Porphyrins are prime candidates for a host of molecular electronics applications. To understand their role in molecular devices, researchers must understand their electronic structure and the role of anchoring groups that will play a crucial role at the molecular junction interface. Here, we discuss the role of functionalization and transition metal incorporation in the free base porphyrin molecule using various levels of Density Functional Theory (DFT) calculations. We incorporated the first-row transition metals (3d-TM = Sc-Zn) and employed different anchoring groups (S, Se, Te, F, Cl, Br, PH2, NH2) connected by phenyl rings. Several structural and electronic parameters, such as HOMO-LUMO energies, band gaps, dipole moments, molecular electrostatic potentials, Mulliken electronegativity, and chemical hardness, were investigated. Our results show that there is a change in the interatomic distance and bond angles of the metalloporphyrin core when modifying the anchoring group. The frontier orbitals of the HOMO-1, HOMO, LUMO, and LUMO+1 remain consistent as one replaces the central TM-atom. The S, Se, Te, PH2, NH2 anchoring groups increased HOMO and LUMO energies, and the change in the energy gap (Eg) was insignificant. Likewise, F, Cl, and Br groups decreased the HOMO and LUMO energies, and the change in Eg was relatively small.

Investigating the electronic structure of first-row transition metal porphyrins: The role of anchoring groups and metal center

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