Innovative use of liquid crystalline acids as color developers in leuco dye-based temperature sensors

Abstract

Novel temperature sensors with unique optical properties, based on 4-alkylbenzoic acid developers (CnBA, where n is the number of carbon atoms in the alkyl chain ranging from 4 to 6), which exhibit a liquid crystalline phase, and 6′-(diethylamino)-1′,2′-benzofluoran (BF) leuco dye are reported. The main aim of this work is to investigate how the molecular packing of CnBA in different phases affects the development of BF. Various techniques were used to study the prepared temperature sensors' phase transitions and thermal stability. The spectroscopic properties of BF : CnBA (1 : 3) were investigated, using temperature-dependent UV-Vis absorption and emission spectroscopy, and the results show that the sensors demonstrate reversible color-changing properties. When the CnBA developers are at room temperature, the materials are pink and emit orange light, while at approximately 105 °C they turn white and emit yellow light. Above that temperature, the sensors return to a pink and orange light emission. Therefore the prepared materials can serve as indicators that inform about not only exceeding a certain temperature threshold but also reaching temperature ranges.