Here, we show that charge-transfer interactions determine whether donor and acceptor ditopic ligands will associate in a complementary or self-complementary fashion upon metal-ion clipping. Anthracene-based ( 9,10 L D and 1,5 L D ) and anthraquinone-based ( 1,5 L A ) ditopic ligands containing two imidazole side arms as zinc coordination sites were designed. The 9,10 L D and 1,5 L A systems associated in a complementary fashion (L A /L D /L A ) upon clipping by two zinc ions (Zn2 +) to form an alternating donor–acceptor assembly [( 9,10 L D )( 1,5 L A )2–(Zn2 +)2]. However, once the charge-transfer interactions were perturbed by subtle modifications of the imidazole side arms ( 9,10 L D ′ ( S ) and 1,5 L A ′ ( S ) ), self-complementary association (L D ′/L D ′/L D ′/L D ′ and L A ′/L A ′/L A ′/L A ′) between the donor ( 9,10 L D ′ ( S ) ) and acceptor ( 1,5 L A ′ ( S ) ) ligands predominantly occurred to form homoassemblies [( 9,10 L D ′ ( S ) )4–(Zn2 +)2 and ( 1,5 L A ′ ( S ) )4–(Zn2 +)2]. As in the case of a homochiral pair ( 9,10 L D ′ ( S ) and 1,5 L A ′ ( S ) ), self-complementary association (narcissistic self-sorting) occurred in the Zn2 + assembly with heterochiral combinations of the donor and acceptor ligands ( 9,10 L D ′ ( S ) / 1,5 L A ′ ( R ) and 9,10 L D ′ ( S ) / 1,5 L A ′ ( R ) / 1,5 L A ′ ( R ) ). Narcissistic self-sorting also took place between the positional isomer of the donor ligands ( 9,10 L D and 1,5 L D ) to form individual homoligand assemblies [( 9,10 L D )4–(Zn2 +)2 and ( 1,5 L D )4–(Zn2 +)2]. Conversely, statistical association took place in the Zn2L4 assembly process of an isomorphous pair of the donor and acceptor ligands ( 1,5 L D and 1,5 L A ).
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)-Im 2 An] containing an anthracene ring as a spacer with two ethynyllinked chiral imidazole groups at the 9,10-positions. The (S,S)-and (R,R)-Im 2 An ligands (L) spontaneously form the assemblies with Zn 2+ ions (M) in solution phase, giving L 4 M 2 -type assemblies with a general formula [(S,S)-or (R,R)-Im 2 An] 4 (Zn 2+ ) 2 . NMR studies revealed that the [(S,S)-Im 2 An] 4 (Zn 2+ ) 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)-Im 2 An] 4 (Zn 2+ ) 2 afforded an alternative Zn 2+ assembly with general formula [(R)-or (S)-Phbox] 2 [(S,S)-Im 2 An] 2 (Zn 2+ ) 2 , where the chiral coligands expel two of the (S,S)-Im 2 An ligands that were singly bound to the Zn 2+ ions in the original [(S,S)-Im 2 An] 4 (Zn 2+ ) 2 assembly. This ligand-exchange reaction causes conformational alteration from a parallel-displaced structure to a twisted stacking between the anthracene rings inside the Zn 2+ assembly, which results in a significant enhancement of CD signals due to excitonic interactions of the chiral anthracene dimer. Dissymmetry factor (g CD ) 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)-Im 2 An] 2 (Zn 2+ ) 2 versus heterochiral cooperative dissociation of [(S,S)-Im 2 An] 4 (Zn 2+ ) 2 by sequestering of Zn 2+ inside the assembly through formation of a heterochiral 2:1 Zn 2+ complex ([(R)-Ph-box][(S)-Ph-box]Zn 2+ ). The presented mechanisms provide a new strategy for generating switch-like OFF/ON states in chiral systems.
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