Water assisted anion chains and anion dependent fluorescence emission in salts of N,N′-bis(3-imidazol-1-ylpropyl)naphthalenediimide

Abstract

In this article we demonstrate the role of anion–π interactions of different anions with protonated N,N′-bis(3-imidazol-1-ylpropyl)naphthalenediimide (L). The compound L shows fluorescence quenching with hydrochloric acid and hydrobromic acid, whereas it shows fluorescence enhancement with perchloric, sulphuric, phosphoric and nitric acid. A similar trend in fluorescence emission of the corresponding salt in the solid state is also observed. The fluorescence quenching is attributed to the anion–π interactions of the chloride and bromide with H₂L, whereas the oxy-anion-containing acids cause fluorescence enhancement by protonation of L. The structures of the salts of N,N′-bis(3-imidazol-1-ylpropyl)naphthalenediimide (L), namely [H₂LCl₂]·2H₂O (1), [H₂LBr₂]·2H₂O (2), [H₂L(ClO₄)₂] (3), [H₂L(HSO₄)₂]·H₂O (4), [H₂L(MeSO₄)₂] (5), [H₂L(HPO₄)]·H₂O (6) [H₂L(NO₃)₂] (7), [H₂L(SiF₆)]·1.5H₂O (8) are studied. Chloride and bromide ions form a hydrogen bonded halide–water chain; whereas the hydrogen phosphate salt forms a one dimensional water bridged polymeric chain. Planar nitrate anions appear as pairs and form planar assemblies with imidazolium ions and such assemblies are sandwiched between naphthalenediimide rings; whereas bisulphate and methylsulphate salts guide formation of helical supramolecular arrangements of H₂L²⁺ ions. The perchlorate anions facilitate formation of a zigzag 2-D network structure and the SiF₆²⁻ anions guide formation of a spiral chain-like structure to hold the SiF₆²⁻ anions.