Occupied and unoccupied electronic levels in organic π-conjugated molecules: Comparison between experiment and theory

Occupied and unoccupied densities of states of π-conjugated molecules measured via ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy, respectively, are compared with corresponding densities of states calculated using the semi-empirical Hartree-Fock intermediate neglect of differential overlap (INDO) method. Excellent agreement is obtained for both occupied and unoccupied levels for PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride), α-NPD (N,N′-diphenyl-N,N′-bis(l-naphthyl)-l,l′ biphenyl-4,4″ diamine), and Alq₃ (tris(8-hydroxy-quinoline)aluminum). The results provide a full description of the electronic structure of these molecules and demonstrate that semi-empirical techniques can be successfully applied to describe the unoccupied levels of these molecules.