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Fluorescence Spectral Study of 9-Acridinecarboxylic Acid and Its Methyl Ester

Fluorescence Spectral Study of 9-Acridinecarboxylic Acid and Its Methyl Ester. Understanding the Unusual Fluorescence Behavior of 9-Anthroic Acid. Dey, Joykrishna; Haynes, Judson L., III; Warner, Isiah M.; Chandra, Asit K. (Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA). Journal of Physical Chemistry A, 101(12), 2271-2278 (English) 1997 American Chemical Society. CODEN: JPCAFH. ISSN: 1089-5639. DOCUMENT TYPE: Journal CA Section: 22 (Physical Organic Chemistry) The absorption and fluorescence spectral characteristics of 9-acridinecarboxylic acid (9-ACA) and 9-(methoxycarbonyl)acridine (9-MCA) were studied in a series of org. solvents and in aq. solns. Fluorescence quantum yields (Ff) and lifetimes (tf) of the compds. were measured in these solvents. Unlike 9-anthroic acid (9-AA), as reported in the literature, no large Stokes-shifted fluorescence emission band was obsd. for 9-ACA and 9-MCA in neutral org. 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 state in 9-AA in which the carboxyl group is nearly coplanar with the arom. ring. The Ff values for both compds. increase as a function of hydrogen-bonding capacity of the solvents. In near neutral to slightly acidic solns., 9-ACA exists mainly in the zwitterionic form. Both 9-ACA and 9-MCA form monoprotonated species in moderately concd. acid solns. The acidium cation of 9-AA formed in the excited state in moderately concd. acid soln. reorganizes to produce a carbocation centered at the carbon atom of the carboxyl group. However, there was no indication of the formation of such acidium cations in the case of 9-ACA and 9-MCA even in concd. perchloric acid medium. The pKas of various prototropic equil. involved in the ground electronic state of the compds. were estd. Semiempirical AM1 calcns. were performed to obtain the energies of the various configurations of 9-AA and 9-ACA in the ground (S0) as well as in the lowest excited singlet (S1) electronic state. The results suggest that the COOH group is oriented at an angle of ~55° with respect to the arom. ring in the S0 state in both the mols. However, in the S1 state, it approaches coplanarity with the arom. ring. The calcd. bond lengths, charge densities, and dipole moments suggest that the resonance charge transfer from the arom. ring to the COOH group increases in the S1 state of 9-AA. However, despite the decrease of twist angle of the COOH group, no significant charge transfer was obsd. in 9-ACA. The charge d.Except for chemicals metioned above, 5336-90-3 and 5132-81-0 are also used. data indicate that the ring nitrogen and the carbonyl oxygen of the COOH group become more basic upon electronic excitation. .