Density functional theory study of the inner hydrogen atom transfer in metal-free porphyrins: Meso-substitutional effects

Density functional theory (DFT) calculations were carried out to study the influence of both electron-withdrawing fluorine and electron-donating amidogen meso-substituents on the inner hydrogen transfer in metal-free porphyrins. Twenty-four stable structures (1-24) and 21 transition states among the stable structures are fully optimized at the B3LYP/6-31G (d) level, and vibration analyses are carried out to verify the optimized structures. It is found that the acidity of the transferred hydrogen atom, the basicity of the nitrogen atoms of the adjacent pyrrole ring, the distance from the transferred hydrogen atom to the nitrogen atom of the adjacent pyrrole ring, and the electronic nature of meso-substituents in transfer paths, all have significant influences on the potential energy barrier of the inner hydrogen transfer in porphyrins. The different transfer paths of each substituted porphyrin are compared, all the transfer barriers are compared to unsubstituted metal free porphyrin, and the infrared and electronic absorption spectra of some important complexes are described. By placing substituents appropriately, the cis-trans transition energy barrier can be greatly decreased, and the cis-porphyrin conformer which can be detected spectroscopically may be significantly stabilized. The present work would shed light on tuning the transition barrier by selecting the most appropriate paths and detecting cis-porphyrins experimentally.