The thermo-responsive behavior in molecular crystals of naphthalene diimides and their 3D printed thermochromic composites†
Herein, we communicate that altering the number of carbon atoms on the alkoxyphenyl substituent in naphthalene diimides results in tunable thermo-salient behavior across a variety of temperatures. Additionally, these compounds were found to display reversible thermochromic behavior in the single crystalline state. We analyzed this behavior using differential scanning calorimetry (DSC), single crystal X-ray diffraction (SXRD), powder XRD (PXRD), and hot-stage microscopy. The heptoxyphenyl-, octoxyphenyl-, and nonoxyphenyl-derivatives exhibited “acrobatic” behavior—namely, bending, jumping, and splitting—upon an irreversible phase transition. This study contributes to a developing paradigm in the understanding of certain naphthalene diimide single crystals that the energy associated with irreversible phase transitions has the potential to perform mechanical work, and that the temperature at which this energy can be fine-tuned by selecting an appropriate alkoxyphenyl substituent. Furthermore, we show that these thermochromic NDI derivatives can be incorporated into commercially-available, polymeric 3D printing materials and the resulting printed mixed polymer-crystalline objects still exhibit thermochromic behavior after incorporation.