Charge control in a model biphenyl molecular transistor.

We study charge control in a gated 4,4'-biphenyl diradical molecular transistor using ab initio density functional theory calculations. I-V curves and intrinsic gate capacitances were derived. We find charge control in this transistor to be strongly affected by polarization of the sigma-states of the molecule, leading to strong electrostatic coupling of the internal potentials to the source and drain electrodes, and relatively weak coupling to the gate. We suggest that this spatially dependent and anisotropic polarization is an essential element in the operation of molecular transistors.