Water-Mediated Liquid Crystalline Ionogel Formation in a Long-Chain Protic Ionic Liquid: An Anion-Centered Design Strategy

We report the synthesis of a novel long-chain protic ionic liquid (PIL), 1,5-diazabicyclo[4.3.0]non-5-ene laurate ([DBNH][H₂₃C₁₁COO]), that undergoes remarkable transformation into a stable ionogel upon controlled water addition. To the best of our knowledge, this is the first demonstration that a PIL forms lyotropic liquid-crystalline mesophases exhibiting birefringence. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) reveal smectic-like phases with temperature-driven structural transitions. The ionogel exhibits clustering triggered emission (CTE) and excitation-dependent luminescence behavior. The microheterogeneity of the gel was probed using Nile red (NR). The steady-state emission spectrum of NR shows a transition from bimodal to unimodal near 80 °C, indicating a temperature-driven gel to sol transition. Wavelength-dependent fluorescence lifetimes of NR reveal slow solvation dynamics, with the rise time observed at longer emission wavelengths, 660 nm, providing a sensitive means to monitor the microheterogeneity of the gel medium. In addition, the material's optical textures can be tunable upon the addition of aqueous inorganic salts, demonstrating its stimuli-responsive nature. Rheological measurements show clear ion-specific effects on the mechanical properties of the ionogels. The neat ionogel shows an elastic response, while the NaCl-containing ionogel exhibits the highest gel strength. In contrast, ionogels containing KCl, CsCl, and RbCl show weaker gel networks and more viscous behavior, indicating the soft nature of these gels. These findings introduce a new class of thermoresponsive, PIL-based lyotropic ionogels and demonstrate an anion-centered strategy for the first time that could be relevant for soft-matter applications.