Anisotropic Poisson's Effect and Deformation-induced Fluorescence Change of Elastic 9,10-Dibromoanthrathene Single Crystals.

Elastic organic crystals have attracted considerable attention as next-generation flexible smart materials. However, the detailed information on both molecular packing change and macroscopic mechanical crystal deformations upon applied stress are still insufficient. Here we report that fluorescent single crystals of 9,10-dibromoanthracene are elastically bendable and stretchable, which allows a detailed investigation of the deformation behavior. We clearly observed a Poisson's effect for the crystal, where the short axes (i.e., b and c-axes) of the crystal are contracted upon elongation along the long axis (i.e., a-axis). Moreover, we found that the Poisson's ratios along the b-axis and c-axis are largely different (i.e., anisotropic Poisson's effect). Theoretical molecular simulation suggests that the tilting motion of the anthracene may be responsible for the large deformation along the c-axis. Spatially resolved photoluminescence (PL) measurement of the bent elastic crystals reveal that the PL spectra at the outer (elongated), central (neutral) and inner (contracted) sides are different from each other. Furthermore, we observe a reversible change in the PL spectra upon applying and releasing the elongation stress.