Surface behavior and imaging of the lowest unoccupied molecular orbital of indolo[2,1-b]quinazoline-6,12-dione (tryptanthrin) via scanning tunneling microscopy

Indolo[2,1-b]quinazoline-6,12-dione (tryptanthrin) was imaged at the solution-graphite interface using scanning tunneling microscopy. STM images of the molecular monolayers show clear lobe-to-lobe resolution of the lowest unoccupied molecular orbitals of the molecules, as well as a somewhat unusual parallel arrangement of the molecular dipoles. Evaluation of the dipole-dipole interactions reveals that the interactions of the closest molecule pairs are reduced due to a magic angle-like effect. These results represent an important step toward a more in-depth understanding of the behavior of tryptanthrins, which are of interest due to their potential as pharmaceutical compounds. •We have achieved sub‐molecular resolution of tryptanthrin using STM.•The molecule image corresponds directly to the calculated LUMO.•The surface molecules display very unusual parallel dipole alignment.•The electrostatic interactions of the dipoles are described.•These results are important for continued STM studies of tryptanthrin as a potential anti‐malarial agent.