Structure and Activity of Phthalic Acid Hydrazides

The parent of the series of which luminol is the best known member, phthalic acid hydrazide itself, provides few useful clues to the understanding of luminol chemiluminescence. It is not chemiluminescent on oxidation by Fe -hemin in alkaline aqueous solution. In aprotic solvents however a yellow (526 nm) emission is seen. As will be seen later, most cyclic hydrazides produce the excited state of the corresponding dicarboxylate. In this case the emission comes from the monoanion of the starting phthalhydrazide, by back-transfer of energy from the product. One can only assume that the primary excited product is the dicarboxylic acid dianion, but there is not direct evidence for this [7]. 

Energy transfer to a linked acceptor (Sect. VII.3) helps to support this interpretation. However no information relating to the primary excitation step results. 

The well-known observation concerning the effect of substituents on the phthalic hydrazide system, that electron-releasing groups increase and electron-withdrawing groups reduce the chemiluminescence quantum yield, is not yet fully understood with respect to its physico-chemical meaning. Three possibilities arise  

The substituents may be essential for the chemical reaction of the hydrazide oxidation itself the excitation step may depend on them or they may affect the fluorescence efficiency of the product. It would appear that all three influences are at work both quantitatively and qualitatively. For example, substitution may result in general instability towards oxidation as in the cases of 3,6-diamino- or 3-amino-6-methoxy phthalhydrazide [8]. The ease of oxidation on the chemiluminescent pathway is certainly significant, although it is difficult to separate this from the influence on the population of the excited state. An example of this effect is that of the 4-dialkylamino-phthalhydrazides. Differences in chemiluminescence quantum yields of up to 600-fold were observed, whereas the fluorescence quantum yields of the corresponding 4-dialkyl-amino phthalates only differ by a factor of about 10 [9]. 

0R is taken, according to the experimental evidence [10], as practically 1.0, the strongest influence of the substituent must be on the excitation step itself. 

---