A Greener Route to Blue: Solid-State Synthesis of Phthalocyanines

Dyes and pigments are the main known forms of colorants. Unlike pigments that are generally insoluble and suspended in the binder, dyes are soluble owing to their relatively smaller particle sizes and differences in bonding properties. Organic dyes have a broad range of applications. There are several methods for synthesizing these dyes. Unfortunately, most existing approaches are energy intensive and generally environmentally unfriendly. Thus, developing sustainable alternatives is urgent.

Creating a sustainable economy has become a priority in nearly all economic sectors and industries. In an effort to achieve the United Nations Sustainable Development Goals (SDGs), especially SDGs #6 (clean water and sanitation) and #12 (responsible consumption and production), environmentally friendly and sustainable alternative methods for synthesizing chemicals have been heavily researched. An emerging greener route involves using solid-state reactions, which allows for a significant reduction of solvent use.

In particular, mechanochemistry has gained prominence in organic synthesis, material chemistry, and polymer science. Previous studies revealed that mechanochemical and solid-state synthesis of dyes could potentially avoid the use of solvent, which has the benefits of enhancing water and environmental quality. Additionally, it could bring more economic benefits to the industry, such as low production costs, and contribute to creating a sustainable economy.

Phthalocyanines (Pcs) are a well-known and prominent type of organic dye. Due to their extended aromatic structure, Pcs exhibit remarkable and tunable optical properties that endow them with potential applications in numerous fields, including nanomedicine, sensing and optoelectronics. Traditionally, the synthesis of Pcs is performed in high boiling organic solvents such as dimethylaminoethanol (DMAE) as well as organic and inorganic catalysts. This approach is, however, flammable, corrosive and environmentally unfriendly. Although Pcs exhibit crystallographic polymorphism under ball-milling processing and have been used in the solid-state preparation of some composite materials, the solid-state synthesis of Pcs is yet to be reported.

Herein, Doctoral candidate Daniel Langerreiter, Prof. Mauri Kostiainen, Dr. Sandra Kaabel and Dr. Eduardo Anaya-Plaza from Aalto University developed and new solid-state synthesis method for Pcs based on ball milling and aging. In their approach, tetra-tertButyl Pc (1) and 4-tertButyl phthalonitrile (2) were selected as model compound and precursor, respectively, in the presence of equimolar amounts of DMAE. The parameters and conditions influencing the solid-state reaction pathway were explored. Their research work is currently published in the journal, Angewandte Chemie.

The researchers showed that the presented solid-state approach was capable of achieving a high-yield synthesis of Pcs. Besides being highly efficient, this method also reduced the amount of DMAE solvent required by up to 100-fold compared to the traditional approach. Instead of harsh conditions that mostly characterize the molten-state reactions, solid-state reaction could be achieved even at lower temperatures with a 4-fold increase in space-time yield.

Through systematic screening, three main parameters influencing the solid-state reaction were identified: the presence of template, temperature, and the role/amount of DMAE in the tBu phthalonitrile to tetra-tBu phthalocyanine conversion. Although reactions progressed during aging at 55 ˚C, higher conversion rates were obtained at 100 ˚C, confirming the homogenizing role of the ball milling process. The solid-state reaction approach could be used to synthesize Zn²⁺, Cu²⁺, Co²⁺, and Mg²⁺ Pcs. The solid-state reaction was easily scalable and did not rely on precursor solubility, suggesting it could be adopted for synthesizing highly insoluble Pc derivatives.

The presented research is supported by FinnCERES competence center in the area of materials bioeconomy. FinnCERES is a part of the Academy of Finland Flagship Programme, and it is a joint effort by Aalto University and VTT Technical Research Centre of Finland. The Flagship redefines the bioeconomy with advanced bio-based materials. These materials leverage the natural properties of wood and other raw materials, thus supporting sustainable economic growth. The Flagship’s research covers four themes: future biorefineries, clean air and water, lignocellulosics beyond plastics, and electronic, optics, and energy applications. FinnCERES pursues answers to fundamental research questions related to the refining, processing and utilization of biomass, and addresses challenges related to resource sufficiency and climate change.  By developing new technologies and processes for biomass utilization, FinnCERES aims to reduce the carbon footprint of various industries and contribute to the mitigation of climate change. Another important aspect of FinnCERES is its emphasis on cross-disciplinary collaboration and knowledge sharing. The Flagship brings together researchers from different fields and encourages them to work together on common challenges. This approach promotes innovation and helps to accelerate the development of new solutions. Moreover, FinnCERES has established partnerships with industry and other research organizations both in Finland and internationally, which enables the Flagship to bring its research results into practice and have a real-world impact.

In summary, an efficient approach for sustainable Pcs production was demonstrated. The findings also provided insights into the critical parameters influencing the synthesis of Pcs. Compared to traditional synthetic approaches, it is a more feasible and greener alternative for synthesizing high-performance dyes. In a statement to Advances in Engineering, Dr. Eduardo Anaya-Plaza stated that by promoting solid-state Pc synthesis, their study would contribute to a more sustainable chemistry without degrading reaction quality and efficiency.