Structural chemistry of polycyclic heteroaromatic compounds. Part 13. Photoelectron spectra and electronic structures of tricyclic hetarenes of the anthracene type

The ultraviolet photoelectron spectra of two tricyclic heteroaromatic compounds (2, 3) that are pi -isoelectronic with anthracene (1) have been recorded and analysed making use of semi-empirical AM1 and PM3, as well as density functional theory (DFT) B3LYP calculations. In compounds 2 and 3, one peripheral benzene ring of compound 1 is substituted by a thiophene ring that is either [b]- or [c]-annellated. In compounds 2 and 3, only small shifts are found for most of the ionization potentials of pi electrons. Since the ionization energies of-all occupied Ti molecular orbitals of compounds 1-3 could be assigned, a direct comparison of their pi electron energy is possible. Compared with compound 1, the pi -electron system of naphtho[2,3-b]thiophene (2) is stabilized by 0.6 eV, while that of naphtho[2,3-c]thiophene (3) is destabilized by 0.2 eV. [b]-Annellation of the thiophene ring is thus favourable while [c]-annellation is unfavourable. (C) 2001 Elsevier Science B.V. All rights reserved.