An NMR study of the conformation of covalently linked porphyrin dimers

✍ Scribed by Lambertus Benthem; Rob B. M. Koehorst; Tjeerd J. Schaafsma

• Publisher: John Wiley and Sons
• Year: 1985
• Tongue: English
• Weight: 560 KB
• Volume: 23
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260230911

✦ Synopsis

Proton NMR spectra of covalently linked tetraphenyl-or tetra-ptolyl-porphyrin h e r s of three Werent types (para-para, ortho-para and ortho-ortho) can be interpreted using a ring current model. For the assignment of the protons of the linking chain a tetraaryl porphyrin monomer has been used, containing this chain at the same position as in the corresponding dimer but without the presence of a second porphyrin group. The porphyrin ring current shift of the para-para dimer was used as a reference for the calculation of the resonance positions of the aromatic propyloxyphenyl protons. Using these NMR results, the dimer conformation in the ground state was determined for the various dimem. From space-filling models of the h e r s in different conformations, the polar coordinates of the various protons of the propyloxyphenyl group of one porphyrin ring with respect to the other ring can be obtained, and the ring current shifts due to the presence of the latter can be calculated for these protons. It emerges that the para-para dimer exists mainly in a stretched conformation, and that the ortho-ortho dimer has an ex0 conformation (unfolded). It is likely that in the ortho-para h e r , the pora-linked porphyrin group has a rotation around the linking chain axis which is fast on the NMR time scale. The degree of folding of the ortho-para dimer is shown to be inlluenced by the solvent polarity.