Phthalocyanine

Phthalocyanine (Template:Chem2) is a large, aromatic, macrocyclic, organic compound with the formula Template:Chem2 and is of theoretical or specialized interest in chemical dyes and photoelectricity.

It is composed of four isoindole unitsTemplate:Efn linked by a ring of nitrogen atoms. Template:Chem2 = Template:Chem2 has a two-dimensional geometry and a ring system consisting of 18 π-electrons. The extensive delocalization of the π-electrons affords the molecule useful properties, lending itself to applications in dyes and pigments. Metal complexes derived from Template:Chem, the conjugate base of Template:Chem2, are valuable in catalysis, organic solar cells, and photodynamic therapy.

Properties

File:Phthalocyanine STM.jpg

STM images of individual phthalocyanine molecules recorded at a bias of −2 V (left) and +1 V (right). Note that STM probes density of electrons in the HOMO/LUMO bands rather than atomic profiles.<ref> Template:Cite journal </ref>

Phthalocyanine and derived metal complexes (MPc) tend to aggregate and, thus, have low solubility in common solvents.<ref name=":0"> Template:Cite journal </ref> Benzene at 40 °C dissolves less than a milligram of Template:Chem2 or CuPc per litre. Template:Chem2 and CuPc dissolve easily in sulfuric acid due to the protonation of the nitrogen atoms bridging the pyrrole rings. Many phthalocyanine compounds are, thermally, very stable and do not melt but can be sublimed. CuPc sublimes at above 500 °C under inert gases (nitrogen, [[carbon dioxide|Template:Chem2]]).<ref> Template:Cite journal </ref> Substituted phthalocyanine complexes often have much higher solubility.<ref> Template:Cite journal </ref> They are less thermally stable and often can not be sublimed. Unsubstituted phthalocyanines strongly absorb light between 600 and 700 nm, thus these materials are blue or green.<ref name=":0"/> Substitution can shift the absorption towards longer wavelengths, changing color from pure blue to green to colorless (when the absorption is in the near infrared).

There are many derivatives of the parent phthalocyanine, where either carbon atoms of the macrocycle are exchanged for nitrogen atoms, such as tetrapyrazinoporphyrazine, or the peripheral hydrogen atoms are substituted by functional groups like halogens, hydroxyl, amine, alkyl, aryl, thiol, alkoxy and nitrosyl groups. These modifications allow for the tuning of the electrochemical properties of the molecule such as absorption and emission wavelengths and conductance.<ref> Template:Cite journal </ref>

History

In 1907, an unidentified blue compound, now known to be phthalocyanine, was reported.<ref> Template:Cite journal </ref> In 1927, Swiss researchers serendipitously discovered copper phthalocyanine, copper naphthalocyanine, and copper octamethylphthalocyanine in an attempted conversion of o-dibromobenzene into phthalonitrile. They remarked on the enormous stability of these complexes but did not further characterize them.<ref> Template:Cite journal </ref> In the same year, iron phthalocyanine was discovered at Scottish Dyes of Grangemouth, Scotland (later ICI).<ref> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref> It was not until 1934 that Sir Patrick Linstead characterized the chemical and structural properties of iron phthalocyanine.<ref> Template:Cite journal </ref>

Synthesis

Phthalocyanine is formed through the cyclotetramerization of various phthalic acid derivatives including phthalonitrile, diiminoisoindole, phthalic anhydride, and phthalimides.<ref> Template:Cite journal </ref> Alternatively, heating phthalic anhydride in the presence of urea yields Template:Chem2.<ref name="Ullmann"> Template:Ullmann </ref> Using such methods, approximately 57,000 tonnes (63,000 Imperial tons) of various phthalocyanines were produced in 1985.<ref name="Ullmann" /> More often, MPc is synthesized rather than Template:Chem2 due to the greater research interest in the former. To prepare these complexes, the phthalocyanine synthesis is conducted in the presence of metal salts. Two copper phthalocyanines are shown in the figure below.

File:Copper phthalocyanine.svg

File:Phthalocyanine Green G.png

Halogenated and sulfonated derivatives of copper phthalocyanines are commercially important as dyes. Such compounds are prepared by treating CuPc with chlorine, bromine or oleum.

Applications

File:Copper Phtalocyanine Blue.JPG

Sample of copper phthalocyanine, illustrating the intense color characteristic of phthalocyanine derivatives.

Template:See also At the initial discovery of Pc, its uses were primarily limited to dyes and pigments.<ref> Template:Cite journal </ref> Modification of the substituents attached to the peripheral rings allows for the tuning of the absorption and emission properties of Pc to yield differently colored dyes and pigments. There has since been significant research on H₂Pc and MPc resulting in a wide range of applications in areas including photovoltaics, photodynamic therapy, nanoparticle construction, and catalysis.<ref> Template:Cite journal </ref> The electrochemical properties of MPc make them effective electron-donors and -acceptors. As a result, MPc-based organic solar cells with power conversion efficiencies at or below 5% have been developed.<ref> Template:Cite journal </ref><ref> Template:Cite journal </ref> Furthermore, MPcs have been used as catalysts for the oxidation of methane, phenols, alcohols, polysaccharides, and olefins; MPcs can also be used to catalyze C–C bond formation and various reduction reactions.<ref> Template:Cite journal </ref> Silicon and zinc phthalocyanines have been developed as photosensitizers for non-invasive cancer treatment.<ref> Template:Cite journal </ref>

Various MPcs have also shown the ability to form nanostructures which have potential applications in electronics and biosensing.<ref> Template:Cite journal </ref><ref> Template:Cite journal </ref><ref> Template:Cite journal </ref> Phthalocyanine is also used on some recordable DVDs.<ref> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref>

Related compounds

File:Relpor.png

Relationship of the phthalocyanine with the porphyrin macrocycle. Two intramacrocyclic N-H groups are omitted.

Phthalocyanines are structurally related to other tetrapyrrole macrocyles including porphyrins and porphyrazines. They feature four pyrrole-like subunits linked to form a 16 membered inner ring composed of alternating carbon and nitrogen atoms. Structurally larger analogues include naphthalocyanines. The pyrrole-like rings within Template:Chem2 are closely related to isoindole. Both porphyrins and phthalocyanines function as planar tetradentate dianionic ligands that bind metals through four inwardly projecting nitrogen centers. Such complexes are formally derivatives of Template:Chem2, the conjugate base of Template:Chem2.

Soluble phthalocyanines

Of fundamental but little practical value, soluble phthalocyanines have been prepared. Long alkyl chains can be added to improve their solubility in organic solvents.<ref>Template:Cite journal</ref> Soluble derivatives can be used for spin-coating or drop-casting. Alternatively, introducing ionic or hydrophilic groups into the structure can confer water solubility.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Solubilization can also be achieved through axial coordination.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> For instance, the axial ligand functionalization of silicon phthalocyanine has been extensively studied.

Toxicity and hazards

No evidence has been reported for acute toxicity or carcinogenicity of phthalocyanine compounds. The Template:LD50 (rats, oral) is 10 g/kg.<ref name=Ullmann/>

External links

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