Valence electronic structure of pi-conjugated materials: Simulation of theultraviolet photoelectron spectra with semiempirical Hartree-Fock approaches

A detailed understanding of the electronic structure of pi-conjugated mater ials can be reached by means of two widely available semiempirical quantum- chemical methods: Austin model 1 (AM1) and intermediate neglect of differen tial overlap (INDO). This is illustrated by calculating the ultraviolet pho toelectron spectra (UPS) of pi-conjugated oligomers and polymers and compar ing the theoretical data to experimental spectra. The approach is applied h ere to a series of compounds with varying molecular topology and chemical c onstitution: oligomers of p-phenylenevinylene and various derivatives, fluo rinated derivatives of polyisothianaphthene, and 4,4'-bis(m-tolyphenylamino )biphenyl (TPD). The AM1- and INDO-calculated UPS spectra are also compared to data obtained with the valence effective Hamiltonian method, which is k nown to provide reliable results for the simulation of UPS spectra of these types of molecules. An easily applicable procedure is proposed to obtain t he best fit to the experimental spectra from the AM1 and INDO molecular orb ital energies. Both techniques accurately reproduce the lower energy part o f the spectrum, which contains the most important part of the pi electronic structure; INDO is also found to perform well for the inner part of the UP S spectrum, which mainly corresponds to the sigma electronic states.