Ionic liquid crystals based on amino acids and gemini surfactants: tunable phase structure, circularly polarized luminescence and emission color

Abstract

Circularly polarized luminescence (CPL)-active ionic liquid crystals (ILCs) featuring excitation wavelength-dependent (Ex-De) photoluminescence have significant application prospects in display devices. In addition, materials with tunable CPL responses are quite attractive from both theoretical and application perspectives. Herein, we successfully developed non-doped CPL ILCs by employing gemini surfactants and natural amino acids. Delightfully, both the phase structure and chiroptical property obviously depended on the type of spacer supplied by the gemini surfactant. When the spacer consisted of two flexible methylene groups, the complex 16-2-16·2Trp self-assembled into chiral smectic (SmC*) ILCs, giving rise to left-handed CPL (L-CPL). However, when the spacer was changed to a rigid butene or m-xylene unit, the complex 16-Butene-16·2Trp or 16-Ph-16·2Trp stacked into columnar hexagonal (Col_h) mesophases, accompanied with L-CPL or right-handed CPL (R-CPL), respectively. Our work also elucidated that adjusting the type of amino acid is an effective method to regulate the chiroptical characters of ILCs. Namely, the SmC* mesophase composed of 16-2-16·2Phe or 16-2-16·2Lys showed R- or L-CPL emission, respectively. It is worth noting that the natural amino acids effectively endowed these ILCs with favorable Ex-De performances based on the clustering-triggered emission (CTE) mechanism, thereby achieving multicolor emission by changing the excitation wavelength in situ. This the first time ILC materials have been constructed with tunable phase structures, CPL handedness, and emission colors. Thus, this work opens a pathway toward the development of novel CPL ILCs with abundant functions and deepens the understanding of the luminescent and chiral properties of amino acids.