Density Functional Theory (DFT) Investigation of Electronic and Optical Properties of Azo Dyes

Abstract

Nonlinear optics is an important field of research owing to its extensive usage in advanced technologies. NLO materials have widespread applications in photonics and optoelectronics. NLO materials are crucial components of optical fibers, optical computing, data processing devices, optical sensors, optical modulators, high density optical storage devices, and ultra-fast optical switches. Five commercially available azo dyes, Acid Orange 5, Acid Orange 61, Direct Red 111, Food Yellow 6 and Metanil Yellow were theoretically investigated for their electronic and nonlinear optical properties using density functional theory (DFT) calculations. Each azo dye exhibits one alkaline and two acidic forms dominantly. So, all the three forms of these azo dyes were considered for calculations. Computational results reveal that these dyes possess high planarity across Ar-N=N-Ar’ linkage. Dihedral angles of alkaline and acidic I forms are found in range of 179o-179.9o. In acidic II forms, dihedral angles are distorted up to 176o owing to their geometrical change due to protonation. FMO analysis shows reduced HOMO-LUMO band gap for acidic forms as compared to alkaline forms. The drop in Eg values is attributed to the protonation which stabilizes the molecule by countering the excessive negative charge due to the sulphonic group. The HOMO-LUMO band gap lies in the range of 2-5 eV. The lowest HOMO-LUMO band gap of 2.54 eV is observed for AO61 AC I form. Low Eg values make azo dyes conductors. Furthermore, the effect of protonation of the dyes on nonlinear optical (NLO) responses was thoroughly investigated. Long range corrected method CAM-B3LYP/6-311-g(d) was employed for computation of polarizability and hyperpolarizability values. The results show that protonation of the dyes brings significant increase in nonlinear response. The highest first hyperpolarizability is observed for DR111 AC II (βo =2.6 x 104 au). TD-DFT calculations were also carried out to find change in dipole moment, crucial excited state energy difference and oscillator strength to estimate first hyperpolarizability using two-level-model. The βo (TLM) values calculated for dyes using two-level-model are comparable to the βo. It was also found that the change in dipole moments and change in excitation energies are the decisive factors for the increased first hyperpolarizabilities. In order to find the practical usability of the dyes in the field of nonlinear optics, FD-DFT calculations were performed at 532nm and 1064nm. The values of β (-2ω, ω, ω) and β (-ω, ω, 0) are estimated of the orders of 106 au and 105 au, respectively. The results are found to be quite supportive for application of the studied azo dyes in SHG phenomena.