Abstract: Meso-substituted porphyrin derivatives have demonstrated great potential as sensing materials for toxic gas detection. In this paper, density functional theory (DFT) and its time-dependent DFT approach (TD-DFT) were employed to investigate the ultraviolet-visible (UV-Vis) or the near-ultravioletvisible (near-UV-Vis) absorption spectra of Meso-tetra (o-nitrophenyl/o-aminophenyl) porphyrins (NO₂PP, NH₂PP) and their corresponding zinc derivatives, NO₂ZnPP and NH₂ZnPP. The geometry optimizations for these four molecules were obtained from two different exchange-correlation functionals, the generalizedgradient approximation functional PBE (Perdew-Burke-Ernzerhof) and the hybrid functional B3LYP (Becke, three-parameter, Lee-Yang-Parr). The excitation energies and oscillation strengths were obtained from TD-DFT calculations. Calculations show that the optical absorptions are associated with numerous electronic transitions. In addition, the PBE-predicted wavelengths of the B and Q bands are more consistent with experiment than those predicted by B3LYP. The B band of NO₂-substituted derivative exhibits a bathochromic shift different from that of NH₂-containing material, also consistent with experimental results. In addition, at the PBE/6-31G(d) level of theory, the calculated energies of the lowest triplet excited states of NO₂PP, NH₂PP, NO₂ZnPP, and NH₂ZnPP are 1.426, 1.469, 1.608, and 1.581 eV, respectively.

Key words: TD-DFT, Electronic absorption spectrum, Electronic transition, Bathochromic shift, Triplet state