Title

Fluorescence spectral study of 9-acridinecarboxylic acid and its methyl ester. Understanding the unusual fluorescence behavior of 9-anthroic acid

Document Type

Article

Publication Date

3-20-1997

Abstract

The absorption and fluorescence spectral characteristics of 9-acridinecarboxylic acid (9-ACA) and 9-(methoxycarbonyl)acridine (9-MCA) were studied in a series of organic solvents and in aqueous solutions. Fluorescence quantum yields (Φf) and lifetimes (τf) of the compounds were measured in these solvents. Unlike 9-anthroic acid (9-AA), as reported in the literature, no large Stokes-shifted fluorescence emission 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 state in 9-AA in which the carboxyl group is nearly coplanar with the aromatic ring. The Φf values for both compounds increase as a function of hydrogen-bonding capacity of the solvents. In near neutral to slightly acidic solutions, 9-ACA exists mainly in the zwitterionic form. Both 9-ACA and 9-MCA form monoprotonated species in moderately concentrated acid solutions. The acidium cation of 9-AA formed in the excited state in moderately concentrated acid solution 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 concentrated perchloric acid medium. The pKas of various prototropic equilibria involved in the ground 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 (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 aromatic ring in the S0 state in both the molecules. However, in the S1 state, it approaches coplanarity with the aromatic ring. The calculated bond lengths, charge densities, and dipole moments suggest that the resonance charge transfer from the aromatic 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 observed in 9-ACA. The charge density data indicate that the ring nitrogen and the carbonyl oxygen of the COOH group become more basic upon electronic excitation.

Publication Source (Journal or Book title)

Journal of Physical Chemistry A

First Page

2271

Last Page

2278

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