Role of sidechain sequence in stereoregular polyalkylterthiophene

To elucidate the role of sidechain sequence for the physical properties of poly(alkyl-thiophenes) the species (1) and (2) were synthesised using octyl as side-groups. The octylthiophene is tail-to-tail connected in (1) while it is head-to-head connected in (2). At room temperature (1) is red while (2) is yellow. Comparative X-ray diffraction and UV-Vis absorption spectroscopy studies of the polymers have been performed. The absorption spectra were obtained for temperature range 20–180°C.The spectrum of (1) obtained at room temperatures show an absorption edge at 2.0 eV. This is similar to regular poly(3-octylthiophene)(P3OT) and as for P3OT the spectrum moves to higher energies when the temperature increases. (2) has an absorption edge at 2.4 eV which is similar to dissolved P3OT. The spectrum of (2) undergoes only a minor shift with increasing temperature. The spectral characteristics can be related to the crystalline order seen by XRD. A sharp, crystalline X-ray pattern is seen for (1) whereas (2) is essentially X-ray amorphous. The different behaviour is believed to be related to chain twisting in (2) due to the head-to-head configuration