Two-Dimensional IR Spectroscopy: Mapping Structure and Dynamics in Complex Molecular Environments

Complex, dynamically heterogeneous environments such as lipid membranes, electrocatalytic interfaces, soft polymer matrices, crowded biological media, and phase-separated assemblies are characterized by spatially distinct microenvironments, multiscale structural dynamics, and short-range interactions. Microscopic interactions are poorly captured by bulk descriptors. In this article, we review how ultrafast two-dimensional infrared (2D IR) spectroscopy and related vibrational methods provide direct access to these complex molecular landscapes by resolving inhomogeneous ensembles, picosecond environmental fluctuations, and exchange between local states. When combined with vibrational probes and modern molecular simulation, these spectroscopic observables reveal how local interactions, such as electrostatics, hydrogen bonding (H-bonding), and ion pairing and aggregation generate microscopic heterogeneity and govern multiscale dynamics. We organize discussions around representative classes of complex environments, including hydration and electrostatics in lipid membranes, molecular environments and dynamics in biomolecular condensates, confinement and crowding in self-assembled and interfacial systems, including electrochemical interfaces, polymers/gels and soft matter, electrolytes and ion-specific effects, and aqueous cosolvent mixtures relevant to solvation and cryopreservation. Looking ahead, ongoing advancements in vibrational probe design, multidimensional spectroscopy, and molecular simulation are expected to enable more quantitative and predictive analyses of heterogeneous molecular environments across chemistry, materials science, and biology.