Pyrazino[2.3-g]quinoxaline-Bridged Indole-Based Building Blocks: Design, Synthesis, Anion-Binding Properties, and Phosphate-Directed Assembly in the Solid State

Strategies for exploring anionic templates to direct sophisticated supramolecular assembly have attracted attention. Herein, a series of new anion receptors 1-3 containing two indole-based binding sites bridged by linking spacer pyrazino[2.3-g]quinoxaline (PQ) have been rationally designed and prepared from the precursors 2,3-diindo1-3'-yl quinoxaline (DIQ) and 5,6-dihydrodiindolo[3,2-a:2',3'-c]-phenazine (DIPZ). X-ray analyses showed a self-connected network and dimeric packing through hydrogen bonding and pi-pi stacking interaction M the solid state in the structures of 1 and 2, respectively. All three receptors exhibited a series of prominent absorption bands from the expanded pi system. The 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-eve detection of fluoride anions in DMSO. These tailored building blocks captured two anions located at far-spaced binding sites, and adopted noninterfering anion-binding processes to guarantee the anion-binding affinity, topology, and dimensionality. Solid-state studies elucidated that the neutral 1-3 interacted with the tetrahedral dihydrogen phosphate anion in proper proportions and designed topologies, thus leading to the formation of a series of multidimensional networks by self-assembly in the solid state. The observations showed a well-characterized phosphate-directed assembly of multidimensional metal-free coordination polymers in the solid state, in which the formed phosphate aggregates. including dimer encapsulated in an indole-mediated hydrogen-bonded pocket and an infinite chain, behaved as anionic templates to direct the self-assembly. However. no evidence proved the presence of such phosphate-directed infinite coordination polymers in solution.