DFT Study of Structural,Electronic, and Charge-TransferProperties of 2‑Naphthol Azo Derivatives: Geometric, Positional,and Substituent Effects

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 <i>cis</i>-azo, <i>trans</i>-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 <i>trans</i>-azo and hydrazo forms of Sudan-MI derivatives show ππ*CT and nπ*CT beside the ππ* and nπ* transitions, the <i>cis</i>-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 <i>trans</i>-azo isomer (λ_max 516.9 nm) is 93.8 nm and the 4-positional Sudan-MI <i>trans</i>-azo isomer (λ_max 447.3 nm) is 24.2 nm red-shifted compared to the 4-positional Sudan I <i>trans</i>-azo isomer (λ_max 423.1 nm). The <i>cis</i>-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.