Structural chemistry of polycyclic heteroaromatic compounds. Part XI. Photoelectron spectra and electronic structures of tetracyclic hetarenes of the triphenylene type

Abstract

The UV photoelectron spectra of several tetracyclic heteroaromatic compounds ( 2– 9) which are π-isoelectronic with triphenylene ( 1) have been recorded and analysed making use of semiempirical AM1 and PM3 as well as ab initio/DFT B3LYP calculations. In one series of compounds ( 2– 7), the peripheral benzene rings of 1 are successively substituted by thiophene rings that are either [b]- or [c]-annellated with the central benzene unit. In 2– 7 only marginal shifts are found for most of the IPs of electrons. In the benzotrithiophenes 5– 7, a systematic variation is displayed by IP( π 7). Compared to 1, the π electron system of benzo[c]trithiophene ( 7) is approximately two times as much destabilized as in the isomers 5 and 6 with [b]annellated thiophene rings. The IP[ n(S)] values of the thiophene derivatives 2– 7 indicate that these orbitals are clearly destabilized relative to thiophene. The same holds for the n(O) orbital of the furane derivative 9 in comparison with that of furane. In 9, only the higher π MOs ( π 7– π 9) are destabilized whereas the lower levels ( π 1– π 4) are stabilized, and those in between ( π 5– π 6) remain essentially unshifted. In the pyrrole derivative 8, all π MOs are substantially destabilized by about 0.5–1.6 eV relative to 1.