Anion and ether group influence in protic guanidinium ionic liquids

Area of Science:

• Materials Science
• Physical Chemistry
• Supramolecular Chemistry

Background:

• Ionic liquids (ILs) are versatile materials with broad applications.
• Protic ionic liquids (PILs) offer unique properties but require detailed structure-property understanding for targeted design.
• Guanidinium cations are promising for PILs, yet data guiding their design is limited.

Purpose of the Study:

• To synthesize and characterize novel protic ionic liquids (PILs) featuring guanidinium cations with ether or alkyl side chains and various anions.
• To elucidate the influence of anion choice and ether substitution on the thermal, transport, and spectroscopic properties of these PILs.
• To establish structure-property relationships for guiding the rational design of guanidinium-based PILs.

Main Methods:

• Synthesis of novel guanidinium-based protic ionic liquids (PILs).
• Experimental measurement of thermal properties (e.g., glass transition temperature) and transport properties (e.g., self-diffusion coefficients).
• Nuclear magnetic resonance (NMR) spectroscopy for structural and dynamic analysis.
• Analysis of liquid and crystalline structures, supported by *ab initio* computational methods.

Main Results:

• Hydrogen bonding between cation and anion is the dominant interaction, with strength correlating inversely to the proton affinity of the acid.
• Anion proton affinity influences proton localization on the cation, observable via NMR shifts and diffusion coefficients.
• Ether side groups and imide anions reduce glass transition temperatures and fragility, accelerating dynamics; these side chains adopt curled conformations due to dispersion interactions.

Conclusions:

• The study provides detailed insights into how anion type and ether side chain modifications affect the properties of guanidinium-based PILs.
• Proton localization versus exchange is controllable via anion choice, impacting material dynamics.
• Ether side chains promote specific conformations and enhance dynamic properties, offering a route for designing task-specific ionic liquids.