Electron Spin Resonance Studies of Anisotropy in Semiconducting Polymeric Films

X-band ESR has been employed to study the structural anisotropy in several polythiophene derivatives. Because of the dominating homogeneous width, mainly Lorentzian shaped spectra were obtained. Information about the structural anisotropy was obtained from the peak position and width of the Lorentzians. Qualitatively, the anisotropy was in full agreement with earlier results from X-ray diffraction, including the 'flip' in molecular orientation between solution and spin cast films with respect to the film substrate. With the Monte Carlo technique, the spectra were fitted with a biaxial g tensor and an anisotropy parameter S, and an intrinsic width s. The simulations showed that the g-tensor could be treated as pseudo-uniaxial, with the unique axis along the side chains rather than along the ring normal. Closed-form analytical expressions relating the g-tensor to the anisotropy were used for a quantitative assessment of the molecular anisotropy. Because the molecular g tensor for these materials are not known, a known value of S for one sample obtained by x-ray diffraction was used for normalization. Fairly consistent values were obtained for both g and S.