Solvent Quality Controls Macromolecular Structural Dynamics of a Dendrimeric Hydrogenase Model

Abstract

We report a spectroscopic investigation of the ultrafast dynamics of the second-generation poly(aryl ether) dendritic hydrogenase model using two-dimensional infrared (2D-IR) spectroscopy to probe the metal carbonyl vibrations of the dendrimer and a reference small molecule, [Fe(μ-S)(CO) 3 ] 2 . We find that the structural dynamics of the dendrimer are reflected in a slow phase of the spectral diffusion, which is absent from [Fe(μ-S)(CO) 3 ] 2, and we relate the slow phase to the quality of the solvent for poly(aryl ether) dendrimers. We observe a solvent-dependent modulation of the initial phase of vibrational relaxation of the carbonyl groups, which we attribute to an inhibition of solvent assistance in the intramolecular vibrational redistribution process for the dendrimer. There is also a clear solvent dependence of the vibrational frequencies of both the dendrimer and [Fe(μ-S)(CO) 3 ] 2 . Our data represent the first 2D-IR study of a dendritic complex and provide insight into the solvent dependence of molecular conformation in solution and the ultrafast dynamics of moderately sized, conformationally mobile compounds.

Tracking Energy Transfer across a Platinum Center

“…Vibrational relaxation and thermalization in molecules occur via an intramolecular vibrational energy redistribution (IVR) process, which has been studied using a variety of experimental methods. − Energy transfer and thermalization in covalent networks − is better understood than the transfer across a metal center in a transition-metal complex for which only a few studies were reported. − The coordination bonds at the metal center are often weak resulting in very inefficient energy transfer across the metal. For example, the lifetime of CO stretching modes in some metal carbonyls reaches 1 ns. − A similar situation is encountered when an organic compound is attached to a metal or semiconductor surface. The surface binding energy is often small resulting in weak thermal conductivity through the interface. − In contrast to surfaces, transition-metal complexes feature high-frequency vibrational modes on each side of the metal atom, which can potentially participate in the exchange of high-frequency quanta across the metal center.…”