A New Synthetic Approach Toward Rylene Dyes

Abstract

A novel approach based on a cascade cross-coupling protocol has been established that enabled the synthesis of symmetric and asymmetric rylene imides and esters. Using this protocol, TDI building blocks were prepared on a gram scale and with minimal efforts required for purification, starting from readily available material. The scope of materials derived from these building blocks ranges from terrylene-based symmetric and asymmetric dyes and pigments to functional materials for photophysical studies into the spin states of coordination complexes and as potential guest-chromophores in host-guest chemosensors for enzymatic redox and hydrolysis reactions. The cascade cross-coupling protocol was also expanded towards PAH-based carboxylic esters and imides including heteroaromatic scaffolds, and a triple cascade reaction consisting of three distinct cross-coupling reactions was discovered.

During this work, several synthetic obstacles encountered were overcome by either improved reaction conditions or alternate molecular designs, but others still remain. This includes the central, unanswered question whether reliable conditions can be established that allow a comprehensive number of amines and anilines to serve as nucleophiles in the imidization reaction with terrylene and naphthalene building blocks. Currently, the unpredictable success rate of the imidization reaction hampers the universal applicability of the cascade approach. Whereas sterically unhindered amines were generally sufficiently reactive, sterically hindered anilines and aromatic amines with poor solubility resulted in low yields or no reaction at all. However, hydrolysis and decarboxylation were identified as the main side-reactions, caused by the necessity for harsh reaction conditions in these problematic cases. Going forward, the largest room for improvement lies in the exploration of nucleophilic catalysts that would allow imidization reactions under milder conditions for low-reactivity amines and anilines.

In conclusion, a new approach towards synthesizing terrylene and PAH imides has been discovered that significantly facilitates the access to rylene- and PAH-based nanocolorants by streamlining the synthesis and purification methods required. These advantages, that minimize the obstacle of lengthy, low-yielding syntheses, could raise the interest of the scientific community in these rylene-based materials to the same level only PDIs have reached so far.

Ein neuer Syntheseansatz für die Herstellung von symmetrischen und asymmetrischen Rylenimiden und -Estern, der auf einer Kaskadenreaktion basiert, wurde entwickelt. Mit dieser Methode konnten TDI Bausteine im Grammmaßstab hergestellt und die Aufreinigung vereinfacht werden. Die Reichweite der darstellbaren Verbindungen reicht von TDI Farbstoffen und Pigmenten für die Untersuchung photophysikalischer Eigenschaften bis zur Synthese von Heteroatom-dotierten polycyclischen aromatischen Kohlenwasserstoffen. Der hier präsentierte neue Reaktionsansatz vereinfacht den Zugang zu Terrylen-basierten Materialien und ermöglicht die Synthese ausgehend von kommerziell erhältlichen Verbindungen in wenigen, simpel aufzureinigenden Stufen.