Highly fused, fully conjugated aromatic compounds are interesting candidates for organic electronics. With higher crystallinity their electronic properties improve. It is shown here that the crystallization of three archetypes of such molecules—pentacenetetrone, indigo, and perinone—can be achieved hydrothermally. Given their molecular structure, this is a truly startling finding. In addition, it is demonstrated that perinone can also be synthesized in solely high‐temperature water from the starting compounds naphthalene bisanhydride and o‐phenylene diamine without the need for co‐solvents or catalysts. The transformation can be drastically accelerated by the application of microwave irradiation. This is the first report on the hydrothermal generation of two fused heterocycles.

Inspired by geological ore formation processes, we apply one-step hydrothermal (HT) polymerization to the toughest existing high-performance polymer, poly(p-phenyl pyromellitimide) (PPPI). We obtain highly-ordered and fully imidized PPPI as crystalline flakes and flowers on the micrometer scale. In contrast to classical 2-step procedures that require long reaction times and toxic solvents and catalysts, HT polymerization allows for full conversion in only 1 h at 200 °C, in nothing but hot water. Investigation of the crystal growth mechanism via scanning electron microscopy (SEM) suggests that PPPI aggregates form via a dissolution–polymerization–crystallization process, which is uniquely facilitated by the reaction conditions in the HT regime. A conventionally prefabricated polyimide did not recrystallize hydrothermally, indicating that the HT polymerization and crystallization occur simultaneously. The obtained material shows excellent crystallinity and remarkable thermal stability (600 °C under N₂) that stem from a combination of a strong, covalent polymer backbone and interchain hydrogen bonding.

Hydrothermal polymerization (HTP) has been recently established as a novel route to synthesize polyimides of outstanding crystallinity. In this contribution, we lay out the basic theoretical and experimental framework for understanding the mechanistic underpinnings of this process. For this purpose, we hydrothermally synthesize two representative polyimides that are known to form amorphous polymers when synthesized classically. Hydrothermal polymerization, in contrast, yields unprecedented crystallinity after only two hours. The co-monomers diamine and dianhydride form monomer salts via acid–base reaction when brought in contact in water. We show that the physicochemical properties of the crystalline monomer salts (i.e. solubility, solid-state polymerization temperature) are important factors for the crystallinity and the morphology of the corresponding hydrothermally synthesized polyimide. We develop a mechanistic model of hydrothermal polymerization processes allowing us to relate the polymerization parameters (concentration, reaction temperature, reaction time) to the obtained polyimide crystallinity and morphology. By adjusting the parameters, the achieved crystallinity can be further increased and high morphological homogeneity can be obtained. We believe that the developed mechanistic picture is applicable for the hydrothermal polymerization of any polyimide.

A green one-pot hydrothermal route quantitatively generates high-purity fluorescence bisimide dyes without the need for catalysts or organic solvents.