Controllably realizing elastic/plastic bending based on a room-temperature phosphorescent waveguiding organic crystal

Abstract

Recently, the study of flexible (elastically bendable and plastically bendable) organic single crystals has become a hot research field in crystal engineering. In general, crystal elasticity and plasticity are incompatible with each other. Different from the applications of fluorescent crystals, the applications of room-temperature phosphorescence (RTP) materials generally ignore the crystallographic nature of large single crystals. Herein, we creatively combine elasticity and plasticity based on one RTP crystal 4,4'-dibromobenzil DBBZL. The in-depth study of the irreversible transformation between elastic bending and plastic bending provided important insights into the mechanism of both elastically bendable crystals and plastically bendable crystals in crystal engineering. The DBBZL crystal exhibits elastic bending (reversible) under external stress, whereas it shows plastic bending (irreversible) after excessive bending. Notably, the first phosphorescent optical waveguides of large single RTP crystals are realized not only in straight state, but also in elastic bent state and plastic bent state.

Fig. 1. Molecular structure of DBBZL (a); photographs of single DBBZL crystals taken under daylight (b) and 365 nm UV light (c); emission spectrum of the DBBZL crystal (d); phosphorescent decay curve of DBBZL measured at 514 nm (e).

Fig. 2. Photographs of elastic bending process of the DBBZL crystal compressed by tweezers.

Fig. 3. Crystal packing in DBBZL. Representative supramolecular synthons of the molecules between bending layers (a); crystal morphology with face indices (b), crystal structure viewed down the major bending face (010), the orange lines indicate the π–π stacking mode (c); and crystal structure viewed down the (–100) face (d).

Fig. 4. Photographs of elastic bending (a) and plastic bending (b) under mechanical stress applied through a pair of tweezers; schematic of the crystal before and after elastic or plastic bending (c); and photographs of the overbending process (d).

Fig. 5. Phosphorescent optical waveguide properties of single DBBZL crystals in straight shape (a–c), in elastic bent shape (d–f) and in plastic bent shape (g–i). Photographs of the crystal under UV light (a, d and g); images collected at different positions of the crystal upon 355 nm laser (b, e and h). Phosphorescent spectra measured at one end of the crystal with constantly changing distances between the end and the laser excitation positions (c, f and i).