Theoretical Studies on Protonation-Induced Inversion of the Rectifying Direction in Dipyrimidinyl–Diphenyl Diblock Molecular Junctions

The protonation effects on electron transport through a conjugated dipyrimidinyl–diphenyl diblock oligomer sandwiched between two gold electrodes were theoretically investigated using the fully self-consistent nonequilibrium Green’s function method combined with density functional theory. All configurations of protonation on the dipyrimidinyl group of the molecule were considered. The numerical results show that the protonation prefers to improve electron transport ability through the molecular junction. The transmission spectra around the Fermi energy are tuned by the number and locations of protons residing in the molecular junction. It is found that the protonation in the outer pyrimidinyl is favorable for enhancing the rectification ratio, while the protonation in the inner pyrimidinyl plays a dominative role for inverting the rectifying direction. This theoretical work for the first time presents a mechanism for the experimental findings of inversion of the rectifying effect in diblock molecular diodes caused by protonation [J. Am. Chem. Soc.2005, 127, 10456].