Phthalocyanines – from Prime Dyes to Hightech Chemistry

Dear Colleagues,

The group of metal phthalocyanine (MPc) substances is fascinating in every respect. Use of these premium-quality blue dyes has quickly spread all around the world. Common MPcs, such as those from phthalic acid anhydride and urea, can be produced in an environmentally friendly manner using a microwave, without any solvents. Due to the extreme physical and chemical robustness of MPcs and their complex chemical structure, research was soon conducted into their use as catalysts. Rational modifiability, fine-tuning, switching effects, and other additional functions are responsible for the increasing significance of MPcs.

Early on, MPcs were used in electrochemical cathodic O₂ reduction using FePc and CoPc and are currently used in CO₂ reduction and in the reductive decomposition of NO_x into N₂. Microorganic pollutants in sea water can be removed through photodegradation using CdPc, ZnPc and, particularly, PdPc. Co^IIPc provides a good chemical model for vitamin B₁₂, particularly in terms of the super-nucleophilic properties of both reduced complexes. The anion [Co^IPc]⁻ is stable with respect to water and inert to most functional groups in organic chemistry and thus reduces highly selectively and nearly solely nitro to primary amino functions. MPcs as catalysts are particularly suited to reduction reactions with NaBH₄. It has transpired that some MPcs, particularly CoPc, are inert to strong catalyst poisons such as thiols and sulfides. Catalytic switching between three catalytic patterns can be achieved using PdPc, thereby permitting the switching of multiple consecutive reactions by means of Pd^IIPc ↔ [Pd^IPc]⁻, saving many synthesis steps. A multiple switching effect was also observed in PcPbPc, Cl-AlPc, and AlPcAl oxide layers.

Some MPcs are gas sensitive and can be used as sensors to detect gas. For instance, CoPc is used to monitor NO_x. FePc cyanide complexes, OTiPc, and OVPC radical anions have remarkable magnetic properties. Quite a few MPcs have optical laser-like properties. The electrical conductivity of MPcs allows them to be intensively used as semiconductors and conductors with different dimensionality as well as in transistors. Electrophotographic processes (xerography) are mainly based on TiO₂ and OTiPc or OVPc.

Photovoltaics (PV) is an area in which MPcs will experience increasingly high demand. Unlimited, fresh, clean, and safe electrical power can theoretically be sourced from the sun’s nuclear fusion for the next 5 billion years. Electricity yield from solar radiation can be increased to over 80% by combination of of MPcs with carbon nanotubes and the use of multiple photoelectric layers with different MPc photosensitizers, which can capture more sunlight frequencies and, therefore, energy. Lithium battery storage, which contains Li₂Pc as its cathode material, thereby increases the number of charging cycles substantially. In terms of quantum information, coherent coupling can be achieved between OVPc spin ensemble and microwave photons. This indicates that molecular spin qubits can be integrated into quantum circuits. Many theoretical calculations add to and expand upon the scope of MPc in high-chem and high-tech applications.

The main aim of this Special Issue “Phthalocyanines – From Prime Dyes to High-Tech Chemistry” is to create an open forum where scientists can share their research. The contributions to this issue, in the form of original research or review articles, may cover all aspects of phthalocyanine metal complexes; research with interdisciplinary input is especially welcome, giving new insights into phthalocyanine metal complexes and generating future-oriented and pioneering products and methods.

Dr. Heiner Eckert
Guest Editor

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