INNER-SHELL EXCITATION OF GAS-PHASE AND POLYMER THIN-FILM 3-ALKYLTHIOPHENES BY ELECTRON-ENERGY LOSS AND X-RAY PHOTOABSORPTION SPECTROSCOPY

Oscillator strengths for C 1s and S 2p excitation have been derived from gas-phase electron energy loss spectra of a series of 3-alkyl-substituted thiophenes. The C 1s oscillator strength spectra are compared to the C 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of electropolymerized thin films of the same materials. In the gas-phase-monomer spectra the width and peak intensity of the C 1s → π* transition are found to be independent of the length of the 3-alkyl substituent so that the integrated oscillator strengths for C 1s → π* excitation are essentially unchanged on alkylation. Analysis of the polymer NEXAFS data using a subunit normalization procedure similar to that used for the gas-phase-monomer data suggests that the C 1s → π* intensity in the poly(3-alkylthiophenes) is also essentially independent of the length of the alkyl side chain. This disagrees with an earlier analysis of the same polymer NEXAFS spectra in which apparent decreases in the C 1s → π* intensity with increased alkyl chain length were attributed to delocalization of the ring-π* orbital onto the alkyl side chain. The significance of these results with regard to the electrical properties of doped alkyl-substituted polythiophenes is discussed. © 1990 American Chemical Society.