Séminaire

Mardi 14 Mars 2017 à 11h00.

From Linear to Nonlinear: Exploiting the Ubiquitous Chirality of Thiolate-Protected Gold Clusters for Second-Order Nonlinear Optics

''The structures of thiolate-protected gold clusters often show inherent chirality, even when protected by achiral ligands.1 In the case of clusters that are not chiral, the symmetry can be broken by protection with chiral thiolate ligands. Second-order nonlinear optics (Second-Harmonic Generation, Hyper-Rayleigh Scattering) require the absence of centrosymmetry. This is ensured in chiral systems. We studied the second-order nonlinear optical properties of a series of thiolateprotected gold clusters,2 and determined their first hyperpolarizabilities in solution using Hyper-Rayleigh Scattering. Different strategies to break the symmetry of the clusters were investigated: a) inherently chiral clusters, b) induced chirality using chiral ligands and c) isoelectronic substitution of gold atoms by silver.3 Thiolateprotected gold clusters can be understood as ultrasmall nanoparticles with defined structures. In contrast to larger nanoparticles, they do not support a localized surface plasmon resonance. The differences of the second-order nonlinear optical properties of clusters and nanoparticles are discussed. This also opens the question for critical minimum sizes that can support plasmonic behavior. Thiolate-protected gold clusters are promising candidates for multiphoton imaging applications. References (1) Knoppe, S.; Bürgi, T. Acc. Chem. Res. 2014, 47, 1318. (2) Knoppe, S.; Vanbel, M. K.; Van Cleuvenbergen, S. J.; Vanpraet, L.; Burgi, T.; Verbiest, T. J. Phys. Chem. C 2015, 119, 6221. (3) van Steerteghem, N.; Van Cleuvenbergen, S. J.; Deckers, S.; Kumara, C.; Dass, A.; Häkkinen, H.; Clays, K.; Verbiest, T.; Knoppe, S. Nanoscale 2016, 8, 12123.''