How substitution tunes the electronic and transport properties of oligothiophenes, oligoselenophenes and oligotellurophenes

The geometrical, electronic and charge-transport properties of a series of unsubstituted and dicyanovinyl (DCV)-substituted oligothiophenes (nT), oligoselenophenes (nS) and oligotellurophenes (nTe) where n = 3-5 are investigated by means of Density Functional Theory (DFT) and Time-Dependent DFT calculations. The intramolecular reorganization energy is found to decrease upon dicyanovinyl substitution, with a larger decrease for holes than electrons. The ground state HOMO and LUMO energies along with the Time-Dependent DFT calculations show smaller HOMO-LUMO gaps for the DCV-substituted oligomers. Calculations of the intermolecular charge transfer integrals in the crystals have been performed to understand the role of substitution on the charge-transport rate. DCV-substitution changes the packing motifs in the crystal and results in larger transfer integrals for holes along the pi-stacldng direction, calculated as -63 and -88 meV for DCV4T and DCV4S respectively. These results suggest that DCV-substituted oligomers studied here are more promising charge transporting materials than their unsubstituted analogues. (C) 2015 Elsevier B.V. All rights reserved.