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

Abstract

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.