Order-disorder transition in anthracene/tetracyanobenzene probed by muonium-substituted radicals

ALC-mu SR measurements on polycrystalline anthracene/tetracyanobenzene (A/T CNB) from 25 K to above room temperature reveal Delta M = 1 transitions cor responding to several adducts of anthracene; the strongest of these show te mperature-dependent behaviour related to orientational order and split at t he phase transition temperature. The complexity of the lineshapes and the e xistence of the splitting indicate that intermolecular effects are importan t in determining the isotropic and anisotropic components of A mu in solid state systems of this kind, Parallel orientation-dependent TF mu SR investi gations on a single-crystal sample confirm that the main ALC signals observ ed correspond to separate radicals and reveal a room-temperature hyperfine tensor in which the molecular-frame anisotropy is partially averaged out. A b initio calculations using Hartree-Fock and density functional methods are used to assign the radical signals. (C) 2000 Elsevier Science B.V. All rig hts reserved.