Heterogeneity in Single-Molecule Observables in the Study of Supercooled Liquids

Abstract

Bulk approaches to studying heterogeneous systems obscure important details, as they report average behavior rather than the distribution of behaviors in such environments. Small-molecule and polymeric supercooled liquids, which display heterogeneity in their dynamics without an underlying structural heterogeneity that sets those dynamics, are important constituents of this category of condensed matter systems. A variety of approaches have been devised to unravel ensemble averaging in supercooled liquids. This review focuses on the ultimate subensemble approach, single-molecule measurements, as they have been applied to the study of supercooled liquids. We detail how three key experimental observables (single-molecule probe rotation, translation, and fluorescence lifetime) have been employed to provide detail on dynamic heterogeneity in supercooled liquids. Special attention is given to the potential for, but also the challenges in, discriminating spatial and temporal heterogeneity and detailing the length scales and timescales of heterogeneity in these systems.

Ideal probe single-molecule experiments reveal the intrinsic dynamic heterogeneity of a supercooled liquid

“…The 92τ fit,med and 173τ fit,med median trajectory length data sets were obtained from five different temperatures from 243 K to 249 K, whereas 415τ fit,med and 765τ fit,med data sets were obtained at the single temperature of 248 K. As median trajectory length increases, the β distribution exhibits a shift to lower values and significant narrowing. The shape of the distribution is reasonably symmetric for the 415τ fit,med and 765τ fit,med trajectory length data sets, whereas shorter trajectory length data sets display tails at high β, likely due to statistical fluctuations and poor fits associated with short trajectories (19). Fig.…”

“…As such, to test whether the measured QE stretched relaxation can be described as a sum of exponentials spread in time and space, an attempt was made to recover the distribution predicted by the ILT of the QE ACF. This test can also validate that the breadth of the measured τ fit distribution reflects dynamic heterogeneities rather than experimental limitations such as finite trajectory length (19).…”

“…To mirror dynamic heterogeneity in a host system, fluorescent probe molecules need to be sufficiently small to reside within distinct dynamic regions, sufficiently fast to report dynamic exchange on the time scale of host structural relaxation, and sufficiently long-lived to capture potential longer-lived heterogeneities and to avoid artifacts associated with finite trajectory length (19,20). Fluorescent molecules with suitable photophysical properties including high quantum yield and photostability are typically large and slow compared with host molecules, potentially obscuring host heterogeneities by averaging over them in space and/or time.…”

“…While such experiments have shown potential in probing dynamic heterogeneity in glassforming systems including molecular (9-13) and polymeric glass formers (14)(15)(16)(17)(18), limitations related to characteristics of the single-molecule fluorescent probes have substantially hindered these experiments and interpretation thereof (8,19).…”

Nano-viscosity of supercooled liquid measured by fluorescence correlation spectroscopy: Pressure and temperature dependence and the density scaling

“…One of the important and unusual features of the dynamics of supercooled liquids is its heterogeneity. In recent years it has been studied theoretically [1] but the experimental evidence remains very scarce [2,3]. It mainly comes from the studies on a single molecule level [3,4] The combination of microscopy and high pressure is challenging and technically difficult since the main requirements for high resolution microscopy and for applying high pressure are in conflict.…”