Theoretical Study on Electrochemical Behavior of meso-substituted Porphyrins and Their Zinc Complexes

• Main Authors: Thai Thi Ha Tran, 陳氏河泰
• Other Authors: Yuhlong Oliver Su
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/j84fk8

博士 === 國立暨南國際大學 === 應用化學系 === 106 === Meso-substituted free base porphyrins and metalloporphyrins, with extensive conjugated -ring systems, have been employed as key components in photoelectric devices, photocatalysis, electron-transfer systems, and biological systems. The design of porphyrinoids and understanding their electrochemistry have been attracting much attention due to their wide applications. This thesis, therefore, aims to study the electrochemical behavior of free base and zinc meso-substituted porphyrins by employing both cyclic voltammetry (CV) and density functional theory (DFT). The role of porphyrin cation radicals, effect of axial ligands and meso-substituents to electrochemistry of porphyrins were included. The results show that the half-wave oxidation potential splitting of the two oxidation states (∆E= 2nd E1/2 - 1st E1/2) of tetraphenylporphyrin (H2TPP) and its zinc complex (ZnTPP) are higher than those of porphyrins and their zinc complexes with meso-substituted five-membered heterocylic rings. The DFT calculations show that the trend of ∆E values is consistent with that of highest spin density (HSD) distribution and HOMO-LUMO energy gaps of cationic radicals as well as the -conjugation between the central porphyrin and meso-substituted rings, defined by dihedral angles () . Regarding to the effect of axial ligands, the findings show that the first half-wave oxidation potentials (1st E1/2) of zinc porphyrins complexed with imidazole all move to negative side while the second ones (2nd E1/2) move positive side, resulting in larger ∆E, in comparison with the zinc porphyrins. The DFT results show that steric hindrance between the central porphyrin and the meso groups is a key factor in determining the half-wave oxidation potentials. Also, imidazole exhibits strong effects on the deformation of porphyrin rings which is dominant in determining the 1st E1/2 while the inter -conjugation between porphyrin rings and meso-substituted phenyl groups mainly contributes to the 2nd E1/2. Without imidazole, the inter -conjugation between porphyrin rings and meso-substituted phenyl groups is the only important criterion which effects both 1st E1/2 and 2nd E1/2 of zinc porphyrins. Meso-tetrasubstituted free base porphyrins with six-membered rings, which exhibit inductive or resonance effects between the substituents and the porphyrin rings, were also examined. The experimental results show that the free-base porphyrin with 4-hexoxy-biphenyl substituents (H2TBoC6P), inductive effects, has the smallest ∆E, compared to that with tetra-naphthalene (TNP) or tetra-phenyl (TPP) substituents. The calculation results again confirm that steric hindrance between the porphyrin ring and meso-substituents is more important than substituent effects in determining the electrochemistry of free-base porphyrins. Furthermore, ∆E is related to HSD distribution on porphyrin rings and HOMO-LUMO energy gaps of free-base porphyrin cationic radicals. Such agreements between DFT calculations and experimental CV have implemented to obtain insights into structures and electrochemical properties of porphyrins.