Valence Electronic Structure of π-Conjugated Materials:  Simulation of the Ultraviolet Photoelectron Spectra with Semiempirical Hartree−Fock Approaches

Abstract

A detailed understanding of the electronic structure of π-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 π-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 INDOcalculated 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 π electronic structure; INDO is also found to perform well for the inner part of the UPS spectrum, which mainly corresponds to the σ electronic states.

A joint theoretical and experimental study on the electronic properties of phenyl-capped 3,4-ethylenedioxythiophene oligomers

“…The disagreement between the HF eigenvalues and the UPS data is unexpected since it has been shown that such an HF theoretical scheme is successful in simulating the electronic structure of other conjugated oligomers. 25 As shown in Fig. 4, there is a one-to-one correspondence between the spectral features recorded in the gas and solid phase.…”

“…The approach has been extensively described in connection with the studies of conjugated molecules, where the DOVS curves were obtained by either ab initio or semiempirical HF methods. 25 Using Koopmans' approach, the peaks in the UPS spectra are associated to eigenenergies calculated for the electronic levels in the neutral molecule. The following steps were taken to facilitate comparison of the theoretical results with the experimental spectra.…”

“…The DOVS curves derived from RHF calculations were scaled along the binding energy axis by a factor of 1.3 for RHF to correct for correlation and relaxation effects upon photoionization. 25 In case of DFT calculated DOVS, the required scaling was negligible ͑0.97͒. Subsequently, the curves were rigidly shifted to match the experimental spectra, which compensate for solid-state relaxation effects.…”

Molecular n-Type Doping of 1,4,5,8-Naphthalene Tetracarboxylic Dianhydride by Pyronin B Studied Using Direct and Inverse Photoelectron Spectroscopies

“…The computed densities of states for the occupied and unoccupied levels were convolved with a Gaussian function and rigidly shifted on the energy scale (by -2.6 and -0.5 eV to match the position of the first peak in each case), in order to simulate the solid-state UPS and IPES spectra, respectively. Extensive studies on a variety of systems have shown that such a simulation procedure, which effectively allows one to include solid-state polarization effects, reproduces experimental spectra very well [23,24].…”

Transparent low-work-function indium tin oxide electrode obtainedby molecular scale interface engineering

“…25 A detailed description can be found elsewhere. 26,27 As discussed above, the presence of TDAE at the ITO surface is revealed by a photoemission signal corresponding to -orbitals of the molecules. Meanwhile, the low binding energy feature at 2.2 eV, associated to the highest occupied molecular orbital of TDAE, is absent in the spectrum of modified ITO.…”