Heat capacities of β-modification phthalocyanines from 4 to 45 K. Anomalous behaviour in copper(II) and nickel(II) phthalocyanine

Abstract

Heat capacities from ca. 4 to ca. 45 K are reported for β-modification metal-free, copper(II) and nickel(II) phthalocyanine (β-H₂Pc, β-CuPc and β-NiPc where Pc = C₃₂H₁₆N₈).

β-CuPc undergoes two essentially first-order transitions implying the existence of three distinct phases, phase III stable below ca. 9 K, phase II stable from ca. 9 to ca. 12 K and phase I stable above ca. 12 K. Moreover, the sum of the two entropies of transition (for III → II and II → I) is so large (ca. R ln 4, possibly more) that it is very likely that phase I is disordered in some way. To explain the value R ln 4, we tentatively propose (i) that the disorder in phase I is positional disorder of the Cu²⁺ ions, to each of which there are presumed to be available four nearly equivalent potential minima in the central area of the Pc^2– ligand, and (ii) that in phase III the Cu²⁺ ions are ordered under the influence of their mutual interaction.

At temperatures between 4 and 8 K, the molar heat capacity of β-NiPc considerably exceeds those of β-H₂Pc and β-CuPc, and the latter heat capacities, unlike the former, are proportional to T³. This suggests that the heat capacity of β-NiPc below 8 K differs from those of β-H₂Pc and β-CuPc in including a substantial non-vibrational contribution.