Spectroscopy and Photophysics of Monoazaphenanthrenes II. Ab initio Analysis of Molecular Parameters and Electronic Spectra

The ground state structures of phenanthrene and its monoaza-deriv atives, phenanthridine and 7,8-benzoquinoline were optimized using the ab initio methods. Both methods of optimization are leading to the results, which are in good agreement with available experimental data. Calculated ground-state electric dipole moments in phenanthridine and 7,8-benzoquinoline were found to have antiparallel orientations, due to the different electronic charge distributions in these molecules. The energies of vertical electronic transitions from the ground to excited singlet states (S$\text{}_{0}$ → S$\text{}_{n}$ transitions) and the corresponding oscillator strengths were calculated within the framework of configuration interaction-singles and time-dependent density functional theory. The last method was found to be more accurate in reproduction of experimental absorption spectra. Very interesting result of these computations is the change of relative orientation of the transition dipole moments for the two lowest ππ* electronic transition in monoazaphenanthrenes - from perpendicular in phenanthrene molecule to nearly parallel orientation in both monoazaphenanthrenes. The observed changes of molecular parameters and spectra can be related to the inductive effects of the substitution of nitrogen atom into the aromatic skeleton of phenanthrene