Giant values obtained for first hyperpolarizabilities of methyl orange: a DFT investigation

Abstract

Advances in photonics and optoelectronics depend on proposing new materials with well-defined nonlinear optics properties. Based on the foundations of density functional theory, this work presents a systematic investigation of linear and nonlinear optical properties of methyl orange, a well-known azo dye. Structural changes from alkaline to acidic structures drastically boost all investigated properties. For instance, the material dipole polarizability starts from an isotropic condition ($\alpha _{\mathrm{iso}}>\varDelta \alpha$) to an anisotropic behavior ($\alpha _{\mathrm{iso}}<\varDelta \alpha$). The first hyperpolarizabilities are also strongly tuned varying from 18.9 $\times\, 10^{-30}$ to 171.7 $\times\, 10^{-30}$ esu. A careful analysis of frontier molecular orbitals indicates proper wide-bandgap semiconductor energy gap (3.22 eV) and associates the highest hyperpolarizabilities to the lowest energy gap, which means semiconductor molecules with intense nonlinear optical activity.