To obtain the subject new compound, having little fear of explosion and decomposable by relatively long wavelength light.

This compound is represented by formula I (R1 and R2 are each an alkyl or an aryl; T is H, a halogen, an alkyl or the like; U and V are each H, a halogen, an alkyl or the like; X is an anion), e.g. a compound of formula II. The compound of formula I is obtained by heating under reflux a mixture of the corresponding aniline [e.g. 3,6-di(n-octyloxy)-2- methoxyacetanilide], an acidic solvent (e.g. concentrated hydrochloric acid) and methanol, cooling the resultant reaction mixture, diazotizing with sodium nitrite, nitrosylsulfuric acid, isoamyl nitrite or the like, adding to the system a suspension of potassium hexafluorophosphate in tap water, agitating, cooling, washing with water and recrystallizing. The compound can be stabilized by forming a complex with zinc chloride, cadmium chloride, tin chloride or the like.