Influence of substituents in aryl groups on the structure, thermal transitions and electrorheological properties of zinc bis(diarylphosphate) hybrid polymers

Abstract

The structures and thermal properties of three new hybrid one-dimensional (1D) polymers based on zinc bis(diarylphosphate)s containing p-substituted phenyl rings are reported. The crystal structures of Zn[O₂P(p-OC₆H₄NO₂)₂]₂ (1), Zn[O₂P(p-OC₆H₄OMe)₂]₂ (2) and Zn[O₂P(p-OC₆H₄CO₂Et)₂]₂ (3) differ from that of their unsubstituted analogue, Zn[O₂P(OPh)₂]₂ (ZnDPhP). Compounds 1 and 3 consist of tetrahedrally coordinated zinc cations connected by double bridges of phosphate groups (2+2 bridging mode) and form polymeric chains that are packed in a distorted hexagonal lattice with six closest neighbours. In compound 2 zinc cations are linked by alternating single and triple phosphate bridges (3+1 bridging mode) and the resulting chains, having only four closest neighbors, are packed in a distorted tetragonal manner. DFT computations revealed that the 2+2 bridging mode, even at the highest energy conformation, is more stable than the 3+1 one. Simultaneous Thermal Analysis, Raman spectroscopy and powder XRD (PXRD) studies show that pyrolysis of the studied hybrid polymers begins above 260 °C, leading to a mixture of zinc condensed phosphates and carbonaceous deposits that may have electron-conducting properties. DSC and PXRD studies provide evidence that crystalline domains in 2 and 3 rearrange and/or disappear at a much lower temperature (ca. 150 °C) leading to an isotropic liquid (in the case of 3) or an amorphous solid material (in the case of 2). Electrorheological measurements indicate that 1–3 are polarized in an external electric field, and the type of electrorheological effect depends on the type of functional group attached to the phenyl ring; this feature can be utilized in designing new electrorheological devices.