Ab initio electronic absorption spectra of para-nitroaniline in different solvents: intramolecular charge transfer effects

Intramolecular charge transfer (ICT) effects of para–nitroaniline (pNA) in eight solvents (cyclohexane, toluene, acetic acid, dichloroethane, acetone, acetonitrile, dimethylsulfoxide, and water) of different polarities are investigated extensively. The second-order algebraic diagrammatic construction, ADC(2), ab initio wave function is employed with an implicit solvation method. We found that the pyramidal dihedral angle of the amine group depends on the solvent, decreasing with increased polarity. The first absorption band involves HOMO→LUMO π→π* transitions with charge transfer from the amine and the benzene ring to the nitro group. ICT effects increase by 10% upon solvation in water compared to the gas phase. A second band of pNA is characterized for the first time. The brightest state of both bands depends on the polarity of the solvent. The second band is primarily a local excitation (LE) on the nitro group, including some CT from the amine group to the benzene ring that decreases with the solvent polarity. The LE character on the nitro group of the second band increases by 36% from the gas phase to water. A -0.32 eV redshift in the first band of cyclohexane increases to -0.84 eV in water, in agreement with experiment. The second band is redshifted by -0.21 eV for cyclohexane and -0.36 eV for water. An exponential correlation between the polarity and spectral properties is found, which saturates for solvents of intermediate polarities.