1,2-methyl orthoesters 2-arylazo-5-aryl-1,3,4-thiadiazoles AIE BODIPY Mn4+ ion PI-88 Technology: general issues azo-coupling reactions azobenzene catalysis colorant crystal structure dye dye chemistry d–d transition electrochemistry energy-electron transfer environment-friendly flavone fluorescence fluorescent labeling fluorophore glycosylation heterocycles inorganic pigments membrane molecular rotors morpholine orange color polymeric blend