Near-room-temperature phase-change fluorescent molecular rotor and its hybrids

A unique molecular rotor consisting of a tetraphenylethylene derivative with a paraffin wax-like long alkyl chain (TPE18) is synthesized and compared to other derivatives containing shorter alkyl chains. In differential scanning calorimetry, TPE18 shows an endothermic peak (enthalpy of fusion, ΔH_fus = 55.3 J g⁻¹) at 45 °C during the heating process and an exothermic peak at 38.6 °C during the cooling process. TPE18 exhibits critical changes in the fluorescence intensity and color during the crystal–melt phase change. TPE18 can be dissolved by an octadecane melt and then absorbed by filter paper. When the waxed paper is exposed to the heat sources of an IR laser or a finger, high-resolution fluorescence images of the characters and fingerprints appear on the paper. TPE18 also serves as a host matrix in a melt state that was easily mixed with the guest fluorophores of Alq3 and rubrene. Homogeneous hybrid solids with defined features are obtained without any significant segregation after cooling to room temperature. Fluorescence images with various colors are drawn onto the surfaces of glass slides or papers using the hybrids as crayons.