X-ray absorption spectra of poly-p-phenylenes and polyacenes: localization of π orbitals

Abstract

Carbon K-edge X-ray absorption near edge structure (XANES) spectra of evaporated films of poly-p-phenylenes and polyacenes were measured by use of synchrotron radiation. Only one dominant resonance associated with a Cls-to-x* transition was observed for each poly-p-phenylene, although there should exist several x* unoccupied orbitals which are energetically well separated from each other. These features were characterized by means of semi-empirical molecular orbital (MO) calculations based on the equivalent core approximation, which leads to a satisfactory agreement between the observed and calculated spectra. The calculations show that the x* orbitals, which are delocalized in the ground state, become localized by the creation of a CIS core hole. Spectral features of polyacenes, which are somewhat different from those of poly-p-phenylenes in their x* regions, indicate a lesser degree of I[* localization.

Unoccupied electronic states in polythiophene as probed by XAS and RAS

“…Such localization processes have been reported previously for phenyl and thiophene oligomers. [10,11] Similar localizations have also been reported in the gas phase. [12,13] We measured RAS of PT as a function of photon energy to investigate more details of such delocalization.…”

Electronic structure of biphenyl on Si(100)

“…The peak at 287.0 eV corresponds to an excitation of a C͑1s͒ electron into the ͑C-H͒ * orbital. The similarity between the biphenyl and benzene NEXAFS spectra has been previously observed in the gas phase 10,12,15 and ascribed to the localization of the unoccupied molecular orbitals after C͑1s͒ excitation.…”

“…This room temperature separation of the 1 * resonances is not expected to originate from different C͑1s͒ ionization energies. Indeed the C͑1s͒ ionization energies of a number of gas phase aromatic hydrocarbon molecules 10 have been observed to broaden the NEXAFS peaks by much less than 0.9 eV. Also, high resolution core level photoelectron spectroscopy of benzene adsorbed on Si͑100͒ ͑Ref.…”

“…Over the past few years, the electronic structure of benzene adsorbed on Si͑100͒ has been studied extensively by using a number of synchrotron radiation based methods, namely, ultraviolet photoemission spectroscopy ͑UPS͒, 6 x-ray photoemission spectroscopy ͑XPS͒, 7,8 and near-edge x-ray absorption fine structure spectroscopy ͑NEXAFS͒. [9][10][11][12] The biphenyl molecule is made of two phenyl rings connected by a C-C bond. The torsional angle and the electronic interaction between the phenyl rings depend on the local environment.…”

Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces

“…It has been applied to the study of electronically functional organic materials such as aromatic hydrocarbons, [123,124] fullerenes, [125±127] Alq 3 , [128] PPV, [129] and poly(3-methylthiophene). [130] Unfortunately, however, the observed spectrum is usually not a replica of the density of unoccupied states, due to the strong effect of core hole.…”