Zinc-Ion-Stabilized Charge-Transfer Interactions Drive Self-Complementary or Complementary Molecular Recognition

Abstract

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 (9,10LD and 1,5LD) and anthraquinone-based (1,5LA) ditopic ligands containing two imidazole side arms as zinc coordination sites were designed. The 9,10LD and 1,5LA systems associated in a complementary fashion (LA/LD/LA) upon clipping by two zinc ions (Zn2+) to form an alternating donor-acceptor assembly [(9,10LD)(1,5LA)2-(Zn2+)2]. However, once the charge-transfer interactions were perturbed by subtle modifications of the imidazole side arms (9,10LD'(S) and 1,5LA'(S)), self-complementary association (LD'/LD'/LD'/LD' and LA'/LA'/LA'/LA') between the donor (9,10LD'(S)) and acceptor (1,5LA'(S)) ligands predominantly occurred to form homoassemblies [(9,10LD'(S))4-(Zn2+)2 and (1,5LA'(S))4-(Zn2+)2]. As in the case of a homochiral pair (9,10LD'(S) and 1,5LA'(S)), self-complementary association (narcissistic self-sorting) occurred in the Zn2+ assembly with heterochiral combinations of the donor and acceptor ligands (9,10LD'(S)/1,5LA'(R) and 9,10LD'(S)/1,5LA'(R)/1,5LA'(R)). Narcissistic self-sorting also took place between the positional isomer of the donor ligands (9,10LD and 1,5LD) to form individual homoligand assemblies [(9,10LD)4-(Zn2+)2 and (1,5LD)4-(Zn2+)2]. Conversely, statistical association took place in the Zn2L4 assembly process of an isomorphous pair of the donor and acceptor ligands (1,5LD and 1,5LA).