Difluorinated tolane-based photoluminescent liquid crystals with imidazolium salt-terminated flexible chains: Thermophysical and photophysical properties

Abstract

Photoluminescent liquid crystal (PLLC) molecules have attracted significant interest because they exhibit both photoluminescent (PL) and liquid crystal (LC) properties within a single-composition molecular system. In the quest to develop PLLC molecules, researchers have explored ionic compounds incorporating ionic moieties in flexible chains or mesogenic structures. Notable examples include disk-shaped columnar PLLCs and smectic (Sm) PLLCs, which exhibit distinct layers of ionic moieties, counterions, and luminescent mesogens. Ionic LC molecules exhibiting the Sm phase at low temperatures and whose phase transition temperatures can be modulated by tuning the ionic moiety show promise as PLLCs. However, reports on Sm PLLC molecules remain scarce. Based on previous findings that difluorinated tolanes can emit fluorescence in the crystalline (Cr) state, we explored their potential as luminescent mesogens for the development of efficient Sm PLLC compounds. In this study, we designed a rod-shaped compound featuring a difluorinated tolane skeleton as a luminescent mesogen, a decyleneoxy chain for stabilizing the LC phase, and an imidazolium ion as an ionic moiety at the chain end to induce the smectic A (SmA) phase. Upon investigating their thermophysical properties, all compounds demonstrated thermotropic LC attributes and contained a SmA phase. Notably, the melting temperature decreased with increasing anion volume. Photophysical property evaluation revealed that the compounds displayed blue photoluminescence in both dilute solutions and Cr states, with no counteranion-dependent variations in photoluminescence observed in the solution. However, the photoluminescence behavior in the Cr-state specimens varies with different counteranions, demonstrating an increased photoluminescence wavelength with increasing anion volume. Furthermore, the Cr ⇄ LC phase transition altered the photoluminescence behavior, highlighting the potential of our molecular design strategy for developing temperature-responsive PL materials. These findings provide valuable insights for designing new PLLC molecules that can serve as effective fluorescent sensors across a range of temperatures, including room temperature.