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

• The largest geometrical change upon oxidation occurs on unsubstituted oligomers.
• No significant geometrical change found in dicyanovinyl-substituted oligomers upon reduction.
• DCV-substituted oligomers undergo similar geometrical modifications upon ionization.
• The effect of DCV-substitution is to significantly reduce the reorganization energy for holes.
• Significant transfer integrals are found between selected dimers in the 4T, DCV4T and DCV4S crystals.

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 π-stacking 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.

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