A theoretical insight into the photophysics of acridine

The electronic absorption and emission spectra of acridine have been studie d by means of a multiconfigurational second-order perturbation method (CASS CF/CASPT2) and its multistate extension (MS-CASPT2). The low-lying valence singlet and triplet pi - pi* and n - pi* excited states have been computed. The location of the lowest Rydberg state (3s) has been also estimated. By optimization of the geometries of the ground and low-lying excited states a nd the calculation of transition energies and properties, the obtained resu lts lead to a complete analysis and assignment of the available experimenta l singlet-singlet and triplet-triplet absorption spectra and to the descrip tion of the basic features of the fluorescence and phosphorescence processe s of acridine. The photophysics of acridine and its protonated form are ana lyzed and the effects of solvation are discussed. The present findings supp ort the model of a state reversal on the lowest singlet excited state upon increasing the solvent polarity.