Solvent Quality Controls Macromolecular Structural Dynamics of a Dendrimeric Hydrogenase Model

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.