End-Group-Induced Charge Transfer in Molecular Junctions: Effect on Electronic-Structure and Thermopower

We analyze triphenyl molecules coupled to gold electrodes through five different end groups to understand the effect of end groups on the thermoelectric properties of molecular junctions. Our investigation suggests that end-group-mediated charge transfer between the molecule and electrodes plays an important role in the resulting thermoelectric properties. We find that the direction of charge transfer, which is governed by the electronegativity of the end-group functionalized molecule, is strongly correlated to the degree of reorganization of frontier molecular orbitals (HOMO–LUMO). In particular, isocyanide, nitrile, and amine end-group molecular junctions, with charge (electron) transfer out of the molecule, exhibit a strong overall downward shift in the energies of frontier molecular orbitals, whereas thiol and hydroxyl end-group molecular junctions, with charge transfer into the molecule, exhibit a smaller overall downward shift. Finally, our study shows that the sign of the thermopower of molecular junctions is closely related to the HOMO–LUMO energies and electronegativity of isolated molecules.