Aza-BODIPY are a class of organic fluorescent dyes which have recently become interesting for organic photovoltaics because of their strong tunable infrared absorption and there high stability. We investigated a tetra-phenyl derivative of aza-BODIPY (difluoro-bora-1,3,5,7-tetraphenyl-aza-dipyrromethene) on the Au(111) surface by low temperature STM and STS. We deposited far less then a monolayer of aza-BODIPY to be able to measure isolated single molecules. STS measurements of these molecules showed a broad prominent peak at a bias of 0.7 V (unoccupied state). DFT calculations indicate that the molecule forms a weak chemical bond with the Au(111) surface with significant charge transfer. Due to the adsorption, the four phenyl groups are twisted into the molecular plane and appear as relatively flat circular protrusions in the experimental (figure 1) and simulated STM images[1]. Furthermore, we measured two other aza-BODIPY derivatives, which only differ by the central metal atom (Zn, Co) to understand how the ferromagnetic Co atom influences the molecular orbitals of the aza-BODIPY. Since these molecules consist of two BODIPY moieties bound to the same metal atom they form a more complex three dimensional structure because of steric hindrance. DFT-simulations of the aza-BODIPY with a cobalt centre adsorbed on Ag(110) predict the appearance of spinpolarized states near the Fermi level.

References:
[1] J. Meyer, A. Wadewitz, Lokamani, C. Toher, R. Gresser, K. Leo, M. Riede, F. Moresco, G. Cuniberti, Phys. Chem. Chem. Phys. 13, 14421 (2011).