Valence electronic structure of π-conjugated materials:: Simulation of the ultraviolet photoelectron spectra with semiempirical Hartree-Fock approaches

A detailed understanding of the electronic structure of pi-conjugated materials can be reached by means of two widely available semiempirical quantum-chemical methods: Austin model 1 (AM1) and intermediate neglect of differential overlap (INDO). This is illustrated by calculating the ultraviolet photoelectron spectra (UPS) of pi-conjugated oligomers and polymers and comparing the theoretical data to experimental spectra. The approach is applied here to a series of compounds with varying molecular topology and chemical constitution: oligomers of p-phenylenevinylene and various derivatives, fluorinated 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 known to provide reliable results for the simulation of UPS spectra of these types of molecules. An easily applicable procedure is proposed to obtain the best fit to the experimental spectra from the AM1 and INDO molecular orbital energies. Both techniques accurately reproduce the lower energy part of 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 UPS spectrum, which mainly corresponds to the sigma electronic states.