Orbital Control and Coherent Charge Transport in Transition Metal Platinum(II)–Platinum(II) Lantern Complexes in Molecular Junctions

Platinum(II) dimers in organometallic complexes exhibit oscillatory vibrational charge transfer with optical excitation and whose nature depends strongly on the Pt–Pt distance. Yet, the electronic transport of such devices has not been studied. We take a well-reported Pt(II) dimer complex that is ideal for transport in a molecular junction and subject it to a battery of electronic structure–property calculations. With DFT, we find the orbital nature of the HOMO level to not be solely induced by the 5dz₂ orbital as long thought, but rather a complex mixture with significant contributions from 6s orbital states from the Au electrodes that have not been considered before. We probe how the chemical tuning of the ligand affects the Pt–Pt bond length and thus conduction, by systematically modifying both the bridging ligands (BL) and cyclometallating ligands (CMLs) with donor (methyl) and acceptor (fluorine) derivatives. With nonequilibrium Green’s functions (NEGF-DFT), we find patterns between the conductivity, substituent and ligand type. Methyl groups tend to increase the conduction and fluorine does vice versa, but it is strongly dependent on the ligand. We modify the atoms on the bridging ligand in contact with the Pt centers and derivatize with N, O, S and Se. The heavy elements such as S and Se give a decreasing conductance due to the interaction of the bonding π orbitals with the antibonding HOMO level. This is evident from the bond orders being ∼1 with no ligands but ≪1 with ligands, as revealed with Mayer, Gophinatan–Jug (G-J) and Nalewajski–Mrozek (N-M) indices. The same heavy elements also bind strongly to Pt, making the ligands a more viable pathway for the conduction and giving rise to destructive quantum interference at the second Pt site. The projected density of states (DOS) show the bonding states to be in the occupied eigenchannel, and the Seebeck thermopower function confirms the majority charge carrier to be p-type. The donor/acceptor correlation with the ligand type and degree of substitution shows enhanced chemical tuning is possible for transition-metal complex (TMC) junctions.