Impact of Nitrogen Substitution and Molecular Orientation on the Energy-Level Alignment of Heteroacene Films

The impact of imine nitrogen (=N-) substitution in hydrocarbon oligoacenes on the charge-transport type has been carefully probed. A N-heteroacene containing one =N- and 7-fused rings, dianthrano [1,2-a: 1',2'-j]pyridine (DAP), has been synthesized as a model molecule for the study. The electronic structure, energy-level alignment (ELA), and the molecular orientation of the molecule on two prototypical substrates, namely, highly oriented pyrolytic graphite (HOPG) and Ag(111), have been investigated using angle-resolved ultraviolet photoelectron spectroscopy. Combining crystal structure, optical property and electronic structure investigations, our results indicate that by introduction of only one =N- into a large oligoacene, the ELA can be effectively tuned and a potential of n-type conductivity can be realized. Furthermore, the permanent electric dipole induced by the intramolecular polar bonds (C-N and C-H) allows tuning the ELA by altering the molecular orientation. Tilted molecular orientations can impose smaller electron injection barriers and ionization energies to DAP thin films than a flat-lying orientation.