DFT Study of Structural, Electronic, and Charge-Transfer Properties of 2-Naphthol Azo Derivatives: Geometric,...

This work represents a systematic computational study of structural and optoelectronic properties of 24 phenylazo-2-naphthol derivatives using the DFT-B3LYP/6-31 + G(d,p) method. The positional isomers of azo compounds have been designed by introducing an azophenyl unit (with and without substituents) at three different (1-, 3-, and 4-) positions of 2-naphthols. This result shows that depending on the linking position of the azophenyl unit and substituents (NO₂ and maleimide), the cis-azo, trans-azo, and hydrazo forms of our substituted azo derivatives possess distinguished UV-vis absorption and charge-transfer properties compared to unsubstituted Sudan I derivatives. Our MO calculations show that all Sudan-MI azo derivatives exhibit unique intramolecular charge transfer from the 2-naphthol-azo group as a donor to the maleimide (MI) group as an acceptor. Interestingly, whereas the trans-azo and hydrazo forms of Sudan-MI derivatives show ππ*CT and nπ*CT beside the ππ* and nπ* transitions, the cis-azo Sudan-MI derivatives exhibit mixed (nπ* + ππ*)CT along with ππ*CT, ππ*, and mixed (nπ* + ππ*) transitions. The nature and order of the main azo ππ* (S₀ → S₂) and nπ* (S₀ → S₁) transitions alter in Sudan-MI derivatives. The respective substitution of NO₂ and MI groups in Para Red and Sudan-MI series leads to the bathochromic shift of λ_max (due to π → π* transitions) in comparison to unsubstituted Sudan I derivatives, for example, the 4-positional Para Red trans-azo isomer (λ_max 516.9 nm) is 93.8 nm and the 4-positional Sudan-MI trans-azo isomer (λ_max 447.3 nm) is 24.2 nm red-shifted compared to the 4-positional Sudan I trans-azo isomer (λ_max 423.1 nm). The cis-azo forms of all positional isomers having twisted geometries show different UV-vis spectral behaviors. In general, our studies demonstrate how the variation in the structure of azo compounds impacts their optoelectronic properties, which could be useful in electronic devices.