Imidazobenzimidazole fused azacalix[4]arenes: Synthesis,structure, and Zn2+-selective colorimetric-fluorometric sensor

Tetra(amino)azacalix4]arene skeleton was functionalized at the bridging NH sites using various aromatic aldehydes via formation of imidazobenzimidazole fused heterocycles. X-ray single crystal analysis revealed distorted 1,3-alternate conformations for the resulting macrocycles. Anthracenyl and pyrenyl modified imidazobenzimidazole fused azacalix4]arenes existed as dimers in the solid state, associated mainly through π-π stacking interactions between the planar polycyclic fluorophores. The tetrapyrenyl modified product was further used as a Zn²⁺-selective sensor, which showed naked-eye detected color change and enhanced excimer emission. The stoichiometry between the sensor and Zn²⁺ was determined to be 1:1 and the association constant was 1.1 × 10⁵ L/mol. The sensing process was highly selective and showed strong anti-interference with presence of other cations. The UV-vis spectral changes in the sensing process were completely reversible by alternate addition of Zn²⁺ and F^?, showing an efficient ‘‘on–off-on’’ result.