Theoretical interpretation of electronic absorption and emission transitions in 9-acridinones.

Stationary absorption, fluorescence excitation and fluorescence spectra for 9(10H)-acridinone, 9(10-methyl)-acridinone, 2-methyl-9(10-methyl)-acridinone, 2-nitro-9(10-methyl)-acridinone, 9(10-ethyl)-acridinone and 9(10-phenyl)-acridinone dissolved in 1,4-dioxane, methyl alcohol or acetonitrile, as well as the available spectral characteristics reported by others were compared with those predicted theoretically at the semi-empirical PM3/CI (including the solvent effect within the COSMO model) or PM3/S levels of theory, in order to interpret spectral features of the compounds, i.e. the energies and probabilities of S0-->Sn, S0-->T1, T1-->T2, S1-->S0, T1-->S0 and S1-->T1 transitions. Calculations at the PM3 and PM3/CI levels of theory enabled the structural changes accompanying S0-->S1, S1-->T1 and T1-->S0 transitions to be investigated; they yielded, moreover, basic physicochemical characteristics of the molecules in the ground and excited electronic states. Theoretically predicted dipole moments and charge distributions in the S0, S1 and T1 states provided further insight into the nature of electronic transitions in 9-acridinones. The predicted characteristics correlate quite well with the available experimental ones, thus providing confirmation of the utility of theory in predicting the features of electronically excited molecules and interpreting the electronic transitions occurring in them.