Absorption Spectra of p-Nitroaniline Derivatives: Charge Transfer Effects and the Role of Substituents

Abstract

Abstract Context Push-pull compounds are model systems and have numerous applications. By changing their substituents, properties are modified and new molecules for different applications can be designed. The work investigates the gas phase electronic absorption spectra of fifteen derivatives of p–nitroaniline (pNA) using theoretical methods. This molecule has applications in pharmaceuticals, azo dyes, corrosion inhibitors, and optoelectronics. Both electron-donor and electron-withdrawing groups were investigated. Overall, electron-donors redshift the absorption spectrum and reduce the HOMO-LUMO energy gap. Conversely, electron-withdrawing groups blueshift the spectrum and increase the energy gap. A comprehensive charge transfer (CT) analysis of the main transition of all systems was also carried out. We found that donors slightly enhance the CT character of the unsubstituted pNA, whereas acceptors decrease it, leading to increased local excitations within the aromatic ring. The overall CT variation is not large, except for pNA–SO2H, which considerably decreases the total CT value. It was found that the strong electron donors pNA–OH, pNA–OCH3, and pNA–NH2, which have small HOMO-LUMO energy gaps, have potential for optoelectronic applications. Methods We investigated theoretically employing the second-order algebraic diagrammatic construction (ADC(2)) ab initio wave function the gas phase electronic absorption spectra of fifteen derivatives of p–nitroaniline (pNA). The substituents considered in this investigation included both electron-donor (C6H5, CCH, CH3, NH2, OCH3, and OH,) and electron-withdrawing (Br, CCl3, CF3, Cl, CN, COOH, F, NO2, and SO2H) substituents. Employing machine-learning-derived Hammett's constants σm, σm0, σR, and σI, correlations between substituents and electronic properties were obtained.