A green route to prepare metal-free phthalocyanine crystals with controllable structures by a simple solvothermal method

Exploring the environmentally friendly and low-cost synthesis strategies of phthalocyanine (Pc) crystals in just one step is an absolute challenge. The solvothermal synthesis of phthalocyanine crystals shows the advantages of high-quality crystalline products, facile reaction and purification, and low cost. Nevertheless, only a few metal phthalocyanine crystals have been successfully synthesized via solvothermal reactions. In this study, we found that the crystalline β metal-free phthalocyanine needles could be directly prepared via the tetrapolymerization of phthalodinitrile catalyzed by DBU in solvothermal reactions. Similar to the preparation of β-phthalocyanine crystals, the α metal-free phthalocyanine crystals with the specific multiply-laminated structures can be obtained through solvothermal reactions assisted by DBN. SEM characterization showed that the individual β metal-free phthalocyanine has a well-defined quadrangular shape with smooth faces. However, the α metal-free phthalocyanine exhibits a distinctive undulating surface morphology. Both phthalocyanines showed satisfactory thermal stability (from room temperature to about 300 °C), excellent resistance to acid/alkali solution, and fast photoelectric response properties (order of magnitude of response time, 10(−6) s) as tested by TG-DSC and TPV, respectively. It is noted that ethanol was used as the reaction medium and the resulting phthalocyanine crystals can be facilely purified using hot ethanol to dissolve the impurities adsorbed on the surfaces of phthalocyanine crystals. Compared to the traditional methods, no re-crystallization operation was carried out for our method. To the best of our knowledge, this is the first report on the solvothermal synthesis of metal-free phthalocyanine crystals with controllable crystal form adjusted by DBU/DBN in one step.