X-ray Powder Diffraction Structure Reinvestigation of the α and β Forms of Cobalt Phthalocyanine and Kinetics of the α → β Phase Transition

Abstract

The structures of the acid-paste-prepared α-form of cobalt phthalocyanine (α-PcCo) and of its corresponding β-form (β-PcCo) have been refined through X-ray powder diffraction using the Rietveld method. The α-polymorph is triclinic, space group P1̄, cell parameters a = 12.090(8) Å, b = 3.754(2) Å, c = 12.800(9) Å, α = 88.96(6)°, β = 90.97(6)°, γ = 95.09(7)°, and Z = 1; the β-form is monoclinic, space group P21/a, cell parameters a = 14.5982(9) Å, b = 4.7937(3) Å, c = 19.4348(11) Å, β = 120.782(3)°, and Z = 2. α-PcCo consists of columnarly arranged molecules, those of adjacent columns being aligned parallel to each other, whereas a nearly perpendicular arrangement is present in β-PcCo. α-PcCo also differs from the α-forms of PcH2 and PcPt in which an angle of ca. 125° is observed between molecules of closely contacting chains. From the refined peak-profile parameters of the powder pattern a crystallite size of ∼150 Å has been estimated for α-PcCo, a value approaching that of nanocompounds. The isothermal α → β phase transition has been followed in-situ real-time by means of EDXD at two different temperatures, 152 and 250 °C. Calorimetric data indicate two different behaviors of the rate constant in the temperature ranges 182.5−220 and 220−270 °C leading to values of the empirical activation energy, Ea, of 36.6(16) and 14.8(9) kcal/mol, respectively. However the various DSC runs lead to values of the phenomenological n parameter in the range 2.34−2.68 (indicating an isokinetic two-dimensional growth). According to the combined EDXD and DSC data a three-step model for the α → β PcCo phase transition may be proposed: (1) disordering of adjacent layers of PcCo molecules from their original α-type arrangement; (2) crystallization of the β-form from the disordered phase through rearrangement of the layers of the phthalocyanine units; (3) crystal growth of the β-phase from the nuclei to an average particle size of ∼ 2500 Å, as indicated by the fitted peak-profile parameters. No evidence of the occurrence of intermediate ordered phases was observed.