A DFT Analysis of Electronic, Reactivity, and NLO responses of a Reactive Orange Dye: the Role of Hartree-fock Exchange Corrections

An experimental and theoretical study based on DFT/TD-DFT approximations is presented to understand the nature of electronic excitations, reactivity, and NLO properties of reactive orange 16 dye (RO16), an azo chromophore widely used in textile and pharmacological industries. The results show that the solvent has a considerable influence on the electronic properties of the material. According to experimental results, the absorption spectrum is noticed by four intense transitions, which have been identified as $\pi\rightarrow\pi^*$ states using TD-DFT calculations. However, the TD-DFT results reveal a weak $n\rightarrow\pi^*$ in the low-lying spectral region. Continuum models of solvation indicate that these states suffer bathochromic and hypsochromic shifts, respectively. However, the expected blue shift for the absorption $n\rightarrow\pi^*$ is only described using long-range or dispersion-corrected DFT methods. RO16 is classified as a strong electrophilic system ($\omega > 1.5$ eV). From vacuum to solvent, the environment is active and changes the nucleophilic status from strong to moderate nucleophile ($2.0 \leq N \leq 3.0$ eV). The results also suggest that all electrical constants are strongly dependent on long-rang and Hartree-Fock exchange contributions, and the absence of these interactions gives results far from reality. In particular, the results for the NLO response show that the chromophore presents a potential application in this field, with a low refractive index, and first-hyperpolarizability ca. 214 times bigger than the value usually reported for urea ($\beta = 0.34 \times 10^{-30}$ esu), which is a standard NLO material. Concerning the solvent effects, the results indicate that $\beta_\textrm{total}$ increases ca. 180\% regarding the gas-phase value. The results suggest RO16 is a potential compound in NLO applications.