Different self-assemblies and absorption-emission properties of the picrate salts of aromatic amine or heterocycle linked oximes

摘要

The supramolecular oxime synthons in the self-assemblies of picrate salts of 4-(N,N-dimethylaminophenyl)aldoxime, quinoline-4-carbaldoxime and pyridine-4-carbaldoxime are different. In these salts, protonation in each case occurs at a nitrogen atom located at a remote site of the oxime. These results suggest that synthon analysis provides descriptors rather than predicting self-assemblies. The self-assembly of the picrate salt of 4-(N,N-dimethylaminophenyl)aldoxime lacks an oxime homodimer. The R-2(2)(8) type homodimeric sub-assemblies of oximes are observed in the picrate salt of quinoline oxime, whereas a similar picrate salt of pyridine-oxime has R-2(2)(4) type oxime homodimers. Water molecules in this salt act as filler molecules for a tight-packed structure and have a role in deciding the nature of oxime synthons. Protonation at a nitrogen atom of the oxime was observed in the picrate salt of indole-3-carbaldoxime. This salt undergoes charge-transfer between the aromatic rings. The differential scanning calorimetry data establish the higher melting points of salts over the parent components and exothermic decompositions just above or nexime cause a color difference from other salts. The emission spectrum of each salt is invariably quenched in solution or in the solid state. The relative ability of different nitro-aromatic compounds ar the respective melting temperature. The charge-transfer interactions of the picrate salt of indole-3-carbaldoto quench emissions of the oxime derivatives enables them to be differentiated in solution. It is shown that the modification of the ground state by charge-transfer interaction is not the sole factor to cause fluorescence quenching in these salts, but excited state proton transfer plays a decisive role.