Molecular Mobility and Electrical Conductivity of Amino Acid-Based (DOPA) Ionic Liquid Crystals in the Bulk State and Nanoconfinement.

Abstract

This study investigates the molecular mobility, phase behavior, and electrical conductivity of dihydroxyphenylalanine-based ionic liquid crystals (DOPAn, with alkyl side chain lengths n = 12, 14, 16) with cyclic guanidiniumchloride headgroups in both bulk and nanoconfined states. Using broadband dielectric spectroscopy, differential scanning calorimetry, and fast scanning calorimetry, the research reveals a complex interplay between molecular structure, self-assembly, and dynamic behavior. In the bulk, DOPAn exhibits a phase sequence from plastic crystalline to hexagonal columnar and isotropic phases, driven by the formation of superdiscs and their columnar organization. Multiple relaxation processes are identified, including localized side-chain dynamics (γ-relaxation), ionic headgroup or core-related motions (α1-relaxation), and cooperative alkyl domain fluctuations (α2-relaxation), with conductivity decreasing as the side chain length increases. Under nanoconfinement in anodic aluminum oxide membranes, the phase behavior is altered: the Colh–Iso phase transition is suppressed, and a new confinement-induced α3-relaxation emerges, attributed to dynamics in an adsorbed interfacial layer. The DC conductivity is significantly reduced—up to four orders of magnitude—due to confinement effects, altered molecular orientation, and phase transitions, particularly the emergence of a nematic-like state in DOPA16. These findings underscore the critical role of molecular design, pore geometry, and surface chemistry in tuning the properties of ionic liquid crystals for advanced applications in nanofluidic, ion transport, and responsive materials.