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)-ImAn] 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)-ImAn ligands (L) spontaneously form the assemblies with Zn ions (M) in solution phase, giving LM-type assemblies with a general formula ( S, S)- or ( R, R)-ImAn. NMR studies revealed that the ( S, S)-ImAn 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)-ImAn afforded an alternative Zn assembly with general formula [( R)- or ( S)-Ph-box]( S, S)-ImAn, where the chiral coligands expel two of the ( S, S)-ImAn ligands that were singly bound to the Zn ions in the original ( S, S)-ImAn assembly. This ligand-exchange reaction causes conformational alteration from a parallel-displaced structure to a twisted stacking between the anthracene rings inside the Zn assembly, which results in a significant enhancement of CD signals due to excitonic interactions of the chiral anthracene dimer. Dissymmetry factor ( g) 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]( S, S)-ImAn versus heterochiral cooperative dissociation of ( S, S)-ImAn by sequestering of Zn inside the assembly through formation of a heterochiral 2:1 Zn complex ([( R)-Ph-box][( S)-Ph-box]Zn). The presented mechanisms provide a new strategy for generating switch-like OFF/ON states in chiral systems.