Indole-Based Anion Receptors: Design, Synthesis, Anion Binding Properties And Anion-directed Self-assembly

In this dissertation a series of well-designed anion receptors based on indole were prepared, their anion binding properties in solution and anion-directed metal-free multidimensional coordination polymer in solid state were investigated. The design, preparation, anion binding properties and anion-directed self-assembly of such anion recetpors were studied in detail, which are summarized as follows:A new series of indolocarbazole-quinoxalines (DIPZs) were prepared and characterized for effective fluoride and acetate anions sensing. The new indole-based system had a highly flat rigid structure with largeπsystem, and exhibited high binding affinity and sensitivity for acetate and fluoride anions. Receptors gave abundant and unique spectral feature in dimethyl sulfoxide (DMSO). Both fluoride and acetate anions caused a bathochromic shift of the absorption peaks of receptor nitro-substituted DIPZ in DMSO, whereas only fluoride anion resulted in a remarkable shift of the absorption peak of receptor DIPZ in DMSO. Both receptors could also operate as efficient colorimetric sensors for naked-eye detecting of fluoride and acetate anions, and their combined use also offered a simple way for naked-eye distinguishing of these two anions. Notably, the DIPZ system offered novel and excellent receptors for Y-shaped anion both in solution and in crystalline solid through the formation of two hydrogen bonds.The easy-to-make anion receptor DIPZ was employed as a versatile building block for constructing anion complexes with spherical fluoride and bromide, Y-shaped acetate and benzoate, trigonal nitrate, tetrahedral hydrogen sulfate and dihydrogen phosphate, and octahedral hexafluorosilicate anions. A systematical solid-state study was performed to investigate the influence of the size, basicity and geometry of anionic substrates on anion-binding affinities and modes, even the assembling structures in the solid state. The combination of conventional hydrogen bonding (N-H hydrogen bonding) and other weak interactions (aryl C-H aryl hydrogen bonding andπ-πstacking interaction) led to the formation of a series of anion-assisted supramolecular architectures with specific aggregates, such as 5:6 binding structure, anionic dimer and chain, cationic channel and sandwich-like structure, which have attracted extensive interests in supramolecular chemistry.Strategies for exploring anionic templates to direct the sophisticated supramolecular assembly have attracted various attentions. Herein, a series of new anion receptors containing two indole-based binding sites bridged by linking spacer pyrazino[2.3-g]quinoxaline were rationally designed and prepared from precursors 2,3-diindol-3’yl quinoxaline and DIPZ. X-ray analysis showed self-connected network and dimeric packing via hydrogen bonding andπ-πstacking interaction in the solid state in the structures. All the three receptors exhibited a series of prominent absorption bands from the expandedπsystem. The novel indole-based expanded receptors were found to strongly and selectively bind F-, AcO- and H2PO4- among the tested anions (F-,Cl-,Br-, AcO-,H2PO4-, HSO4-,NO3-, and ClO4-) and operated as efficient colorimetric sensors for naked-eye detecting of fluoride anion in DMSO solution. These tailored building blocks captured two anions located at far-spaced binding sites, adopting non-interfering anion-binding processes to guarantee the anion-binding affinity, topology and dimensionality. Solid studies elucidated that the neutral receptors interacted with tetrahedral dihydrogen phosphate anion in proper proportions and designed topologies, leading to the formation of a series of multidimensional networks by self-assembly in the solid state. The observation represented the well-characterized phosphate-directed assembly of multidimensional metal-free coordination polymers in the solid state, where the formed phosphate aggregates including dimer encapsulated in indole-mediated hydrogen-bonded pocket and infinite chain behaved as anionic templates to direct the self-assembly. However, no evidences proved such phosphate-directed infinite coordination polymers in solution.