Molecular Rectification Enhancement Based on Conformational and Chemical Modifications

Design principles for molecules with intrinsic directional charge transport will likely prove crucial for breakthroughs in nanotechnology and other emerging fields like biosensors and advanced photovoltaics. Here, we perform a systematic computational study to characterize the electronic rectification induced by conformational and chemical modifications at low bias potentials and elucidate design principles for intrinsic molecular rectifiers. We study donor-bridge-acceptor (D-B-A) systems that consist of phenylene units with geometrical rotation of the rings and representative electron-donating and -withdrawing substituent groups at the donor and acceptor sites. We calculate transport properties using the non-equilibrium Green's function technique and density functional theory (DFT-NEGF) and obtain I-V characteristics and rectification ratios. Our results indicate that efficient intrinsic rectification at low bias voltages can only be obtained by combining dihedral angles of 60° between phenyl rings and asymmetric chemical substitution. Together, these structural features cause rectification enhancement by localizing the molecular orbital closer to the Fermi level of the electrode in one end of the molecular device. Our designed systems present rectification ratios up to 20.08 at 0.3 V in their minimum-energy geometry and are predicted to be stable under thermal fluctuations.