Freezing of Crankshaft Motion of trans-Stilbene Molecule in Charge Transfer Complexes, STB-TCNQ and STB-TCNQF4

Kazuya SAITO, Mizuho Okada, Hiroki AKUTSU, Akane SATO and Michio SORAI

Heat capacities of crystalline charge-transfer (CT) complexes between trans stilbene (STB) and TCNQ or TCNQF4 were precisely measured below room temperature by adiabatic calorimetry.  Although orientational disorder is seemingly absent, a glass transition due to freezing of the crankshaft motion is observed in STB-TCNQF4 around 240 K as in the case of highly disordered STB TCNQ around 250 K.  Assessment of the degree of CT by IR and structural methods indicates that STB-TCNQF4 is not a fully but weakly (only partially) ionic complex with a similar degree of CT of 0.1 - 0.2 to that in STB-TCNQ, in spite of the large difference in acceptor ability.
(J. Phys. Chem. B, 108, 1314-1320 (2004))

Can the Molecule Involving a CT Interaction Reorient Itself in the Crystal Lattice? Phase Transition and Glass Transition in trans-Stilbene-TCNQ

Kazuya SAITO, Mizuho OKADA, Hiroki AKUTSU and Michio SORAI

Heat capacity of the title compound was measured below room temperature by adiabatic calorimetry. A structural phase transition and a glass transition, both probably relating to the intramolecular crankshaft motion, were detected at 273 K and 250 K, respectively. The degree of charge transfer (CT) was determined from IR spectroscopy as ca. 0.2 e-. Since the central C=C moiety of the stilbene molecule has a significant density in the HOMO, the observation of the phase and glass transitions means that the part involved in the CT interaction reorients itself in the crystal lattice above the glass transition temperature.
(Chem. Phys. Lett., 318, 75-78 (2000))

Glass Transition Due to Freezing of Intramolecular Motion: Crystalline trans-Azobenzene and trans-Stilbene

Kazuya SAITO, Yasuhisa YAMAMURA, Koichi KIKUCHI and Isao IKEMOTO

The heat capacities of crystalline trans-azobenzene and trans-stilbene were measured by adiabatic calorimetry. No anomalies were detected in both compounds at any temperature regions where possible phase transitions have been previously suggested. Anomalies due to glass transitions were observed at about 170 and 110 K for trans-stilbene and trans-azobenzene, respectively. The magnitude of the heat capacity jump at the transition is analyzed and the transitions are attributed to the freezing of the intramolecular crankshaft motion. Thermodynamic functions are tabulated for trans-azobenzene.
(J. Phys. Chem. Solids, 56(6), 849-857 (1995))

See also Glass Transition in Organic Conductor

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