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 π-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, λem =573 nm, ΦF =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, λem =500 nm, ΦF =0.38), the original waveguide performance decreased under an applied bending stress; however, this crystal showed a unique bending mechanofluorochromism.

Synthesis and Field-Effect Transistor Application of π-Extended Lactam-Fused Conjugated Oligomers obtained by Tandem Direct Arylation.

Five π-extended lactam-fused conjugated oligomers (5FO, 5FS, 4FPO, 4FPS, and R-4FPO) were synthesized by the tandem direct arylation. The intermolecular oxidative direct arylation was applied in the second step. These conjugated oligomers had fine-tuned FMO energies predictable by the theoretical calculation and excellent thermal stabilities. 4FPO and 4FPS bearing tetrafluoropyridine exhibited lower LUMO energy levels (-3.20 eV and -3.39 eV, respectively) compared with others. Based on the X-ray crystallography, 4FPO was found to have a herringbone crystal packing and a considerably large electron transfer integral value (137 meV). 4FPO-based bottom-gate, bottom-contact FET device demonstrated an electron mobility of 5.2×10-3  cm2 V-1 s-1 as a result of an edge-on alignment on the SiO2 substrate.