Multicyclopropyl-functionalized hypergolic ionic liquids with enhanced energetic performance

This study presents a class of hypergolic ionic liquids featuring cyclopropyl groups as key structural motifs. The synthesized ionic liquids were comprehensively characterized, and showed wide liquid-phase range with melting points below −40 °C and thermal decomposition temperatures over 289 °C. The intrinsic folded geometry of cyclopropyl groups facilitates dense molecular packing, resulting in high densities ( ρ  = 1.04–1.09 g∙cm −3 ). The inherent strain energy in the cyclopropyl rings enhances their energetic performance, as indicated by heat of formation (Δ f H  = 1.52–2.79 kJ∙g −1 ), specific impulse ( I sp  ≈ 320 s), and notably density-specific impulse ( ρI sp  > 440 s∙g∙cm −3 ), which significantly surpasses that of conventional unsymmetrical dimethylhydrazine (UDMH, 402.9 s∙g∙cm −3 ). Additionally, all ionic liquids exhibit hypergolic behavior upon contact with white fuming nitric acid. This work provides an innovative strategy for the design of strained-ring hypergolic ionic liquids as high-energy propellants.