A flexible crystal was designed and synthesised. The irreversibly deformable crystals were successfully used as waveguides.
Source: BAM, division Structure Analysis
Crystals are often brittle and break under mechanical stress. This behaviour greatly limits their potential applications. In recent years, mechanically flexible crystals have been discovered. These crystals can be bent either irreversibly (plastic) or reversibly (elastic). The mechanical behaviour of the crystal depends on its crystal structure.
The compound 4-bromo-6-[(6-chloropyridin-2-ylimino)methyl]phenol (CPMBP) was designed with the aim of obtaining a mechanically plastically deformable crystal. The synthesis was performed mechanochemically, allowing CPMBP to be prepared rapidly and in an environmentally friendly manner. The powder obtained after synthesis was crystallized out and the mechanical properties of the single crystals were investigated. It was found that some crystals could be plastically deformed, but others broke. X-ray crystal structure analyses proved that two different polymorphs were obtained. The crystal structures of both forms were solved. The different mechanical properties could be explained based on the crystal structures. The crystals of Form I (brittle) have an interlocked structure that causes the brittle behaviour. In the case of Form II (plastic), slip planes have formed, which allow permanent displacement of molecules within the crystal lattice. An investigation of the optical properties revealed that Form II (compared to Form I) has a blue-shifted emission maximum as well as an approximately 20-fold higher quantum yield. Density functional theory (DFT) calculations suggest that the different optical properties result from the different π...π distances within the crystal structures.
The plastic crystals could be successfully used as optical waveguide. For this purpose, the experiments were carried out in the straight and bent crystal, respectively. The results indicate that the bent crystal does not lose its ability as a waveguide. By changing the wavelength of the light source, active or passive waveguiding can be performed. In active waveguiding, absorption of the incident light takes place, which excites photoluminescence. Passive waveguiding involves the use of a wavelength that is not absorbed and therefore passes through the crystal unchanged.
Tuning the mechanical flexibility of organic molecular crystals by polymorphism for flexible optical waveguides
Torvid Feiler, Biswajit Bhattacharya, Adam Michalchuk, S.-Y. Rhim, V. Schöder, E. List-Kratochvil, Franziska Emmerling
published in CrystEngComm, Vol. 23, issue 34, pages 5815 - 5825, 2021
BAM, Department Materials Chemistry, Division Structure Analysis