Quantum interference is a well-known phenomenon that dictates charge transport properties of single molecule junctions. However, reports on quantum interference in donor-bridge-acceptor molecules are scarce. This might be due to the difficulties in meeting the conditions for the presence of quantum interference in a donor-bridge-acceptor system. The electronic coupling between the donor, bridge, and acceptor moieties must be weak in order to ensure localised initial and final states for charge transfer. Yet, it must be strong enough to allow all bridge orbitals to mediate charge transfer. We present the computational route to the design of a donor-bridge-acceptor molecule that features the right balance between these contradicting requirements and exhibits pronounced interference effects.

Rectification has been at the foundation of molecular electronics. Most single-molecule diodes realized experimentally so far are based on asymmetries in the coupling with the electrodes or using the donor-acceptor principle. In general, however, their rectification ratios are usually small (<10). Here, we propose a single-molecule diode based on an orbital resonance while using the highest occupied molecular orbital (HOMO) and HOMO-1 as transport channels. Our proposed diode design is based on an asymmetric two-site model and analyzed with DFT + NEGF calculations. We find high rectification ratios, even in the case of symmetric coupling to the electrodes. In addition, we show that diode parameters such as the operating voltage and rectification ratio can be tuned by chemical design.