Rational design and synthesis of zwitterionic liquid dielectrics

Abstract

Zwitterions (ZIs) are emerging candidates for soft dielectrics but are limited by high melting points (T_m), glass transition temperatures (T_g), and viscosities (η) dramatically exceeding those of ionic liquids. To overcome these limitations, we synthesized 18 imidazolium-derived zwitterions with systematically varied composition at the (1) imidazolium tail (R_t), (2) imidazolium 2 position (R₂), (3) inter-charge spacer (R_s), and (4) anion (R_a). We found that long, flexible spacers yield stable zwitterionic liquids (ZILs), which we attribute to amplified entropy of fusion. Remarkably, stable ZILs with an elongated (6–16 atom length) inter-charge spacer, flexible tail, and a CF₃-sulfonimide anion are 100- to 500-fold less viscous at room temperature than a benchmark supercooled ZI with a 4-atom spacer and a sulfonate anion. Moreover, these previously unreported ZILs exhibit high permittivities ranging from ε_r,s = 290 (6-atom spacers) to ε_r,s = 404 (16-atom spacers), highlighting the promise of this class of polarizable soft matter.