A PBC-DFT study of electronic properties of substituted polythiophenes

The electronic properties of polythiophenes substituted with various electron-withdrawing and electron-donating groups are investigated by employing periodic density functional calculations. The polymer is modeled as infinite one-dimensional system with periodic boundary condition along the molecular direction. The effect of substitution on bandgaps is studied with various substituents like alkyls, halogens, aromatic and alkoxy groups in different regioregular forms. The alkoxy groups are found to substantially lower the bandgap of unsubstituted polythiophene, and aromatic groups in head-head/tail-tail forms are found to increase the bandgap. Irrespective of the type of substituents, groups that show deviations in planarity in oligomeric forms have shown an increase in bandgap in contrast to the general perception that head-head linkage always results in an increase in bandgap. Our study also confirms the previous findings that scaling from the oligomer bandgaps poses serious limitation to the prediction of polymeric bandgaps and that it should be evaluated with the infinite polymer system employing periodic boundary condition. Copyright (C) 2015 John Wiley & Sons, Ltd.