpt)(μ-OH)(L4)]·1.5H2O}n (5, H4L4 = 5-sulfobenzene-1,2,4-tricarboxylic acid), and {[Zn3(H2O)0.5(tpt)2(μ-OH)(L5)1.25]·5H2O}n (6, H4L5 = 3,3′-disulfonyl-4,4′-biphenyldicarboxylic acid). More significantly, the changeable donor surroundings around the Zn(II) ions have optimized the ligand-field geometries, giving rise to variable band gaps and band potentials. Resulting from the effective isolation and migration of the charge carriers and much stronger oxidizing ability of the valence band, the cuboid-shaped framework 6 has exhibited the best photodegradation activity toward the MB pollutant, up to 89% after 4 h UV illumination. These observations have systematically revealed the consequence of the ligand field on the CP photocatalysts in organic dye photodecomposition.