Determining the conformation of a phenylene-linked porphyrin dimer by NMR spectroscopy and quantum chemical calculations

To determine the conformation of a substituted phenylene-linked porphyrin dimer, we observed the NMR spectra of this dimer and carried out quantum chemical calculations for an unsubstituted phenylene-linked porphyrin dimer. While the observed spectra could not establish the conformation, they did clarify the confirmation of the structural formula and demonstrate anomalously large chemical shifts of the pyrrole protons adjacent to the phenylene linker. The plane angles between the porphyrin and phenyl rings and between the two-porphyrin rings in the calculated stable structure were, respectively, 64.3 and 51.5°. The calculated proton chemical shifts for this structure were consistent with the observed ones. Moreover, the anomalously large chemical shifts of the pyrrole protons adjacent to the phenylene linker were found from the calculations for parts of the dimer to be caused by the remote porphyrin ring. These findings led us to the following conclusions: (1) The planes of the phenyl and two porphyrin rings were neither coplanar nor perpendicular to one another in the stable conformation of the phenylene-linked porphyrin dimer, and (2) a strong diamagnetic current of π electrons of the remote porphyrin ring resulted in magnetic deshielding on the pyrrole protons adjacent to the phenylene linker.