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-diindol-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 π-π stacking interaction in the solid state in the structures of 1 and 2, respectively. All three receptors exhibited a series of prominent absorption bands from the expanded π 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-eye 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.