Unraveling Confined Dynamics of Guests Trapped in Self-Assembled Pd₆L₄ Nanocages by Ultrafast Mid-IR Polarization-Dependent Spectroscopy

Self-assembled coordination cages form host–guest complexes through weak noncovalent interactions. Knowledge of how these weak interactions affect the structure, reactivity, and dynamics of guest molecules is important to further the design principles of current systems and optimize their specific functions. We apply ultrafast mid-IR polarization-dependent pump–probe spectroscopy to probe the effects of two Pd₆L₄ self-assembled nanocages on the properties and dynamics of fluxional group-VIII metal carbonyl guest molecules. We find that the interactions between the Pd₆L₄ nanocages and guest molecules act to alter the ultrafast dynamics of the guests, restricting rotational diffusional motion and decreasing the vibrational lifetime.