Abstract
Predictable stereoselective formation of supramolecular assembly is generally believed to be an important but complicated process. Here, we show that point chirality of a ligand decisively influences its supramolecular assembly behavior. We designed three closely related chiral ligands with different point chiralities, and observe their self-assembly into europium (Eu) tetrametallic tetrahedral cages. One ligand exhibits a highly diastereoselective assembly into homochiral (either ΔΔΔΔ or ΛΛΛΛ) Eu tetrahedral cages whereas the two other ligands, with two different approaches of loosened point chirality, lead to a significant breakdown of the diastereoselectivity to generate a mixture of (ΔΔΔΔ and ΛΛΛΛ) isomers. The cages are highly emissive (luminescence quantum yields of 16(1) to 18(1)%) and exhibit impressive circularly polarized luminescence properties (|glum|: up to 0.16). With in-depth studies, we present an example that correlates the nonlinear enhancement of the chiroptical response to the nonlinearity dependence on point chirality.
Highlights
Predictable stereoselective formation of supramolecular assembly is generally believed to be an important but complicated process
Utilization of this type of linker for potential guest encapsulated-tetrahedral cage formation was first demonstrated with transition metals a few years ago;[32] here we aim to engineer a similar topology of luminescent supramolecular assemblies with lanthanide ions: Ln4L6 tetrahedral cages (Fig. 1b, c)
The significant differences in NMR, circular dichroism (CD), and circularly polarized luminescence (CPL) observations arising from the tetrahedral cage formation with ligands L1 vs. L2 or L3 can be associated to the different extent of stereoisomer formation
Summary
Predictable stereoselective formation of supramolecular assembly is generally believed to be an important but complicated process. Great effort has been devoted to investigating the long-range linker effect[17, 18] and chiral nonlinear effect[19] in order to achieve stereoselective control of tetrahedral cages, but these studies mainly focused on transition metals (M)-related polyhedra, M4:L6 or M4:L4 (L is either a C2- or C3-symmetric ligand), in which the metals usually have well-defined and predictable coordination geometries[20]. Introducing a predisposed point chiral moiety (point chirality) onto the ligand is a general approach to relay chiral information to an overall stereochemistry of self-assembled tetrahedral cages[21,22,23], but the challenge lies in designing a suitable point chirality to selectively favor formation of one stereoisomeric structure without any depreciation of chiral information. The driving force and formation mechanism behind achieving prominent stereoselectivity for higher order coordination geometry edifices—such as in lanthanide tetrahedral cages—remain unclear and more studies are necessary
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