Heat capacities of crystalline charge-transfer (CT) complexes between
trans stilbene (STB) and TCNQ or TCNQF4 were precisely
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))
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))
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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