STRUCTURAL CHEMISTRY OF POLYCYCLIC HETEROAROMATIC-COMPOUNDS .4. ELECTRONIC-STRUCTURES OF ANGULAR DITHIENOPYRIDINES

The Hel photoelectron spectra of nine isomeric [b,d]-annellated dithienopyridines (5-13) are reported and discussed including those of some closely related compounds such as phenanthridine (2), dithieno[3,4-a:3',4'-c]benzene (4) and its tetrahydro derivative 3. The ionization potentials are assigned to molecular orbitals using the results of MNDO. AM1 and PM3 calculations. The ability of PE spectroscopy as an analytical method in the analysis of the isomers 5-13 is investigated. From the spectra as well as from the theoretical results, the isomers can be divided into two groups. depending on the mode of annellation. The crystal and molecular structures of one member of each group (isomers 9 and 12) are determined by X-ray diffraction. The mode of annellation is also reflected in the bond lengths of the pyridine and thiophene rings.