Experimental and theoretical study of 3-carboxy-5,6-benzocoumarin

The electronic absorption and emission spectra of 3-carboxy-5,6-benzocoumarin in organic solvents and in aqueous buffered media are reported and discussed. The experimental data allowed for the estimation of the ground state acidity constant, pKa = 4.6. A significant larger fluorescence quantum yield was found for the undissociated form than for the carboxylate species. The solvent-dependent MO calculations, at both semiempirical and ab initio levels, outlined the role of the solvation processes in the ground state properties especially for the dissociated species. Considering the sequence of states calculated by the AM1 method at the geometry of S0 and S1, the wavelength of the absorption and emission bands were estimated. The agreement is good for the absorption maxima and only satisfactory for the fluorescence bands. The difference in the quantum fluorescence yields for the undissociated and dissociated species was explained in terms of a different probability of the ISC, S1 → T2 process.