Elastic orange emissive single crystals of 1,3-diamino-2,4,5,6-tetrabromobenzene as flexible optical waveguides

Single crystals of monoaromatic compounds exhibiting both mechanical softness and optical properties have attracted significant scientific interest in recent years, but they are very scarce. Herein, single crystals of 1,3-diamino-2,4,5,6-tetrabromobenzene (compound 1) were shown to exhibit excellent and unexpected elastic behaviour upon mechanical deformation. The crystal structure analysis of 1 illustrates the intermolecular interactions (Br⋯Br, N⋯Br and N–H⋯Br) that are responsible for such flexible behaviour. Furthermore, the elastic modulus of crystals of 1 is found to be around 12 GPa, which was determined through nanoindentation experiments on the bendable major (100) face. In addition, interestingly, compound 1 was also found to exhibit optical properties in the longer wavelength region (orange emission) both in the solid and solution states despite not having any significant extended conjugation. To rationalize the experimentally observed optical behaviour, time-dependent density functional theory (TDDFT) calculations have been performed. The potential use of single crystals of 1 has been tested in optical waveguide applications. The present work demonstrates the significance of soft interaction guided packing in the origin of both mechanical and optical properties of organic single crystals.