Lipophilic ion aromaticity is not important for permeability across lipid membranes

To clarify the contribution of charge delocalization in a lipophilic ion to the efficacy of its permeation through a lipid membrane, we compared the behavior of alkyl derivatives of triphenylphosphonium, tricyclohexylphosphonium and trihexylphosphonium both in natural and artificial membranes. Exploring accumulation of the lipophilic cations in response to inside-negative membrane potential generation in mitochondria by using an ion-selective electrode revealed similar mitochondrial uptake of butyltricyclohexylphosphonium (C₄TCHP) and butyltriphenylphosphonium (C₄TPP). Fluorescence correlation spectroscopy also demonstrated similar membrane potential-dependent accumulation of fluorescein derivatives of tricyclohexyldecylphosphonium and decyltriphenylphosphonium in mitochondria. The rate constant of lipophilic cation translocation across the bilayer lipid membrane (BLM), measured by the current relaxation method, moderately increased in the following sequence: trihexyltetradecylphosphonium ([P_6,6,6,14]) < triphenyltetradecylphosphonium (C₁₄TPP) < tricyclohexyldodecylphosphonium (C₁₂TCHP). In line with these results, measurements of the BLM stationary conductance indicated that membrane permeability for C₄TCHP is 2.5 times higher than that for C₄TPP. Values of the difference in the free energy of ion solvation in water and octane calculated using the density functional theory and the polarizable continuum solvent model were similar for methyltriphenylphosphonium, tricyclohexylmethylphosphonium and trihexylmethylphosphonium. Our results prove that both cyclic and aromatic moieties are not necessary for lipophilic ions to effectively permeate through lipid membranes.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Photoswitchable Layer-by-Layer Coatings Based on Photochromic Polynorbornenes Bearing Spiropyran Side Groups

Herein, we present the synthesis of linear photochromic norbornene polymers bearing spiropyran side groups (poly(SP-R)) and their assembly into layer-by-layer (LbL) films on glass substrates when converted to poly(MC-R) under UV irradiation. The LbL films were composed of bilayers of poly(allylamine hydrochloride) (PAH) and poly(MC-R), forming (PAH/poly(MC-R)) _n coatings. The merocyanine (MC) form presents a significant absorption band in the visible spectral region, which allowed tracking of the LbL deposition process by UV-vis spectroscopy, which showed a linear increase of the characteristic MC absorbance band with increasing number of bilayers. The thickness and morphology of the (PAH/poly(MC-R)) _n films were characterized by ellipsometry and scanning electron microscopy, respectively, with a height of ∼27.5 nm for the first bilayer and an overall height of ∼165 nm for the (PAH/poly(MC-R))₅ multilayer film. Prolonged white light irradiation (22 h) resulted in a gradual decrease of the MC band by 90.4 ± 2.9% relative to the baseline, indicating the potential application of these films as coatings for photocontrolled delivery systems.

Self-propelled chemotactic ionic liquid droplets

[P_6,6,6,14][Cl] droplets show self-propelled movement within open fluidic channels along the liquid–air interface in directions determined by external chemical gradients.

Multiscale modeling of the trihexyltetradecylphosphonium chloride ionic liquid

Hierarchical trihexyltetradecylphosphonium cationic and chloride anionic models.