Exceptionally flexible quinodimethanes with multiple conformations: polymorph-dependent colour tone and emission of crystals

Tetraarylated quinodimethanes, which are one of the overcrowded ethylenes, can adopt multiple conformations, such as folded, twisted, and other forms. However, it is difficult to obtain all of these conformations due to energy differences among them. To tackle this issue, we designed flexible polyaryl-substituted tetraazaanthraquinodimethanes (N(4)AQDs), in which a steric hindrance in the overcrowded fjord region was moderately reduced. As a result, X-ray analyses revealed that N(4)AQDs could adopt not only folded and twisted forms but also intermediate structures, e.g., planar and twisted-folded forms, in pseudopolymorphs. Since there is a large difference in the electronic configuration among conformers, the colour tone of crystals varied from yellow to red depending on the conformation of overcrowded N(4)AQDs. Notably, red to near-infrared emission was observed for the crystal containing twisted conformers. This is the first example of exceptionally flexible quinodimethanes with tunable photophysical properties and multi-chromic behaviour based on multiple conformations.

Automated summary

Tetraarylated quinodimethanes have various conformations but obtaining all of them is challenging. To overcome this, flexible polyaryl-substituted tetraazaanthraquinodimethanes were designed with reduced steric hindrance. X-ray analysis showed that these molecules could adopt folded, twisted, and intermediate structures in pseudopolymorphs. Due to different electronic configurations, the crystal colors varied from yellow to red, with the crystal containing twisted conformers emitting red to near-infrared light. This represents the first flexible quinodimethanes with tunable photophysical properties and multi-chromic behavior based on multiple conformations.