Zurich: High-yield synthesis of metal-free phthalocyanines

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Their photophysical properties and extreme chemical, thermal and photostability make phthalocyanines (Pcs) ideal dyestuffs and useful components of synthetic catalysts, photovoltaic devices, chemical sensors data storage devices. Pcs also have medical applications as sensitisers for photodynamic therapy.

Metal-free phthalocyanines are normally prepared from dicyano or diiminoisoindoline precursors, or from phthalic anhydride and phthalimid precursors by heating a strong base. However, only moderate yields can be achieved by using those synthetic routes.

It is well known that strongly chelating metal ion templates can dramatically improve the yields of cyclotetramerization using readily available starting materials, but no conditions for the subsequent removal of such ions to generate metal-free phthalocyanines have previously been reported.

A new demetallation reaction using ionic liquids has been developed at the University of Zurich that provides Zn(II) removal for substituted and unsubstituted zinc phthalocyanines. Zn(II)-templated cyclotetramerization followed by Zn(II) removal therefore provides a new high-yielding route to diverse, metal-free phthalocyanines.

These products are, in turn, important starting materials for making Pcs with variable metal centres.

The method allows for the high yield synthesis of metal-free Pcs or Pcs with metal centres such as Cu, Co, Ni, Fe, Pd, with applications as dyes, in photochemistry, or as catalysts.