-5-Sulfono-7-(4-x phenyl azo)-8-hydroxy quinoline (SAHQ)-x ligands, x - NO2 or CH3 or Cl have polycrystalline structure in as synthesized condition. Thermally evaporated thin films of SAHQ-x have crystal structure depending on the substitution group; SAHQ-NO2 and SAHQ-Cl films have nano-crystalline structure with different degree of crystallinity and SAHQ-CH3 film has amorphous structure. The changes in optical functions and therefore optical constants with substitution group variation have been calculated by using spectrophotometer measurements of the transmittance and reflectance at nearly normal incidence of light in the wavelength range 200-2500 nm. Substitution group variation influences the refractive index, dispersion parameters, optical functions and bond length of SAHQ. It has no influence on mobility, relaxation time and plasma frequency of charge carriers. The obtained optical energy gaps for SAHQ-x ligands, x - NO2 or CH3 or Cl are 1.89, 2 and 2eV, respectively. (C) 2013 Elsevier B. V. All rights reserved.