Solution and Solid State Study on the Recognition of Hydroxyaromatic Aldoximes by Nitrogen Containing Compounds

Abstract

Structural aspects and fluorescence behaviors of a series of cocrystals of hydroxyaromatic aldoximes, namely, 2(H2NAP)·(4,4′-bipyridine), 2(H2NAP)·(HMTA), H2NAP·caffeine, and (H3OHPA)·theophylline·2H2O, where H3OHPA = 2,3-dihydroxyphenylaldoxime, H2NAP = 2-hydroxynaphthaldoxime, HMTA = hexamethylenetetramine, were studied. A difference in fluorescence behavior of oxime molecules in solution and solid states was observed. The H2NAP·caffeine cocrystals are fluorescence active in the solid state but not in solution. The combinations of H3OHPA and 4,4′-bipyridine, HMTA or caffeine in the solid state did not yield cocrystals; however, these combinations in solution caused fluorescence quenching of H3OHPA. On the other hand, a hydrated cocrystal of H3OHPA with theophylline, namely, (H3OHPA)·theophylline·2H2O, was isolated and characterized. It was found that this combination causes fluorescence quenching of 2,3-dihydroxyphenylaldoxime (H3OHPA) in the solution state but not in the solid state as a cocrystal. The fluorescence change of H3OHPA interacting with theophylline can be identified among several aldoximes such as 2-hydroxynaphthaldoxime (H2NAP), 2-hydroxyphenylaldoxime (H2PA), and 2,4-dihydroxyphenylaldoxime (H3PHPA). The synthons involved in each cocrystal are identified, and the fluorescence emission differences leading to molecular recognition are determined. The differences in fluorescent behavior among aldoximes were exploited for their identification and separation.