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

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 STB-TCNQ 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.