Bending the Molecular Wires
Recently, large conjugated π-systems have been a great interest due to their unique properties such as electrical conductivity, or they are commonly known as molecular wires. Taking a step further, it can be closed into a large cyclic system, i.e. macrocyle, or known as a closed loop. One of the examples of this system is porphyrin oligomers where linear conjugated porphyrin oligomers show strong long-range electronic coupling and rich non-linear optical behaviour. These unique properties provide a motivation for synthesising its macrocycle and probing its properties.
Henderson's group from Oxford University successfully synthesised a fully conjugated butadiyne-linked cyclic porphyrin octamer 2. This octamer has unique properties as it exhibit a highly symmetric structure (D8h) and high affinity between 1 and template 3 (Kf = 1037 M-1). Besides that, its belt-shaped structure resembles some natural light-harvesting chlorophyll arrays.
The synthesis of 2 was done by template-directed synthesis between linear octamer 1 and template 3. The linear oligomer 1 wraps around the template where the pyridine side group coordinate to Zn centre at the porphyrin unit of 1. As mentioned earlier, the affinity between the "rigid rod" 1 and 3 is surprisingly high. The template was designed to complement the octamer where the calculated N-N distance is 2.96 nm.
The template 3 was synthesised in 5 steps where the synthesis of the key pyrrole intermediate was done using Barton-Zard reaction.
In this template, eight flexible butyloxy groups provides high solubility for the complex.
After the formation of complex 3.1, oxidative coupling was used under Pd/Cu catalyst with iodine as oxidant to give complex 3.2 with 14% isolated yield. The removal of the template was done by adding pyridine to give 2.
1H-NMR spectrum of 2 showed a consistency with its highly symmetric structure. Interestingly, the internal and external faces of porphyrin rings in 3.2 are non-equivalent as expected. However, in 2 both faces become equivalent on the NMR time scale.
Another interesting point from the formation of 2 is the equilibrium constants for formation 3.1 and 3.2 cannot be measured directly because the values are too large. However, those values can be evaluated indirectly from the displacement of the template by pyridine.
Another parameter of the complex formation equilibrium is effective molarity (EM) which can be calculated using equation below.
Where K0 is the binding constant of one arm of the template for one site on the octamer and it can be approximated to the binding constant of 4-(phenylethynyl)pyridne to 5,10-diethynylporphyrin zinc monomer. From that value, EM for 3.1 and 3.2 are 0.28 M and 5.4 M, and from those values the difference between energies of template binding for 2 and 1 can be evaluated to give an estimate energy required to bend the linear octamer to cyclic conformation.
From this study, it shows the power of noncovalent self-assembely for controlling the conformation of porphyrin-based molecular wires. The synthesis also challenges the common perception such as 1 has rigid structure. Furthermore, 2 is suspected not in strained conformation as it has similar absorption spectrum 1.
Reference
M. Hoffmann, C. J. Wilson, B. Odell, and H. L. Anderson. Angew. Chem. Int. Ed., 2007, 46, 3122-3125.
The synthesis of 2 was done by template-directed synthesis between linear octamer 1 and template 3. The linear oligomer 1 wraps around the template where the pyridine side group coordinate to Zn centre at the porphyrin unit of 1. As mentioned earlier, the affinity between the "rigid rod" 1 and 3 is surprisingly high. The template was designed to complement the octamer where the calculated N-N distance is 2.96 nm.
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| Self-assembly reaction for the formation of 3.1 |
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| The synthesis of template 3 |
After the formation of complex 3.1, oxidative coupling was used under Pd/Cu catalyst with iodine as oxidant to give complex 3.2 with 14% isolated yield. The removal of the template was done by adding pyridine to give 2.
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| Formation of macrocyle 2 |
Another interesting point from the formation of 2 is the equilibrium constants for formation 3.1 and 3.2 cannot be measured directly because the values are too large. However, those values can be evaluated indirectly from the displacement of the template by pyridine.
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| Indirect measurement for equilibrium constant for formation of the complexes |
Another parameter of the complex formation equilibrium is effective molarity (EM) which can be calculated using equation below.
From this study, it shows the power of noncovalent self-assembely for controlling the conformation of porphyrin-based molecular wires. The synthesis also challenges the common perception such as 1 has rigid structure. Furthermore, 2 is suspected not in strained conformation as it has similar absorption spectrum 1.
Reference
M. Hoffmann, C. J. Wilson, B. Odell, and H. L. Anderson. Angew. Chem. Int. Ed., 2007, 46, 3122-3125.
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