Comparing the Structural Aspects of the Second-order Transitions in Three π-Molecular Compounds – Similarities and Differences

Abstract

Although there are good reasons to believe that second-order phase transitions occur widely among crystalline mixed-stack π-molecular compounds, only three examples (pyrene-pyromellitic dianhydride (PYRPMA; T _c = 160 K), naphthalene-1,2–4,5-tetracyanobenzene (NAPTCB; T, = 73 K), and anthracene-1,2–4,5-tetracyanobenxene (ANTCYB; T _c = 213 K)) have been studied intensively. In PYRPMA the space group changes from P2₁/a to P2₁/n, with doubling of one cell edge, while in the other two examples the space group changes from C2/m to P2₁/a, without change of cell dimensions. For all three systems there are experimental measurements (to different degrees of completeness) of C _p , cell dimensions and intensities of superlative reflections, and of NMR and ESR spectra as functions of temperature. The crystal structures of ordered and disordered phases have been determined. These results have been analysed in terms of current descriptions of phase transitions (Ehrenfest order of transition, Landau theory, determination of critical exponents) by various authors and the results are critically compared here at the phenomenological and molecular levels. All indications are that these three phase transitions, although showing many differences of detail, resemble one another in the sense that subtle intermolecular packing interactions are the driving force for the transitions rather than electronic interactions.

Key Words:

• Second order phase transitions
• π-molecular compounds
• pyrene-pyromellitic dianhydride
• naphthalene-1,2–4,5-tetracyanobenzene
• anthracene-1,2–4,5-tetracyanobenzene
• Landau theory