A crystalline molecular gyrotop with a biphenylene dirotor and its temperature-dependent birefringence

Biphenyl contains a direct bond between two phenyl functional groups, and the dihedral angle between the two phenyl groups has been discussed in terms of electronic and steric stabilization. Facile rotation about the C-C bond between the two phenyl components is expected to occur at high temperatures. Here, we report observations of the rotational dynamics of a biphenylene dirotor in a crystalline state in a novel molecular gyrotop. The structure and rotational dynamics of the rotor exhibit remarkable temperature dependence. In accordance with the amplification of the angular displacement of biphenylene inside a crystal with increasing temperature, the birefringence of the single crystal decreased.