Bridge Effect on Molecular Rectification: Linearly Conjugated, Cross- Conjugated, and Saturated Bridges

The bridge is a crucial component of the donor-bridge-acceptor (D-B-A) architecture when studying electron transport properties and molecular electronics. In this work, we explore the influence of the bridge on molecular rectification using the nonequilibrium Green's function coupled with density functional theory. The systems considered here are phenylene-based structures with and without amino (-NH2) and nitro (-NO2) substituents at the D and A subunits, respectively. We define six different bridge structures with linearly conjugated, cross-conjugated, and saturated bonds. We calculate electron transport properties to obtain the current-voltage I-V characteristics and rectification ratios (R) at low bias voltages. Our results show that the I-V characteristics of the systems without substituents are symmetrical, suggesting a wire-like behavior. As expected, rectification is only present in the systems with substituents. The R is maximum in the D-XC-A system, with a value of 2.32 at 0.15 V, where XC is a cross-conjugated bridge. However, we conclude that the best rectification performance, with a R of 1.62, is that of the D-double-A system since the conductance is not compromised to promote rectification.