Excitonic effects in the optical properties of molecular crystals of meta- and para-nitroaniline: A comparative theoretical exploration

Using the ab initio methods for solving the Bethe-Salpeter equation on the basis of the full potential linearized augmented plane wave method (FPLAPW), optical properties of crystalline meta-nitroaniline (m-NA) and para-nitroaniline (p-NA) were calculated and compared. It was found that despite the similarity of the structural and electronic properties, m-NA and p-NA show significant differences in their optical spectra in the energy range below band gaps. The crystalline m-NA has two spin singlet excitonic features whereas p-NA shows three. The singlet and triplet binding energies of m-NA were found to be larger than p-NA. A comparison has been made among the optical spectra obtained by the full BSE and by random phase approximation with and without local field effects. This demonstrates that in comparison to m-NA, local field effects play more significant roles in the optical responses of p-NA in a way that they can modify both peak positions and spectral intensities, and thereby they can redistribute the oscillator strengths. In addition, by means of the BSE-derived exchange-correlation (xc) functional in TDDFT, excitonic structures in the optical absorption spectra of p-NA were reproduced well.