Bending It Further: Porphyrin Nanotube
The synthesis of supramolecules has become an intense interest in the recent years, especially conjugated π-system. In the previous post, a cyclic octamer of porphyrin was synthesised and it gives optical interesting properties. Increasing its dimensionality, such as into barrel-like structure, could enhance electronic delocalisation and perhaps leading to exotic properties such as Aharonov-Bohm oscillation. This property has been observed in single-walled nanotubes (SWNTs) but it hasn't yet been detected in molecular materials, so this gives motivation for synthetic chemist to synthesise the molecular SWNT to observe this exotic property. Anderson's group from Oxford University successfully synthesised porphyrin tube 1 which consists of 12 porphyrins, each of which is directly conjugated to its three neighbours.
The structure 1 can be described as a rim-to-rim dimer of cyclic porphyrin hexamer, so 1 was synthesised using template-directed coupling reaction.
The key intermediate of this synthesis is the functionalised porphyrin 6 which was prepared from 4 using Senge arylation to introduce bulky aryl group for solvation. Then, followed by bromination at the remaining vacant position which then underwent Sonogashira coupling reaction with Me3SiC2H to give 6. Pd-catalysed coupling reaction is used to give 7, which is dimer unit in tube assembly, and another similar reaction in the presence of template T3 gave the porphyrin tube 1.(T4)2 in 32% yield and strikingly it has highly symmetric D6h structure.
At the first glance, the synthesis of 1 was not clear if it is possible or not. In the presence of 7 and T3 in ratio of 3:1 could also form (7),3.T3, which would have wrong spatial arrangement to form 1.(T4)2. The formation of (7),3.T3 might be favoured entropically but the repulsion of aryl group might disfavour this formation. Anderson's group suggested it forms a belt-like structure first then bind to the template to give the correct orientation for 1.(T4)2 formation.
From the UV/Vis/Near-IR absorption and fluorescence spectra, 1.(T4)2 has similar split absorption and emission bands with 2.T4 which implies they have similar electronic structures. However, the absorption and emission maxima of 1.(T4)2 are red-shifted showing greater π-conjugation in the system.
DFT calculation shows HOMO of 1.(T4)2 is distributed over the π-system while the LUMO is spread over both rings, the coefficient on butadyne staves connecting the two rings in . Furthermore, comparing the frontier orbitals with the four Gouterman orbitals of a simple porphyrin unit shows that HOMO and LUMO of 1 are derived from a2a and egx orbitals of the monomer respectively; this explains the presence of a node along each stave in the LUMO.
In short, a template-directed of alkyne homo-coupling was used in the synthesis of porphyrin nanotube with yield of 32%. This reaction shows a new type of cooperative template directed synthesis where the binding mode of the templates switches as the reaction progress. Besides that, this study gives a new opportunity to synthesise longer nanotubes using the same strategy.
Reference
P. Neuhaus, A. Cnossen, J. Q. Gong, L. M. Herz, and H. L. Anderson, Angew. Chem. Int. Ed., 2015, 54, 7344-7348.
The structure 1 can be described as a rim-to-rim dimer of cyclic porphyrin hexamer, so 1 was synthesised using template-directed coupling reaction.
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| The synthesis of 1.(T3)2 |
At the first glance, the synthesis of 1 was not clear if it is possible or not. In the presence of 7 and T3 in ratio of 3:1 could also form (7),3.T3, which would have wrong spatial arrangement to form 1.(T4)2. The formation of (7),3.T3 might be favoured entropically but the repulsion of aryl group might disfavour this formation. Anderson's group suggested it forms a belt-like structure first then bind to the template to give the correct orientation for 1.(T4)2 formation.
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| The mechanism of the formation of 1.(T3)2 |
From the UV/Vis/Near-IR absorption and fluorescence spectra, 1.(T4)2 has similar split absorption and emission bands with 2.T4 which implies they have similar electronic structures. However, the absorption and emission maxima of 1.(T4)2 are red-shifted showing greater π-conjugation in the system.
 |
| HOMO and LUMO orbitals of 1 |
In short, a template-directed of alkyne homo-coupling was used in the synthesis of porphyrin nanotube with yield of 32%. This reaction shows a new type of cooperative template directed synthesis where the binding mode of the templates switches as the reaction progress. Besides that, this study gives a new opportunity to synthesise longer nanotubes using the same strategy.
Reference
P. Neuhaus, A. Cnossen, J. Q. Gong, L. M. Herz, and H. L. Anderson, Angew. Chem. Int. Ed., 2015, 54, 7344-7348.
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