Exceptionally Large Chiroptic Dissymmetry Factors in Films of Supramolecular Magic-Sized Clusters

A key goal of optical materials science is to develop self-assembling hierarchical materials from nanoparticle building blocks with interesting properties. Understanding the structure-property relationships within these systems will enable the rational design of advanced optical devices. Recently, our group has discovered that CdS magic-sized clusters organize into hexagonal fibers which can self-assemble into ordered films that have a hierarchical structure spanning seven orders of length-scale[1]. These films have demonstrated strong linear and circular dichroism (LD, CD) responses originating from transition dipole alignment and cluster-to-cluster coupling, respectively, but further characterization of the optical properties using lab-based measurements are obfuscated by the samples' mixed linear and chiral anisotropies[2, 3]. In this work, we pair two synchrotron techniques, microfocussed small-angle x-ray scattering (SAXS) and synchrotron radiation Mueller Matrix polarimetry (MMP) mapping, to unravel the structural and optical anisotropies present in the self-assembled nanocrystal films. These studies have revealed local chiroptical properties stronger than previous measurements from lab instruments; in fact, our measured dissymmetry factors (i.e., absorption g-factor) are the largest reported for inorganic semiconductors, and only an order of magnitude smaller than the theoretical maximum limit of 2. The MMP mapping reveals that the self-organized films contain millimeter-scale domains with a constant CD magnitude and handedness, whereas isotropic films (non-self-assembled) present much smaller micron-scale domains. Further, MMP and SAXS measurements show that within self-assembled, anisotropic films the transition dipoles of the clusters are macroscopically oriented in the same direction as the nm-scale fibers. In summary, these experiments have revealed a relationship between the self-assembly process, the orientation of supramolecular nanocrystal fibers, and the handedness of a film's response. These results - anomalously large chiral anisotropy, orientational control of the clusters and their transition dipoles, and enantiomeric control of chiral domains within nanocrystal films - enable chiral light control for a range of emerging technologies.<br/>[1] H. Han, S. Kallakuri, Y. Yao, C.B. Williamson, D.R. Nevers, B.H. Savitzky, R.S. Skye, M. Xu, O. Voznyy, J. Dshemuchadse, L.F. Kourkoutis, S.J. Weinstein, T. Hanrath, R.D. Robinson, Multiscale hierarchical structures from a nanocluster mesophase, Nat. Mater. 21 (2022) 518–525. https://doi.org/10.1038/s41563-022-01223-3.<br/>[2] Y. Yao, T.J. Ugras, T. Meyer, M. Dykes, D. Wang, A. Arbe, S. Bals, B. Kahr, R.D. Robinson, Extracting Pure Circular Dichroism from Hierarchically Structured CdS Magic Cluster Films, ACS Nano. (2022) acsnano.2c06730. https://doi.org/10.1021/acsnano.2c06730.<br/>[3] T.J. Ugras, Y. Yao, R.D. Robinson, Can We Still Measure Circular Dichroism with Circular Dichroism Spectrometers: the Dangers of Anisotropic Artifacts, Chirality. (2023) DOI: 10.1002/chir.23597