Creating Elastic Organic Crystals of π-Conjugated Molecules with Bending Mechanofluorochromism and Flexible Optical Waveguide

To create low band-gap, fluorescent, and elastic organic crystal emitters, we focused on an extended pi-conjugated system based on: a) a planar conformation,b) a rigid structure, and c) controlled intermolecular interactions. Herein, we report on two fluorescent and highly flexible organic crystals (1 and 2) which could bend under an applied stress. The bent crystals rapidly recover their straight shape upon release of the stress. Crystal 1 with a tetrafluoropyridyl terminal unit and a lower band-gap energy (orange emission, lambda(em) = 573 nm, Phi(F) = 0.50), showed no bending mechanofluorochromism and had superior performance as an optical waveguide with reddish orange emission. The waveguide performance of the crystal did not decrease under bending stress. For crystal 2 with a pentafluorophenyl terminal unit (green emission, lambda(em) = 500 nm, Phi(F) = 0.38), the original waveguide performance decreased under an applied bending stress; however, this crystal showed a unique bending mechanofluorochromism.