From an analysis of the infrared dichroic ratio of several bands for triphenylene and truxene derivatives, a major influence of the substrate’s structure on the bulk alignment of discotic liquid crystals has been found. Two different substrates : Si (amorphous) and ZnSe (polycrystalline) are used for these studies. The method of the sample being tilted out of the normal incident plane of IR beam has been used for investigating a 3-dimensional arrangement of the various segments of molecules in their discotic phases.