Several hydrocarbon linked mesogenic dimers are known to exhibit an additional nematic phase (NTB) below a conventional uniaxial nematic phase (N). The correlation between molecular structure and the nematic - nematic phase transitions (N-NTB) is not yet well understood. Nematic-nematic transitions are in thermotropic liquid crystals associated in most cases with the formation of a columnar structure in the low temperature nematic phase or the onset of cluster formation or with folding processes of polymer chains, there is evidence too for the formation of twist-bend structures as well as the formation of domain assemblies. The synthesis of a range of such dimeric materials, symmetric as well non-symmetric systems with both positive and negative dielectric anisotropy, containing linear and bent structures in the aromatic core systems is described in this thesis. The characterisation of these systems by the use of differential scanning calorimetry (DSC), optical polarizing microscopy (OPM) and X-ray diffraction (XRD) studies is described in detail and the results for these systems are compared and structure properties relationships and the impact on the formation of nematic-nematic transitions are discussed.