Assembly of (l+d)-Tryptophan Derivatives Containing an Imidazole Group Selectively Forms a Rare Purple Ni²⁺-Hydrogel

Design of metal-selective hydrogels is attractive due to potential applications in materials and biological sciences. Although much progress has been made, assembly of both l- and d-amino acid derivatives was less explored for design of metallohydrogels. In this study, we synthesized a facile and small tryptophan derivative containing an imidazole ligand with both l- and d- configurations (denoted as l/d-ImW). Intriguingly, the assembly of (l+d)-ImW gelators was found to selectively form a Ni²⁺-hydrogel in aqueous medium at room temperature, which shows a rare purple color and exhibits excellent multi-responsiveness. In addition to insights into the gelation mechanism, this study provides a novel approach to the design of metallohydrogels, by the assembly of (l+d)-amino acid derivatives containing both aromatic rings and multiple metal coordination sites.