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.