Thursday July 11, 10h
Amphithéâtre de l’ISMO
Two-dimensional infrared spectroscopy (2D-IR) of metal-carbonyl complexes: from solution to cryogenic matrices
We have implemented an original two-dimensional infrared (2D-IR) spectroscopy setup, with high spectral resolution, using the femtosecond infrared laser source at ISMO. Combined with a cryogenic apparatus, it enables the study of the behaviour of molecular systems trapped in cryogenic matrices through the probing of their vibrational dynamics. The concept of “vibrational dynamics” encompasses the set of structural properties (vibrational modes, anharmonicities, couplings…) and dynamic properties (relaxation mechanisms, dephasing, decoherence, energy transfer…) related to molecular bond vibrations, which are sensitive to the various interactions between the molecular system and its environment. Cryogenic matrices provide inert, weakly interacting environments, making it possible to study a wide variety of molecular systems — stable or unstable — ranging from isolated small molecules to complexes or nanometric aggregates. The 2D-IR spectroscopy technique, by spreading the spectral information across two frequency dimensions and probing its time evolution after initial excitation, makes it possible to disentangle the various contributions to the vibrational dynamics of the system: it can thus distinguish between different modes or different species (either chemically distinct or differing in their local environments). We focused in particular on two molecules, the metal carbonyls W(CO)6 and Fe(CO)5, for which we studied the carbonyl bond stretching vibrations. These two molecules served as test systems for the development and characterization of our experimental setup. We first studied them in solution, then in various cryogenic matrices (N2, Ar, Kr). With the high spectral resolution of our setup, we revealed fine features in the vibrational structures of these two molecules. For Fe(CO)5 in solution, we analyzed the effects of solvents on molecular rotation and pseudorotation phenomena. In cryogenic matrices, we observed that the molecules are trapped in a multitude of sites, within which their symmetry is more or less perturbed. These perturbations manifest in phenomena such as the lifting of degeneracies or the infrared activation of certain vibrational modes. Based on the couplings between these different modes observed in the 2D-IR spectra, we were able to rationalize our observations and propose clear assignments into different site families. We also investigated, specifically for each mode and within the different site families, the various relaxation and intramolecular transfer dynamics. These results demonstrated that 2D-IR spectroscopy is particularly effective for exploring site and environmental effects in cryogenic matrices.