Freezing of crankshaft motion of trans-stilbene molecule in charge-transfer complexes, STB-TCNQ and STB-TCNQF

Heat capacities of crystalline charge-transfer (CT) complexes between trans-stilbene (STB) and tetracyanoquinodimethane (TCNQ) or (TCNQ)F-4 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-(TCNQ)F-4 around 240 K as in the case of highly disordered STBT-CNQ around 250 K. Assessment of the degree of CT by IR and structural methods indicates that STB(TCNQ)F-4 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, despite the large difference in acceptor ability.