Reversible molecular rearrangement of nickel phthalocyanine between α- and x-forms in a crystalline solid by chemical treatment

Abstract

We investigated the molecular arrangement of nickel phthalocyanine (Ni(Pc)) in a solid obtained by removing iodine atoms from the charge-transfer complex crystal of x-Ni(Pc)I, in which the π-electron system is partially oxidized. While thermal treatment of x-Ni(Pc)I yielded β-Ni(Pc), chemical reduction of x-Ni(Pc)I produced α-Ni(Pc) while retaining its morphology. Single-crystal structure analysis was performed on the obtained α-Ni(Pc), revealing that Ni(Pc) molecules form a parallel columnar structure along the a-axis in a slipped-stack manner, with an interplanar distance of 3.43 Å and a tilt angle of 65.0°. These features are consistent with the proposed crystal structure of α-Cu(Pc) based on powder X-ray diffraction measurements. Furthermore, x-Ni(Pc)I could be reproduced by exposing α-Ni(Pc) to I₂ vapor, indicating that the molecular arrangement of Ni(Pc) transforms from the α-form to the x-form during penetration of iodine atoms and oxidation by iodine. Similar parallel columnar structures in the α- and x-forms likely enable the chemically reversible transformation of the molecular arrangement of Ni(Pc) in the solid state.