A DFT Study of Linear and Nonlinear Optical Properties of 2-Methyl-4-nitroaniline and 2-Amino-4-nitroaniline Crystals

The electronic structure and linear and nonlinear optical susceptibilities of 2-methyl-4-nitroaniline (MNA) and 2-amino-4-nitroaniline (ANA) crystals have been studied using the full potential linear augmented plane wave method within density functional theory. In addition, we have investigated the excitonic effects by means of the bootstrap exchange-correlation kernel within time dependent density functional theory. Our calculations show indirect band gaps for MNA and ANA crystals. Both crystals have band structures with low dispersion which is a characteristic behavior of organic crystals. The crystalline ANA shows larger band dispersion, compared to MNA, due to the higher intermolecular interactions. Findings show that the substituent groups play major roles in enhancing the optical response of push−pull organic crystals. On the other hand, the intermolecular interactions make the band dispersion increase and the optical response, especially the nonlinear one, decrease. The MNA crystal shows larger values of nonlinear response, since all the constitutive molecules are mainly polarized along the same axis and there is less overlap between them. Moreover, the considerable below-band-gap anisotropy as well as the high values of nonlinear susceptibilities make these crystals suitable candidates for nonlinear purposes. In addition to the high potential of excitonic effects, both crystals have extremely small wavelengths of plasmon peaks. Finally, this study gives reliable results for the optical spectra in both linear and nonlinear regimes.