Electronic Spectra and Electronic Structures of Prototropic Species Derived from the 9-Anthrol-Anthrone System

Electronic absorption and fluorescence spectroscopies are employed to show that anthrone and its cation are closer to carbonyl derivatives of benzene in electronic structure than to derivatives of anthracene. The anion derived from anthrone is, however, a phenolic anthracene anion by electronic structure and by chemical reactivity. The phenolic 9-anthranol tautomer observed in some nonaqueous media is not measurably present in water at room temperatures in the ground electronic state. However, although neither anthrone or 9-anthrol fluoresce in water, there is evidence from the pH dependence of the fluorescence of the anion, to indicate that at least some of the 9-anthrol tautomer is present in the lowest excited singlet state. Although there has been extensive study of the electronic spectra and prototropic behavior of hydroxyaromatics derived from benzene and naphthalene, relatively little information is available about the hydroxyanthracenes. Baba and Suzuki (1,2) have examined the electronic absorption spectra of 1-anthrol and 2-anthrol in several solvents and of the 1- and 2-anthrolate anions in basic aqueous media. These authors have shown that in the 1-isomer the lowest excited singlet state is 1 L a as in anthracene while in the 2-isomer the 1 L b state drops below the 1 L a state by virtue of intramolecular charge transfer in the 1 L b state. The remaining isomer, 9-anthrol, is unusual in that it appears to exist in equilibrium with its keto tautomer, 9(10H)-anthrone (anthrone)(3), the equilibrium being solvent dependent but substantially favoring the keto tautomer in most solvents. The dissociation constant of the protonated keto tautomer has been reported as -5.0(4). One attempt has been made to calculate the dissociation constant of protonated anthrone in the lowest excited singlet state (5), with the Förster cycle (6), employing the averages of the low frequency absorption and fluorescence maxima of the neutral and protonated keto tautomer. However, we have found that the low fre-4 quency absorption maximum of the neutral anthrone of 3.64 × 10 4 cm −1 reported by these investigators (5) is much higher in frequency than the lowest frequency absorption band we observed.Moreover, it was not clear whether the fluorescence maximum of 2.60 × 10 4 cm −1 reported by the previous investigators (5) originated from the excited anthrone or the 9-anthrol tautomer. In order to better understand the electronic spectra and prototropic properties of the 9-anth