Most of the chemical systems are photoluminescence. These systems can be excited by electromagnetic radiation and subsequent radiations with different wavelengths are obtained. Two of the most popular aspects of photoluminescence are fluorescence and phosphorescence. The fluorescence spectra can be used for measuring the trace amounts of many inorganic and organic species. In addition, the fluorescence spectroscopy can be used in a wide range of issues in applied science, chemistry and biology. For example, fluorescence measurements can also provide a wide range of information about the molecular processes such as the interaction of the solvent molecules with fluorophore, the distance among the sites in the biological systems. Salicylic acid is widely used as relieve pain, inflammation. Recently, salicylic acid and its derivatives are employed as intermediates in the production of agricultural chemicals, paint and paint products. These compounds have recently become more attractive to researchers because they are particularly important in the pharmaceutical and chemical structure of enzyme. In this thesis, the fluorescence emission of some azo dyes pairing 2,4-dihydroxy benzoic acid and salicylic acid have been calculated. For the calculations, the time-dependent density functional theory (TD-DFT) was employed. Two functional including B3LYP and M062X employing 6-311++G(d,p) were used for calculations. The excited electronic states with absorption intensities higher than 0.1, were selected for calculating the emission spectra. Theoretical calculations showed that the M062X functional is more accurate than B3LYP for calculating the absorption and emission spectra. In addition, it was observed that by changing the functional from B3LYP to M062X, the excited electronic states responsible for emission change which attributed to the accuracy of functional. Change in the geometry of the molecule in the considered excited electronic states was observed and causes a large change in the energy and intensity of emission compared to absorption. The change in the intensity of emission compared to absorption was attributed to the varying of the wave function of the excited electronic due to the relaxation in the potential energy surface of excited state. Finally, the effect of different substitutes on the fluorescence intensity and energy shift was investigated. It was observed that the electron withdrawing groups decrease the fluorescence intensity while the electron donating groups have opposite effect.
