DFT study on the influence of meso-phenyl substitution on the geometric, electronic structure and vibrational spectra of free base porphyrin

The influence of meso-phenyl substitution on the geometric, electronic structure and vibrational spectra of free base porphyrin were studied by DFT calculation and experimental observation on a series of meso-phenylporphyrins: 5-monophenylporphine (H2MPP), 5,15-diphenyporphine (H2DOPP), 5,10-diphenyl porphine (H2DAPP), 5,10,15-triphenylporphine (H2TrPP), and 5,10,15,20-tetraphenylporphine (H2TPP). Theoretical calculation indicates that meso-phenyl substitution brings about significant in-plane distortion to porphyrin ring, whereas the resulted out-of-plane distortion is negligible. Thus, the observed redshift of electronic absorption peaks (both B and Q bands) upon increasing meso-phenyl substitution, which is also indicated by theoretical calculation, was attributed to in-plane nuclear reorganization (IPNR) induced by phenyl-substitution rather than nonplanarity mechanism, though the contribution of the latter could not be excluded completely. The vibrational spectra analysis (both Raman and IR spectra) indicates that meso-phenyl substitution results in different evolution for different vibrational modes of porphyrin ring. For example, both experiment and calculation shows the great downshift of ν10 and ν28 modes and the upshift of some other structure-sensitive bands (for instance ν2, ν6, ν15, and ν8 modes) in different extent. This evolution differs from the downshift of structure-sensitive bands (for instance ν2 and ν3 mode) caused by nonplanarity mechanism, and was also attributed to IPNR mechanism rather than nonplanarity mechanism. Further comparison indicates that calculation predicted shift of vibrational frequencies are consistent well with experimental observation, which indicates that DFT calculation at B3LYP/6-31G(d) level give convincible prediction on substitution-resulted frequency evolution of porphyrin.