Synthesis, crystal structure and charge transport properties of one-electron oxidized zirconium diphthalocyanine, [ZrPc2]IBr2

Crystals of the new one-electron oxidized zirconium diphthalocyanine, [ZrPc2]IBr2 (Pc = C32H16N8) were grown directly from phthalonitrile and pure zirconium powder under the oxidation conditions of IBr at 220 °C. [ZrPc2]IBr2 crystallises in the space group P21/m of the monoclinic system, with lattice parameters of: a = 6.697(1), b = 25.483(5), c = 16.918(3) Å, β = 100.03(3)° and Z = 2. The crystals of [ZrPc2]IBr2 are built up from one-electron oxidized [ZrPc2]+ units that form stacks parallel to a axis of the crystal and mixed electron-rich trihalide ions of IBr2-. The iodine atom of the IBr2 ions is located at the inversion center, so the IBr2- ions are linear and symmetrical. The mutual arrangement of [ZrPc2]+ and IBr2- ions is different to that found in the one-electron oxidized [ZrPc2]I3 · I2 complex as well as to that found in the tetragonal crystals of partially oxidized [ZrPc2](I3)2/3. EPR experiment shows that the oxidation of the diamagnetic ZrPc2 complex by IBr is ligand centered. The UV–Vis spectrum of [ZrPc2]IBr2 shows, besides the bands observed in the spectrum of ZrPc2, one additional band at ∼496 nm, which indicates the existence of the one-electron oxidized phthalocyaninato(1−) π-radical ligand and it is assigned to the electronic transition from a deeper level to the half-occupied HOMO level. The single-crystal electrical conductivity data shows anisotropy and non-metallic character in conductivity (dσ/dT > 0). The charge transport mainly proceeds along the stacks of one-electron oxidized [ZrPc2]+ units. The relatively high conductivity along the [ZrPc2]+ stack results from the staggered configuration of the Pc-rings (rotation angle 45.0(3)°) that makes short inter-ring Cα(pyrrole)–Cα(pyrrole) contacts (2.858(2)–3.017(3) Å) and greatest overlap of the HOMO orbitals that form the conduction band of the molecular materials.

Graphical abstractCrystals of [ZrPc2]IBr2 were obtained by cyclic tetramerization of phthalonitrile in the presence of powdered zirconium under oxidative conditions of IBr. Single crystal X-ray analysis supported by Raman and EPR spectroscopies shows a one-dimensional arrangement of one-electron oxidized [ZrPc2]+ units. Single-crystal conductivity experiments show its non-metallic character (dσ/dT > 0) and anisotropy.Figure optionsDownload full-size imageDownload as PowerPoint slide