Structural parameters and vibrational spectra of a series of zinc meso-phenylporphyrins: A DFT and experimental study

The influences of meso-phenyl substitution on the geometric structure and vibrational spectra have been studied by DFT calculation (B3LYP/6-31G(d)) and experiment on a series of zinc porphyrins (ZnTPP: zinc 5,10,15,20-tetraphenylporphyrin; ZnTrPP: zinc 5,10,15-triphenylporphryin; ZnDPP: zinc 5,15-dipenylporphyirn; ZnMPP: zinc 5-monophenylporphyrin; ZnP: zinc porphine). Calculation indicates that meso-phenyl substitution gives rise to slight out-of-plane distortion but significant in-plane distortion, especially for the configuration around Cm atom, to zinc porphyrin. The assignment of experimental vibrational spectra was proposed mainly on the basis of calculation. Different shifting tendency upon meso-phenyl substitution is observed for different structure-sensitive bands, such as the shifting of ν2, ν3, ν6, and ν8 modes toward higher frequencies and ν4 and ν28 modes toward lower frequencies, upon meso-phenyl substitution. This is attributed primarily to in-plane nuclear reorganization effect (IPNR), though the contribution from out-of-plane distortion cannot be excluded completely. Analysis on vibrational structure reveals that asymmetric meso-phenyl substitution, especially the 5,15-diphenyl substitution of ZnDPP, brings about asymmetric vibrational displacement, or even splitting of vibrational structure to normal modes involving mainly the motion of meso-Cm. This is ascribed to the reduction of symmetry of porphyrin skeleton caused by asymmetric meso-phenyl substitution.