The electronic structures of Co and Ni tetraazaannulenes.

We compare the electronic structure of two metal-centered tetramethyldibenzo-tetraazaannulene (TMTAA) macrocyclic complex molecules: 5,7,12,14- tetramethyl-2,3:9,10-dibenzo[b,i]-1,4,8,11-tetraazacyclotetradecine nickel (II) and 5,7,12,14-tetramethyl-2,3:9,10-dibenzo[b,i]-1,4,8,11-tetraazacyclotetradecine cobalt (II). The experimental gap between the highest occupied molecular orbital to the lowest unoccupied molecular orbital for both molecules, obtained from combined ultraviolet photoemission and inverse photoemission studies, is close to the value of 6.6 eV expected from simple model calculations, but with the Fermi level placed closer to the lowest unoccupied molecular orbital. While both the Co(II) (s = 1/2) and Ni(II) (s = 0) TMTAA molecular electronic structures are very similar, the Ni(II) adopts a high-symmetry molecular configuration upon adsorption, with a strong preferential orientation.