Calculations of electron transport through simple π-and σ-type radicals

Organic radicals are of interest in molecular electronics because unpaired electrons lead to degeneracy splitting in other energy levels and such molecules may act as spin filters. This work employs first principles transport calculations using a combination of density-functional theory and a nonequilibrium Green's function technique to model the electron transport properties of 1,4-benzenediamine (BDA) molecules bridging two Au electrodes. These molecules were substituted in the 2-position with -CH₃, -NH₂, and -OH, and also with their radical analogues -CH ₂•, -NH•, and -O•, which have π-type singly occupied molecular orbitals (SOMO). Spin filter efficiency (SFE) values for these radicals vary as 49%, 27%, and 1% for the -CH₂•, -NH•, and -O• containing systems, respectively. The large difference is due to the electron affinity of each radical. We found that the radical can indeed accept some charge once it is connected to electrodes, thereby reducing the fraction of excess spin which, in turn, reduces the amount of MO level splitting and the SFE. The transport properties of a radical with a σ-type SOMO were also calculated for a BDA molecule with the H atom in the 2-position of the benzene ring removed. A SFE of 34% was calculated for this system, but most importantly we found that a significant amount of electron transport can indeed occur through a σ-type MO.