Estudo espectroscópico do ácido p-aminobenzóico através da teoria do funcional da densidade

Abstract

Lanthanide complexes have been receiving increasing attention due to its many potential ap-plications, ranging from immunofluorescence to the manufacture of electroluminescent devic-es. This interest is due to the lanthanide emission characteristics, which have narrow and in-tense bands. However, as the photon absorption 4f-4f transition is forbidden by parity, good quantum yield of the complex depends on the efficiency of antenna ligands, wich absorb pho-tons and transfer its energy from the ligand triplet excited state to the emitting metal state. The energy transfer is efficient when the triplet state of the ligand is located slightly above the emitting state of the metal, and the separation between them is sufficient to help prevent ener-gy back transfer from the metal to the ligand. We can predict the theoretical value of the low-est triplet excited state of antenna ligands using density functional theory, obtaining quantita-tive results. At first, we evaluated the performance of some methods based on density func-tional theory in performing excited state calculations for ethylene and benzene and compared the results with ab initio (CCSD) and experimental data. The functionals that better performed were the M06-2X and PBE0, providing vertical excitation energies accurately on average in 0.2 eV with respect to experimental results. The 6-311++G (3df, 3pd) and aug-cc-pVTZ basis sets produced results with the same order of accuracy. We then calculated the electronic spec-trum of p-aminobenzoic acid with both methods and it was found that the PBE0 functional produced results closer to the experimental ones and faster than by using M06-2X. We con-cluded that the p-aminobenzoic acid is a promising antenna ligand for highly luminescent Tb3+ complexes. We also found that the proposed method is a reliable tool to calculate the lowest triplet state of other antenna ligands in order to know if they are suitable for producing terbium complexes with high quantum yield.