Competing Allosteric Mechanisms for Coordination-Directed Conformational Changes of Chiral Stacking Structures with Aromatic Rings.

This work revealed that significant asymmetric nonlinear effects can be found in a coordination-directed conformational alteration through competing allosteric mechanisms. Toward this aim, we have prepared new chiral bridging ligands [( S, S)- and ( R, R)-Im2An] containing an anthracene ring as a spacer with two ethynyl-linked chiral imidazole groups at the 9,10-positions. The ( S, S)- and ( R, R)-Im2An ligands (L) spontaneously form the assemblies with Zn2+ ions (M) in solution phase, giving L4M2-type assemblies with a general formula [( S, S)- or ( R, R)-Im2An]4(Zn2+)2. NMR studies revealed that the [( S, S)-Im2An]4(Zn2+)2 assembly has an anthracene dimer structure with a parallel-displaced geometry, leading to relatively small circular dichroism (CD) signals, as expected for nonchiral objects. Conversely, subsequent addition of chiral coligands [( R)- or ( S)-Ph-box] to [( S, S)-Im2An]4(Zn2+)2 afforded an alternative Zn2+ assembly with general formula [( R)- or ( S)-Ph-box]2[( S, S)-Im2An]2(Zn2+)2, where the chiral coligands expel two of the ( S, S)-Im2An ligands that were singly bound to the Zn2+ ions in the original [( S, S)-Im2An]4(Zn2+)2 assembly. This ligand-exchange reaction causes conformational alteration from a parallel-displaced structure to a twisted stacking between the anthracene rings inside the Zn2+ assembly, which results in a significant enhancement of CD signals due to excitonic interactions of the chiral anthracene dimer. Dissymmetry factor ( gCD) for CD due to chiral stacking structures shows a significant inverse sigmoidal response to the enantiomeric excess of the chiral coligands. The observed nonlinear phenomena are results of the two conflicting mechanisms, homochiral cooperative association (homochiral self-sorting) to form CD-active assemblies [( S)- or ( R)-Ph-box]2[( S, S)-Im2An]2(Zn2+)2 versus heterochiral cooperative dissociation of [( S, S)-Im2An]4(Zn2+)2 by sequestering of Zn2+ inside the assembly through formation of a heterochiral 2:1 Zn2+ complex ([( R)-Ph-box][( S)-Ph-box]Zn2+). The presented mechanisms provide a new strategy for generating switch-like OFF/ON states in chiral systems.