10-Hydroxy-7-arylindeno[1,2-b]-1,2,5-oxadiazolo[3,4-d]pyridines and 7- aryl-10-oxoindeno[1,2-b]-1,2,5-oxadiazolo[3,4-d]pyridines-synthesis, spectra, and polymorphism

The novel dyes 7-aryl- 10-oxoindeno[1,2-b]-1,2,5-oxadiazolo[3,4-d]- pyridines (4) and 7-aryl-10-hydroxyindeno[1,2-b]-1,2,5-oxadiazolo[3,4-d]- pyridines (5) have been prepared from acetophenone derivatives. While compounds (4) exhibit a dark red color, they are only weakly fluorescent. Compounds (5) is fluorescent. Of interest is that 10-hydroxy-7-phenylindeno- [1,2-b]-1,2,5-oxadiazolo[3,4-d] pyridine (5a) can take four polymorphic forms in the solid state, of which two are yellow (designated as 5a-Y-1 and 5a-Y-2) and two are red (5a-R-1 and 5a-R-2). Two of them are interconvertible (yellow/red) upon exposure to different solvents. X-Ray crystal structure analysis of 5a-R-2 shows the phenyl ring and the indenooxadiazolopyridine ring to be coplanar.