MO THEORETICAL INVESTIGATION OF STRAINED PAH MOLECULES - EFFECTS OF METHYL SUBSTITUENTS IN GROUND AND EXCITED-STATES

The capabilities of AM1 and MNDO methods for the determination of ground-state geometries and vertical excitation energies of perylene, phenanthrene, 1,2-benzanthracene, and 18 of their planar and nonplanar methyl-substituted derivatives has been investigated. It was found that the MNDO method is not suitable for the sterically strained PAH molecules investigated here. The AM1 calculation of the vertical excitation energies was performed using aa CI treatment with 225 singly excited configurations. For the spectral shifts of the S1 excitation energies induced by methyl substitution, a good correlation between calculated and experimental values was obtained. The deformation-induced spectral shift of the twisted 1-methylperylene was correctly obtained as hypsochromic shift but was calculated too small. The potential energy curves for the twisting motion of the naphthalene moieties in the perylene ground and excited states were evaluated and a vibrational analysis of the low-frequency modes was performed.