Occupied and unoccupied electronic levels in organic pi-conjugated molecules: comparison between experiment and theory

Occupied and unoccupied densities of states of pi-conjugated molecules meas ured via ultraviolet photoemission spectroscopy and inverse photoemission s pectroscopy, respectively, are compared with corresponding densities of sta tes calculated using the semi-empirical Hartree-Fock intermediate neglect o f differential overlap (INDO) method. Excellent agreement is obtained for b oth occupied and unoccupied levels for PTCDA (3,4,9,10-perylenetetracarboxy lic dianhydride), alpha-NPD (N,N'-diphenyl-N,N'-bis(l-naphthyl)-1,1' biphen yl-4,4 " diamine), and Alq(3) (tris(8-hydroxy-quinoline)aluminum). The resu lts provide a full description of the electronic structure of these molecul es and demonstrate that semi-empirical techniques can be successfully appli ed to describe the unoccupied levels of these molecules. (C) 2000 Elsevier Science B.V. All rights reserved.