Multi-stimuli, color switchable emitter by solvent induced polymorph synthesis

Abstract

Polymorphism has drawn great attention in tuning organic solid-state luminescent materials by alteration of molecular packing structure and has considerable potential application in optoelectronic devices and fluorescence sensors. Herein, two polymorphs based on dicyanodistyrylbenzene (DCS) compound were obtained by solvent induced synthesis. We have comprehensively investigated photophysical properties (such as photoluminescence colors, lifetime and PL quantum yield) of the two polymorphs, blue one αDCFDCS (Aqua) and green one αDCFDCS (Green). The single crystal structure analysis and XRD powder pattern shows that there is a slightly packing dislocation in a-axis account for the difference between the two phases. Mechanical grinding can lead to an alternation in photoluminescence color from blue to green. The differential thermal analysis (DTA) measurement discloses αDCFDCS (Aqua) is the metastable state of the compound. Both of two polymorphs show thermochromic and can be reversibly switched upon heating and cooling, which is attributing to the crystal cell expansion with the temperature increasing revealed by in situ XRD. Moreover, fuming with NH3 also causes the color change via Lewis acid base reaction. Upon heating or being exposed to HCl, it will return to the original state. The results of the polymorphism not only provide an insight into molecular packing and emission properties, but also pave the way for understanding the solid luminescence change by the multi-stimuli response.