Abstract
We present the first predictions of meso-aryl flipping pathways in porphyrin oligomers. In the context of cyclic oligoporphyrins this flipping results in a paddle rotation of each porphyrin monomer in the oligomeric ring. If the monomer porphyrin units are asymmetric, this flipping will have consequences for their supramolecular behaviour. Desymmetrisation of synthetic porphyrins leads to synthetic challenges, and hence these species are not as well studied as the more accessible, symmetric counterparts. We have both simulated and synthesized novel, desymmetrised monomeric and cyclic trimeric porphyrins and we predict that the flipping barrier for a porphyrin monomer within the trimer is 36.7 kJ mol(-1) higher than that for meso-aryl flipping in the monomer. The flipping rates estimated from Variable temperature NMR data are consistent with these results. We have also carried out a systematic investigation of how porphyrinic substituents will affect the dynamics, revealing that adding steric bulk in the right place can facilitate meso-aryl flipping. While supramolecular chemistry often focuses on highly symmetric assemblies, evolution can break molecular symmetry in subtle ways, leading to many pseudosymmetric assemblies in biology, especially protein-porphyrinic complexes that are important for energy harvesting and electron transport systems. The dynamic behaviour we have characterized can be critical for the design and function of these molecules, and hence our results will help inform future efforts in the synthesis of asymmetric porphyrinic assemblies that interact with biomolecules.
Highlights
In this work we present a desymmetrised analogue of a well-studied cyclic porphyrin oligomer, which was designed to aid our understanding of porphyrin dynamics
We present a detailed analysis of the predicted pathways between conformational isomers for a number of porphyrin systems
Our investigation begins with the novel porphyrin P1 and is extended to the cyclic trimer, P2, whose synthesis and NMR characterisation have allowed us to experimentally corroborate the computational results
Summary
Interesting to ask how we might integrate and exploit such nonnatural prosthetic groups within natural systems and what structural and dynamic factors are important. The 6-31G(d,p) basis set has been used to investigate meso-aryl rotation in small porphyrin monomers.[26] another single point energy calculation with chloroform was represented by the polarizable continuum solvent model (PCM)[54,55] in Gaussian 03. Similar results have been obtained in the last decade using NMR, molecular mechanics, and higher level calculations.[26,27,28,29] to the best of our knowledge, there have been no investigations into the flipping of a single macrocycle in a porphyrin oligomer. A large substituent at the b-pyrrolic position is shown to decrease the flipping barrier, with important implications for the design and synthesis of porphyrin oligomers
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