Detecting the saddling deformations in nickel meso-phenyl substituted porphyrins using low-frequency Raman characteristic peaks

The out-of-plane (OOP) deformations of metalloporphyrins macrocycle are closely related to their biological functions, and Raman spectroscopy is a powerful tool for investigating OOP deformations. However, due to the interplay of electronic structure, substituents, porphyrin macrocycle in-plane (IP) and OOP deformations, it is challenging to measure the OOP deformations directly, or, establish a confirmative correlation between the frequency shifts of characteristic peaks and specific OOP deformation changes. In this work, we first selected the model porphyrin Ni-P and employed DFT calculations to explore the relationship between the ruffling and saddling deformation changes and their corresponding Raman spectral differences. Subsequently, we focused on nickel meso-tetraphenyl porphyrin (NiTPP), nickel meso-tetrachlorophenylporphyrin (NiTClP), and nickel meso-tetramethoxyphenyl porphyrin (NiTMeOP), which are structurally similar in nature, and investigated the relationship between their OOP deformations and Raman spectra bands based on both experiments and DFT calculations. The results indicate that the ruffling deformation magnitudes of the three nickel porphyrins are almost identical, while the saddling deformation magnitudes differ remarkably. The frequency change of the characteristic peak ${\gamma }_{18}$ in relation to saddling deformation of the three porphyrins is $7.7{cm}^{-1}/\AA$, which is close to that of the model porphyrin Ni-P $10.6{cm}^{-1}/\AA$. Therefore, the characteristic peak ${\gamma }_{18}$ can be used as a “reporter” for the change in saddling deformation. As such, this work demonstrates how to utilize the frequency shifts of low-frequency Raman characteristic peaks to identify the OOP deformation changes of the porphyrin macrocycle caused by variations in the external environment, thereby providing a theoretically assisted tool for revealing the reasons for their biological function variations.