Porphyrin colour

Both magnesium and porphyrin are essentially colourless, but mixing solutions of magnesium and haem together causes a colour to form. The magnesium ion resides at the centre of the haem ring, which acts as a polydentate ligand. 

Photodynamic therapy for the treatment of tumours involves the selective uptake and retention of a highly-coloured porphyrin sensitiser (Figure 6.17) in the tumour. Irradiation by a laser with a wavelength corresponding to the absorption maximum of the porphyrin (D) causes excitation of the porphyrin to the excited singlet state. 

In a recent example metalloporphyrins are used as the sensing dyes for a wide variety of odorants, including alcohols, amines, arenes, ethers, halocarbons, ketones, phosphines, thioethers and thiols. An array of four different metalloporphyrins are placed on a substrate and exposed to the vapours for 30 s. The various vapour molecules coordinate onto the central metal atoms of the porphyrins, causing them to change colour and producing a unique four-colour array. The resulting colour array is compared with a library of known chemicals or mixtures and identification is achieved. 

Phthalocyanines 392 are porphyrin analogues embellishing Nature with exciting colours. Their pentamer 393 obtained in a one-step reaction has a cavity, thus it can play the role of host [12]. 

Porphyrins are intensely coloured, with a strong absorption band at 400 nm (e 105), the Soret band. The large ring current effect in the H NMR (< NH x — 5, <5, 10p.p.m.), the stable parent mass peaks, electrophilic substitutions on the ring peripheries, the planarity and intermediate bond lengths in their X-ray structures and the large heat of combustion all reflect the aromaticity of porphyrins,2... 

The Colours of Life. An Introduction to the Chemistry of Porphyrins and Related Compounds, Milgrom, L.R. Oxford University Press New York, 1997. 

Milgrom LR (1997) The colours of life. An introduction to the chemistry of porphyrins and related compounds. Oxford University Press, Oxford... 

Figure 16.20 Examples of sensor molecules (a) tetraphenyl porphyrin, solvatochromic Rei-chardt s dye and pH indicator (phenol red) (b) image of the 36-dye matrix before exposure, after exposure, and the resulting difference map and (c) colour change profiles for common beverages. (Reprinted with permission from Zhang and Suslick [90], Suslick et at. [91], and Janzen et al. [147]. American Chemical Society 2006-2007)...

But we re not going to discuss them and we hope you re grateful. In fact, it s about time to stop, and we shall leave you with a hint of the complexity that is possible. If pyrrole is combined with benzaldehyde a good yield of a highly coloured crystalline compound is formed. This is a porphyrin. 

Pyrrole occurs in bone oil and imparts a bright red colour to pine wood moistened with mineral acid this characteristic behaviour led to its discovery and is used as a qualitative test for pyrrole derivatives. The bile pigments are metabolic products having chains of four pyrrole rings. Their precursors are porphyrins, which include the blood pigments (haem), the chlorophylls, and vitamin B12, and consist of four pyrrole units joined in a macro ring (Section 2.3.1.2). The trisubstituted pyrrole 6 (porphobilinogen) is the biosynthetic precursor of haem and chlorophyll. 

The substances from the tetrapyrrole class are remarkable both through the very deep colour and through the extreme stability to chemical and physical agents, which naturally go together according to the resonance theory. To this class belong the coloured constituent of the blood haemin and of plants chlorophyll, all derivatives of porphyrin and also the artificial pigments discovered by Linstead, the phthalocya-... 

Catalyst instability can be a problem that may be tackled by developing an immobilized catalyst. Organic catalysts do exist that slowly decompose under the conditions necessary for their reaction and release trace amounts of by-products that must be separated from the products. For instance, in photooxigenation reactions catalysed by porphyrin the release of highly coloured materials derived from the catalysts made product purification a real problem. This problem can be solved by immobilization of the catalyst because the decomposed material is also supported and can thus be removed from the reaction medium during the process of catalyst recovery. 

Anderson H. A. Building molecular wires from the colours of life conjugated porphyrin oligomers. Chem. Commun. (1999) pp. 2323-2330. 

H. L. Anderson, Building Molecular Wires from the Colours of Life Conjugated Porphyrin Oligomers , Chem. Commim., 2323 (1999)... 

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