Ultrafast 2D IR microscopy

Infrared Chemical Nano-Imaging: Accessing Structure, Coupling, and Dynamics on Molecular Length Scales

This Perspective highlights recent advances in infrared vibrational chemical nano-imaging. In its implementations of scattering scanning near-field optical microscopy (s-SNOM) and photothermal-induced resonance (PTIR), IR nanospectroscopy provides few-nanometer spatial resolution for the investigation of polymer, biomaterial, and related soft-matter surfaces and nanostructures. Broad-band IR s-SNOM with coherent laser and synchrotron sources allows for chemical recognition with small-ensemble sensitivity and the potential for sensitivity reaching the single-molecule limit. Probing selected vibrational marker resonances, it gives access to nanoscale chemical imaging of composition, domain morphologies, order/disorder, molecular orientation, or crystallographic phases. Local intra- and intermolecular coupling can be measured through frequency shifts of a vibrational marker in heterogeneous environments and associated inhomogeneities in vibrational dephasing. In combination with ultrafast spectroscopy, the vibrational coherent evolution of homogeneous sub-ensembles coupled to their environment can be observed. Outstanding challenges are discussed in terms of extensions to coherent and multidimensional spectroscopies, implementation in liquid and in situ environments, general sample limitations, and engineering s-SNOM scanning probes to better control the nano-localized optical excitation and to increase sensitivity.

A molecular view of the role of chirality in charge-driven polypeptide complexation

Polyelectrolyte molecules of opposite charge are known to form stable complexes in solution. Depending on the system conditions, such complexes can be solid or liquid. The latter are known as complex coacervates, and they appear as a second liquid phase in equilibrium with a polymer-dilute aqueous phase. This work considers the complexation between poly(glutamic acid) and poly(lysine), which is of particular interest because it enables examination of the role of chirality in ionic complexation, without changes to the overall chemical composition. Systematic atomic-level simulations are carried out for chains of poly(glutamic acid) and poly(lysine) with varying combinations of chirality along the backbone. Achiral chains form unstructured complexes. In contrast, homochiral chains lead to formation of stable β-sheets between molecules of opposite charge, and experiments indicate that β-sheet formation is correlated with the formation of solid precipitates. Changes in chirality along the peptide backbone are found to cause "kinks" in the β-sheets. These are energetically unfavorable and result in irregular structures that are more difficult to pack together. Taken together, these results provide new insights that may be of use for the development of simple yet strong bioinspired materials consisting of β-rich domains and amorphous regions.

Structural dynamics inside a functionalized metal-organic framework probed by ultrafast 2D IR spectroscopy

The structural elasticity of metal-organic frameworks (MOFs) is a key property for their functionality. Here, we show that 2D IR spectroscopy with pulse-shaping techniques can probe the ultrafast structural fluctuations of MOFs. 2D IR data, obtained from a vibrational probe attached to the linkers of UiO-66 MOF in low concentration, revealed that the structural fluctuations have time constants of 7 and 670 ps with no solvent. Filling the MOF pores with dimethylformamide (DMF) slows the structural fluctuations by reducing the ability of the MOF to undergo deformations, and the dynamics of the DMF molecules are also greatly restricted. Methodology advances were required to remove the severe light scattering caused by the macroscopic-sized MOF particles, eliminate interfering oscillatory components from the 2D IR data, and address Förster vibrational excitation transfer.