Tetraphenylporphyrin

Tetraphenylporphyrin
Lewis structure for meso-tetraphenylporphyrin
Ball-and-stick model of the tetraphenylporphyrin molecule
Names
Other names
5,10,15,20-Tetraphenylporphin, TPP, H2TPP
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.011.842
MeSH C509964
Properties
C44H30N4
Molar mass 614.74 g/mol
Appearance dark purple solid
Density 1.27 g/cm3
insoluble in water
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Tetraphenylporphyrin, abbreviated TPP or H2TPP, is a synthetic heterocyclic compound that resembles naturally occurring porphyrins. Porphyrins are dyes and cofactors found in hemoglobin and cytochromes and are related to chlorophyll and vitamin B12. The study of naturally occurring porphyrins is complicated by their low symmetry and the presence of polar substituents. Tetraphenylporphyrin is hydrophobic, symmetrically substituted, and easily synthesized. The compound is a dark purple solid that dissolves in nonpolar organic solvents such as chloroform and benzene.

Synthesis and structure

Tetraphenylporphyrin was first synthesized in 1935 by Rothemund, who caused benzaldehyde and pyrrole to react in a sealed bomb at 150 °C for 24 h.[1] Adler and Longo modified the Rothemund method by allowing benzaldehyde and pyrrole to react for 30 min in refluxing propionic acid (141 °C) open to the air:[2]

8 C4H4NH + 8 C6H5CHO + 3 O2 → 2 (C6H5C)4(C4H2N)2(C4H2NH)2 + 14 H2O

Despite low yields, the synthesis of H2TPP is a common experiment in university teaching labs.[3][4]

The conjugate base of the porphyrin, TPP2−, belongs to the symmetry group D4h while its hydrogenated counterpart H2(TPP) is D2h.[5] Unlike natural porphyrins, H2TPP is substituted at the oxidatively sensitive "meso" carbon positions, and hence the compound is sometimes called meso-tetraphenylporphyrin. Another synthetic porphyrin, octaethylporphyrin (H2OEP) does have a substitution pattern that is biomimetic. Many derivatives of TPP and OEP are known, including those prepared from substituted benzaldehydes. One of the first functional analogues of myoglobin was the ferrous derivative of the "picket fence porphyrin," which is structurally related to Fe(TPP), being derived via the condensation of 2-nitrobenzaldehyde and pyrrole.

A picket-fence porphyrin complex of Fe, with axial coordination sites occupied by methylimidazole (green) and dioxygen (R = amide groups).[6]

Sulfonated derivatives of TPP are also well known to give water-soluble derivatives, e.g. tetraphenylporphine sulfonate:

4 SO3 + (C6H5C)4(C4H2N)2(C4H2NH)2

→ (HO3SC6H4C)4(C4H2N)2(C4H2NH)2 + 4 H2O

Complexes

Complexation can be thought of as proceeding via the conversion of H2TPP to TPP2−, with 4-fold symmetry. The metal insertion process proceeds via several steps, not via the dianion. The resulting complexes are symmetrical with simple NMR or EPR spectra. For example, Cu(TPP) has D4h symmetry. The corresponding iron complexes are more complex owing to variable oxidation states and coordination numbers. Well-studied derivatives include the ferric compounds, e.g. Fe(TPP)Cl and the oxide [Fe(TPP)]2O, and ferrous compounds, e.g. Fe(TPP)CO(L) (L = imidazole, pyridine).

Optical properties

Optical properties of tetraphenylporphyrin in toluene

Tetraphenylporphyrin has a strong absorption band with maximum at 419 nm (so called Soret band) and four weak bands with maxima at 515, 550, 593 and 649 nm (so called Q-bands). It shows red fluorescence with maxima at 649 and 717 nm. The quantum yield is 11%.[7]

Applications

Hydrogen can be removed from individual H2TPP molecules by applying excess voltage to the tip of a scanning tunneling microscope (a); this removal alters the I-V curves of TPP from diode like (red curve in b) to resistor like (green curve). Image (c) shows a row of TPP, H2TPP and TPP molecules. While scanning image (d), excess voltage was applied to H2TPP at the black dot, which instantly removed hydrogen, as shown in the bottom part of (d) and in the re-scan image (e).[8]

H2TPP is a photosensitizer for the production of singlet oxygen.[9] Its molecules have potential applications in single-molecule electronics, as they show diode-like behavior that can be altered for each individual molecule.[8]

References

  1. ↑ P. Rothemund (1936). "A New Porphyrin Synthesis. The Synthesis of Porphin". J. Am. Chem. Soc. 58 (4): 625–627. doi:10.1021/ja01295a027.
  2. ↑ A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and L. Korsakoff (1967). "A simplified synthesis for meso-tetraphenylporphine". J. Org. Chem. 32 (2): 476–476. doi:10.1021/jo01288a053.
  3. ↑ Falvo, RaeAnne E.; Mink, Larry M.; Marsh, Diane F. (1999). "Microscale Synthesis and 1H NMR Analysis of Tetraphenylporphyrins". J. Chem. Educ. 1999 (76): 237. doi:10.1021/ed076p237.
  4. ↑ G. S. Girolami, T. B. Rauchfuss and R. J. Angelici (1999) Synthesis and Technique in Inorganic Chemistry, University Science Books: Mill Valley, CA.ISBN 0935702482
  5. ↑ Moghadam; et al. (January 2012). "Computing Group Theory and Character Table of Non Rigid Tetraphenylporphyrin H2 (Tpp) and Metalloporphyrin Mii (Tpp)" (PDF). Global Journal of Science Frontier Research Chemistry. 12 (1).
  6. ↑ S. J. Lippard, J. M. Berg “Principles of Bioinorganic Chemistry” University Science Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3.
  7. ↑ J. B. Kim, J. J. Leonard and F. R. Longo (1972). "A mechanistic study of the synthesis and spectral properties of meso-tetraphenylporphyrin.". J. Am. Chem. Soc. 94 (11): 3986–3992. doi:10.1021/ja00766a056.
  8. 1 2 Vinícius Claudio Zoldan, Ricardo Faccio and André Avelino Pasa (2015). "N and p type character of single molecule diodes". Scientific Reports. 5: 8350. PMC 4322354 Freely accessible. PMID 25666850. doi:10.1038/srep08350.
  9. ↑ Karl-Heinz Pfoertner (2002) "Photochemistry" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_573
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