Red-Emissive Organic Crystals of a Single-Benzene Molecule: Elastically Bendable and Flexible Optical Waveguide

Organic
crystals are easily cracked into pieces or powders under
applied stress because of their intrinsic brittle nature. This undesired
mechanical property directly limits their application in flexible
optical and optoelectronic devices. Herein, we developed a compact
single-benzene molecule dimethyl 2,5-bis­((2-hydroxyethyl)­amino)­terephthalate,
which was easily crystallized to form two polymorphs, A and B. Featuring
a single-benzene π-system, both polymorphs A and B display red
fluorescence in crystals. More importantly, crystals of polymorph
A are flexible and can be elastically bent under mechanical force.
Given these advantages, a flexible optical waveguide has been realized
in the crystal of polymorph A with a bent shape, highlighting its
potential application in flexible devices. In addition, the thermal
transformation of crystals from polymorph A to polymorph B, which
was accompanied by the change of optical property as well as mechanical
elasticity, has been observed.