Solid-State Photoluminescence Modulation of trans-Alkoxy-Nitrostilbene Dyes by Triggering the Solidification of Mesophases via External Stimuli

Stimuli-responsive luminescent materials in solid state have many useful applications such as optical recording. However, reversible solid-solid phase transitions are not easy due to the restricted movements of molecules. In order to modulate solid-state emission properties via external stimuli, we aimed to induce various crystallization processes from the liquid-crystalline states of trans-alkoxy-nitrostilbene dyes. The structure-property relationships in the crystalline and liquid-crystalline states of the nitrostilbenes were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SCD), and photoluminescence spectroscopy (PL). Three types of slipped-stacking structures, which display distinct luminescent intensities, were found in their crystalline state. The photoluminescence color of these dyes in the crystalline state changes from green to yellow depending on the length of their substituted alkyl chains. Various external stimuli, including heat, mechanical force, and ultrasound, were applied to the liquid-crystalline state at supercooled temperatures to trigger different crystallization processes. Accordingly, a number of polymorphs were obtained that displayed various luminescent properties. Among these solids, enhanced luminescence of trans-4-undecyloxy-4'-nitrostilbene (NS11) was obtained through packing changes caused by mechanical force and ultrasonication. The reversible fluorescent properties can be applied to fluorescent recording and sensing.