FTIR spectroscopic determination of quadrupolar order parameters in smectic C phases

By means of FTIR spectroscopy the quadrupolar order parameter has been determined for the smectic C and C* phases of three mesogenic compounds where the angle psi specifies the rotation around the molecular long axis. The CO vibration and gamma-CH vibration of phenyl groups have been used as spectroscopic probes. The quadrupolar ordering is rather large and different for different parts of the molecules, indicating an internal flexibility. From the spectroscopic data it is deduced that the phenyl rings of the aromatic cores are oriented mainly within the tilt plane. Using the microscopic model of Zeks, the quadrupolar parts of the rotational potential have been found to be in the order of magnitude of the thermal energy, but differ for different parts of the molecules.