FLUORESCENCE SPECTRAL STUDY OF 9-ACRIDINECARBOXYLIC ACID AND ITS METHYL-ESTER - UNDERSTANDING THE UNUSUAL FLUORESCENCE BEHAVIOR OF 9-ANTHROIC ACID

The absorption and fluorescence spectral characteristics of 9-acridine carboxylic acid (9-ACA) and 9-(meth-oxycarbonyl)acridine (9-MCA) were studied in a series of organic solvents and in aqueous solutions. Fluo rescence quantum yields (Phi(f)) and lifetimes (tau(f)) of the compoun ds were measured in these solvents. Unlike 9-anthroic acid (9-AA), as reported in the literature, no large Stokes-shifted fluorescence emiss ion band was observed for 9-ACA and 9-MCA in neutral organic solvents or water. The absence of large Stokes-shifted emission in the case of 9-ACA and 9-MCA suggests the existence of a charge-transfer emitting s tate in 9-AA in which the carboxyl group is nearly coplanar with the a romatic ring. The Phi(f) values for both compounds increase as a funct ion of hydrogen-bonding capacity of the solvents. In near neutral to s lightly acidic solutions, 9-ACA exists mainly in the zwitterionic form . Both 9-ACA and 9-MCA form monoprotonated species in moderately conce ntrated acid solutions. The acidium cation of 9-AA formed in the excit ed state in moderately concentrated acid solution reorganizes to produ ce a carbocation centered at the carbon atom of the carboxyl group. Ho wever, there was no indication of the formation of such acidium cation s in the case of 9-ACA and 9-MCA even in concentrated perchloric acid medium. The pK(a)s of various prototropic equilibria involved in the g round electronic state of the compounds were estimated. Semiempirical AM1 calculations were performed to obtain the energies of the various configurations of 9-AA and 9-ACA in the ground (S-0) as well as in the lowest excited singlet (S-1) electronic state. The results suggest th at the COOH group is oriented at an angle of similar to 55 degrees wit h respect to the aromatic ring in the S-0 state in both the molecules. However, in the S-1 state, it approaches coplanarity with the aromati c ring. The calculated bond lengths, charge densities, and dipole mome nts suggest that the resonance charge transfer from the aromatic ring to the COOH group increases in the S-1 state of 9-AA. However, despite the decrease of twist angle of the COOH group, no significant charge transfer was observed in 9-ACA. The charge density data indicate that the ring nitrogen and the carbonyl oxygen of the COOH group become mor e basic upon electronic excitation.