Electronic absorption and fluorescence spectrophotometry of quinacrine

The absorption and fluorescence spectra of quinacrine and its dealkylated derivative, 2-methoxy-6-chloro-9-aminoacridine, were studied as a function of pH and Hammett acidity. The relative stability and high quantum yield of the dication derived from quinacrine make it the most desirable species for fluorimetric determination. This species predominates in the region pH 8-H_o-6. The alkylamino side-chain of quinacrine appears to interfere with excited state protonation and dissociation of the dication. Moreover, in the dication, the steric interaction between the side-chain and the 9-amino group may prevent coplanarity between the 9-amino group and the acridine ring, thereby favoring the aminoacridine over the iminoacridan structure.