Facile Synthesis of Donor–Acceptor Heterocycloarenes Based on Pyrazine Derivatives Possessing Intriguing Iodide Ion Capture Properties

Abstract

Donor-acceptor (D-A) conjugated systems have been extensively investigated and play important roles in organic electronics. Incorporating D-A structures into (hetero)cycloarenes endows them tunable electronic properties, while the well-defined cavity remains. However, the synthetic complexity of introducing electron-acceptor moieties into (hetero)cycloarenes limits their development and applications. In this paper, the first family of electronically tunable D-A heterocycloarenes (<b>DAHC</b><i><b>n</b></i>, <i>n</i> = 1-5) based on pyrazine derivatives was facilely synthesized through cyclocondensation reaction from a tetraketone-functionalized heterocycloarene precursor prepared using the ketal-protection strategy. The effect of expanded conjugation and the inserted electron-withdrawing group on the electronic structures of the D-A heterocycloarenes was studied systematically by X-ray crystallographic analysis, various spectroscopic measurements, and theoretical calculations. Interestingly, the presence of an electron-withdrawing group polarizes the inner C(sp²)-H and significantly increases the binding affinities of D-A heterocycloarenes to the iodide anion. Meanwhile, the anion affinity can be further modulated by the type of attached substituents and the distance of polarization. More importantly, the dicyanopyrazine derivative <b>DAHC3</b> shows the highest binding strength to the iodide ion as a 2:1 sandwich complex (log β₂ = 12.3 and Δ<i>G</i> = -69.1 kJ mol^-1), which is the strongest iodide receptor using C(sp²)-H hydrogen bonding interactions reported to date. Our finding provides a new strategy to design and synthesize D-A heterocycloarenes and strong anion receptors.