Turn-on and color-switchable red luminescent liquid crystals based on pyrrolopyrrole derivatives

A novel nitro-substituted pyrrolopyrrole derivative (TPPP-NO2) was synthesized and displayed liquid crystalline properties with red-emissive color. The luminescent liquid crystalline materials exhibited turn-on emission when transformed into the liquid crystalline (LC) phase at 135 °C, while turn-off at 185 °C when the LC phase melted into the isotropic liquid. The heat or mechanical force induced molecular arrangement from short-range to long-range ordered packing and the decreased intermolecular π–π interaction were responsible for the luminescence turn-on phenomenon. An application for detecting the changes of temperature was displayed by preparing TPPP-NO2 mixed with a polydimethylsiloxane (PDMS) thin film. This stimuli-responsive film could give high-temperature alert by switching the emission color from red to yellow when the temperature was increased to 150–200 °C.