Adsorption of diatomic molecules on iron tape-porphyrin: A comparative study

We investigated the adsorption of diatomic molecules, CO, NO, and O₂, denoted by XO on iron tape-porphyrin (FeTP) using first-principles calculations based on density functional theory. In this work, we present the structure and electronic properties of iron tape-porphyrin and its complexes with CO, NO, and O₂. The binding of such diatomic molecules to FeTP gave rise to significant changes on both the geometric structure and electronic properties of FeTP. We found that in all FeTP complexes with diatomic molecules, the Fe atom moves out of the porphyrin plane toward the adsorbed molecule. The energy of the iron-diatomic molecule bond increases in the order of $Fe{{-}}{O}₂{{0.3em}{0ex}}(0.554{{0.3em}{0ex}}eV)<Fe{{-}}CO{{0.3em}{0ex}}(1.225{{0.3em}{0ex}}eV)<Fe{{-}}NO{{0.3em}{0ex}}(1.719{{0.3em}{0ex}}eV)$. At its stable position, the $Fe{{-}}X{{-}}O$ angle increases in the order of $Fe{{-}}C{{-}}O>Fe{{-}}N{{-}}O>Fe{{-}}O{{-}}O$. The changes in geometric structure are in accordance with other density functional theory calculations and experimental results for porphyrin molecule. As regards the electronic properties, there appears an energy gap between the conduction band and valence band in CO-adsorbed and NO-adsorbed cases with a more pronounced energy gap in the former, while none was observed in the O₂-adsorbed case. We attribute these changes in electronic properties to the strong interaction involving the d orbitals of the Fe atom and the π* orbitals of diatomic molecules. Specifically, for the CO and NO adsorbed on FeTP cases, the change in electronic properties is attributed to the strong hybridization of dxz and dyz orbitals of the Fe atom and π* orbitals. For O₂ adsorbed on FeTP, the hybridization of the d_z² orbital of the Fe atom and π* orbitals plays the key role in O₂--iron tape-porphyrin interaction.