Absorption spectra of p-nitroaniline derivatives: charge transfer effects and the role of substituents.

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 15 derivatives of push-pull para-nitroaniline (pNA). This molecule has applications in pharmaceuticals, azo dyes, corrosion inhibitors, and optoelectronics. Both electron-donor and electron-withdrawing groups were investigated. Employing machine learning-derived Hammett's constants σ, σ, σ, and σ, correlations between substituents and electronic properties were obtained. Overall, the σ constants presented the best correlation with HOMO and LUMO energies, whereas the σ constants best agreed with the transition energy of the first band and HOMO-LUMO energy gap. Electron-donors, which have lower σ values, redshift the absorption spectrum and reduce the HOMO-LUMO energy gap. Conversely, electron-withdrawing groups (higher σ's) blueshift the spectrum and increase the energy gap. The second band maximum energies, studied here for the first time, showed no correlation with σ but tended to increase with σ. A comprehensive charge transfer (CT) analysis of the main transition of all systems was also carried out. We found that donors (lower σ's) slightly enhance the CT character of the unsubstituted pNA, whereas acceptors (higher σ's) decrease it, leading to increased local excitations within the aromatic ring. The overall CT variation is not large, except for pNA-SOH, which considerably decreases the total CT value. We found that the strong electron donors pNA-OH, pNA-OCH, and pNA-NH, which have the smallest HOMO-LUMO energy gaps and lowest σ's, have potential for optoelectronic applications. The results show that none of the studied molecules is fluorescent in the gas phase. However, pNA-NH and pNA-COOH in cyclohexane and water reveal fluorescence upon solvation.

METHODS

We investigated theoretically employing the second-order algebraic diagrammatic construction (ADC(2)) ab initio wave function and time-dependent density functional theory (TDDFT) the gas-phase electronic absorption spectra of 15 derivatives of p-nitroaniline (pNA). The investigated substituents include both electron-donor (CH, CCH, CH, NH, OCH, and OH,) and electron-withdrawing (Br, CCl, CF, Cl, CN, COOH, F, NO, and SOH) substituents.