Effect of Fluoroalkyl-Substituent in Bistolane-Based Photoluminescent Liquid Crystals on Their Physical Behavior

Photoluminescent liquid crystals (PLLCs) have attracted significant attention owing to their broad applicability in thermosensing and PL switching. Extensive efforts have been made to develop bistolane-based PLLCs containing flexible units at both molecular terminals, and it has been revealed that their PL behavior can switch with the phase transition between the crystalline and LC phases. Although slight modulation of the flexible unit structure dramatically alters the LC and PL behaviors, few studies into the modification of the flexible units have been conducted. With the aim of achieving dynamic changes in their physical behaviors, we developed a family of bistolane derivatives containing a simple alkyl or a fluoroalkyl flexible chain and carried out a detailed systematic evaluation of their physical behaviors. Bistolanes containing a simple alkyl chain showed a nematic LC phase, whereas switching the flexible chain in the bistolane to a fluoroalkyl moiety significantly altered the LC phase to generate a smectic phase. The fluoroalkyl-containing bistolanes displayed a stronger deep blue PL than their corresponding non-fluorinated counterparts, even in the crystalline phase, which was attributed to the construction of rigid molecular aggregates through intermolecular F···H and F···F interactions to suppress non-radiative deactivation.

Fluorinated Tolane Dyads with Alkylene Linkage: Synthesis and Evaluation of Photophysical Characteristics

Light-emitting materials have received considerable attention because of their broad applications as substrates in bio-imaging and sensing components, light-emitting displays, and lighting devices. Herein, we developed fluorinated tolane and bistolane derivatives containing fluorinated aromatic rings and demonstrated their intense photoluminescence (PL) characteristics in crystalline powder states. We focused on molecules showing varied PL behavior with a change in the molecular aggregated structures. We synthesized novel fluorinated tolane dyads consisting of fluorinated tolane-based π-conjugated scaffolds and flexible alkylene linkages to control both the electron-density distribution and molecular aggregated states. Fluorinated tolane dyads connected with an alkylene linkage showed blue PL in a dilute solution, but the PL efficiency achieved was low. In contrast, the crystalline powder of tolane dyad substrates exhibited dual emission—relatively intense blue to deep blue PL—originating from monomer and aggregate emission. The PL behavior changed significantly with the alkylene linkage and the application of a mechanical stimulus to the crystalline powder sample. The fluorinated tolane dyads developed in this study could serve as stimulus-responsive photoluminescent materials suitable for optical applications.