Electroabsorption measurements of conjugated organic materials

Abstract

This thesis reports the results of electroabsorption measurements undertaken on three materials that are all, to some degree, conjugated: polymeric and oligomeric emeraldine base, and polysquaraine. The aim of these experiments has been to investigate the nature of the optical excitations occuring within the materials. Electroabsorption (EA) spectroscopy involves the measurement of the change in absorption coefficient of a material with the application of an external field. The fields required are high and the resulting signals small, hence to perform such experiments a dedicated spectrometer was constructed. To achieve high sensitivity lock-in amplification techniques were used, and the sample kept at low temperatures. Such techniques resulted in the spectrometer being able to resolve changes in absorption of the order 1 in 5x10(^7).The sample configuration consisted of thin films of the materials which were spun coated onto sapphire substrates, with interdigitated gold electrodes deposited on top. This configuration allowed the absorption of the material to be measured while alternating fields of up to 200 kVcm(^-1) were applied. The EA data of the oligomeric and polymeric emeraldine base are seen to closely resemble each other - indicating that the same photoexcitation processes are occurring. Using existing theories the spatial extent of the 2 eV excitation is calculated as -0.4 nm, i.e. greater than one phenyl ring repeat unit. This is consistent with previously suggested models of 2 eV photoexcitation in emeraldine base. Similar calculations suggest a spatial extent of the 4 eV transition of -0.25 nm, i.e. restricted to one phenyl ring. A feature in the EA spectra of the oligomeric emeraldine base has been observed at 1.35 eV - below the onset of linear absorption, and it is suggested that this may be evidence of a normally one photon forbidden transition becoming allowed in the presence of an external field. Due to the fully conjugated nature of polysquaraine a different model has been used to interpret the EA spectrum. An energy level scheme for the material is suggested, including the possible location of a normally one photon forbidden transition at 1.75 eV.