Theoretical study of structure and non-linear optical properties of Zn(II) porphyrin adsorbed on carbon nanotubes

The structure and stability of a series of porphyrin-nanotube complexes (ZnP-SWNT, H2P-SWNT, ZnP-pp-SWNT and H2P-pp-SWNT) were computed at the density functional theory (DFT) level. In addition, the first hyperpolarizability (β) was calculated using a coupled-perturbed-HF approach. The results indicate that complex stability is mainly dictated by the presence of Zn(II), with push-pull substituents also improving the stability. By taking the average interaction energy throughout the series of isomers found on the PES, the following stability order was predicted: ZnP-pp-SWNT > ZnP-SWNT ∼ H2P-pp-SWNT > H2P-SWNT. In addition, the push-pull groups, namely NH2 and NO2 in the present work, are essential to the first hyperpolarizability enhancement. For the free porphyrins ZnP-pp and H2P-pp, the β values were (in 10−30 esu−1 cm5) 55 and 68, respectively. These values rose to 93 and 121 (increasing by around 40%) when the complexes with SWNT were formed. Thus, these results indicate that the hybrid nanocomposites represented by H2P-pp-SWNT and ZnP-pp-SWNT might be interesting systems to investigate as lead compounds for NLO properties.