Substituent Effect on Porphyrin Ring Distortion

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A series of porphyrin with meso-(2,6-difluoro- or dichlorophenyl) substituents has been synthesized by mixed condensation of pyrrole, diphenylpyrrole, and 2,6-difluoro- or 2,6-dichlorobenzaldehyde. The porphyrins reported here are abbreviated as follows: tetraaryl-lporphrins (TXPP), hexaarylporphyrins (HXPP), trans-octarylporphyrins (trans-OXPP), cis-octarylporphyrins (cis-OXPP), decaarylporphyrins (DecXPP), and dodecaarylporphyrins (DXPP), where X=F or Cl.Increase of the phenyl substituents have large effect on the UV-visible spectra, gradually red-shifting the Soret bands in the order of TXPP<HXPP<trans-OXPP,cis-OXPP<DecXPP<DXPP. However, the magnitude of the red shift is different between X=F series and X=Cl series. It is tempting to suppose that the magnitude of porphyrin ring distortion is different between X=F seris and X=Cl seris. It is possible that interaction between lone pair of fluorin atoms and porphyrin π orbitals reduce the porphyrin ring distortion.From the acid-base properties of these porphyrins, it is supposed that conjugation between porphyrin ring and meso-phenyl ring has influence upon the porphyrin ring distortion. Furthermore, an unusual stability in the acid monocation state of TXPP (X=F and X=Cl) is observed from the acid-base titration experiments.