The dynamics of organic cations in metal halide hybrid perovskites (MHPs) have been investigated using numerous experimental and computational techniques because of their suspected effects on the properties of MHPs. In this Perspective, we summarize and reconcile key findings and present new data to synthesize a unified understanding of the dynamics of the cations. We conclude that theory and experiment collectively paint a relatively complete picture of rotational dynamics within MHPs. This picture is then used to discuss the consequences of structural dynamics for electron−phonon interactions and their effect on material properties by providing a brief account of key studies that correlate cation dynamics with the dynamics of the inorganic sublattice and overall device properties.M etal halide perovskites (MHPs) are enjoying considerable academic and industrial interest due to their high photovoltaic power conversion efficiencies (22% as of July 2017), 1 ease and low cost of production, and broad material tuneability. 2 Besides applications in photovoltaics, MHPs may find use as solar thermoelectric materials, 3 LEDs, lasers, 4 and nonvolatile memory. 5 The macroscopic properties of MHPs emerge from diverse microscopic phenomena, including crystal structure, 6 defects, cation disorder, 7 ion migration, 8 and spin−orbit coupling. 9 Linking these phenomena to macroscopic performance is challenging because of the complex interplay between them. In this Perspective, we will address cation dynamics, their interactions with lattice vibrations (phonons), and their hypothesized effects on device performance.MHPs share a common ABX 3 (perovskite) structure as shown in Figure 1. This consists of (A) an organic molecule (e.g., methylammonium (MA) or formamidinium (FA)) or a large inorganic atom (e.g., caesium), (B) a metal dication (e.g., Pb (II) or Sn (II) ) and, (X) halide anions (Cl, Br, I, or some combination thereof). The M and X species form a cornersharing octahedral framework. The charge-balancing A-site cation occupies the central cavity generated by this framework and has a strong effect on the MHP structure, quantified by the Goldschmidt tolerance factor. 10 A-site ions just above or below the optimum size may induce tilting of the octahedra within the BX 3 sublattice away from a cubic perovskite. Mixing of ions will lead to inhomogeneity in the local structure, which may result in coupling of cation motion and equilibrium distribution to more complex local lattice dynamics.
The macroscopic mechanical properties of biological hydrogels are broadly studied and successfully mimicked in synthetic materials, but little is known about the molecular interactions that mediate these properties. Here, we use two-dimensional infrared spectroscopy to study the pH-induced gelation of hyaluronic acid, a ubiquitous biopolymer, which undergoes a transition from a viscous to an elastic state in a narrow pH range around 2.5. We find that the gelation originates from the enhanced formation of strong interchain connections, consisting of a double amide–COOH hydrogen bond and an N–D–COO– hydrogen bond on the adjacent sugars of the hyaluronan disaccharide unit. We confirm the enhanced interchain connectivity in the elastic state by atomic force microscopy imaging.
Enhancing the thermal stability of proteins is an important task for protein engineering. There are several ways to increase the thermal stability of proteins in biology, such as greater hydrophobic interactions, increased helical content, decreased occurrence of thermolabile residues, or stable hydrogen bonds. Here, we describe a well-defined polymer based on β-helical polyisocyanotripeptides (TriPIC) that uses biological approaches, including hydrogen bonding and hydrophobic interactions for its exceptional thermal stability in aqueous solutions. The multiple hydrogen bonding arrays along the polymer backbone shield the hydrophobic core from water. Variable temperature CD and FTIR studies indicate that, on heating, a better packed polymer conformation further stiffens the backbone. Driven by hydrophobic interactions, TriPIC solutions give fully reversible hydrogels that can withstand high temperatures (80 °C) for extended times. Cryo-scanning electron microscopy (cryo-SEM), small-angle X-ray scattering (SAXS), and thorough rheological analysis show that the hydrogel has a bundled architecture, which gives rise to strain stiffening effects on deformation of the gel, analogous to many biological hydrogels.
We investigate the molecular geometry of the carboxyl group of formic acid in acetonitrile and aqueous solutions at room temperature with two-dimensional infrared spectroscopy (2D-IR). We found that the carboxyl group adopts two distinct configurations: a configuration in which the carbonyl group is oriented antiparallel to the hydroxyl (anti-conformer), and a configuration in which the carbonyl group is oriented at an angle of ∼60° with respect to the hydroxyl (syn-conformer). These results constitute the first experimental evidence that carboxyl groups exist as two distinct and long-living conformational isomers in aqueous solution at room temperature.
Tardigrades are microscopic animals that survive desiccation by inducing biostasis. To survive drying tardigrades rely on intrinsically disordered CAHS proteins that form gels. However, the sequence features and mechanisms underlying gel formation and the necessity of gelation for protection have not been demonstrated. Here we report a mechanism of gelation for CAHS D similar to that of intermediate filaments. We show that gelation restricts molecular motion, immobilizing and protecting labile material from the harmful effects of drying. In vivo, we observe that CAHS D forms fiber-like condensates during osmotic stress. Condensation of CAHS D improves survival of osmotically shocked cells through at least two mechanisms: reduction of cell volume change and reduction of metabolic activity. Importantly, condensation of CAHS D is reversible and metabolic rates return to control levels after CAHS condensates are resolved. This work provides insights into how tardigrades induce biostasis through the self-assembly of CAHS gels.
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