Hopping Transport and Rectifying Behavior in Long Donor−Acceptor Molecular Wires

We have developed a series of long donor (D)−acceptor (A) block molecular wires (DmAn or DmCAn: C,cyclohexane bridge; m, n = 1−4) attached to Au surfaces with lengths ranging from 3 to 10 nm in order to probe electrical rectification in the hopping regime. In each wire, the donor block was synthesized from the Au surface by stepwise imine condensation between 4,4′(5′)-diformyltetrathiafulvalene electron donors (D) and 1,4 diaminobenzene linkers, followed by the stepwise synthesis of the acceptor block using N,N′-di(4-anilino)-1,2,4,5-benzenebis(dicarboximide) electron acceptors (A) and terephthaldehyde linkers. Molecular junction measurements by conducting probe atomic force microscopy (CP-AFM) revealed that the DmCA1 (m = 1−4) wires exhibited electrical rectification with current rectification ratios as high as 30 at ±1.0 V when contacted with Au-coated tips and Au substrates; DmAn wires did not rectify, suggesting electronic decoupling of the D and A blocks is necessary for diode behavior. The forward bias condition for DmCA1 corresponded to negative potential on the acceptor block and positive potential on the donor block, as anticipated. Furthermore, the rectification ratio was a function of the wire architecture, length, and measurement temperature. Density functional theory (DFT) calculations of ground state neutral and ionized electronic structures and the experimental data for DmCA1 suggest that under forward bias the rate limiting transport step in these diodes is activated hole hopping from the HOMO level of the D block to the HOMO level of the A block; that is, hole- only transport pertains and it is sensitive to energy level alignment. Under reverse bias, the rate limiting transport step is relatively insensitive to temperature, which is consistent with a change in the rate limiting mechanism from hopping to tunneling. We propose a simple energy level model that rationalizes the change in transport mechanism and we suggest how these molecular diode structures might be further improved to achieve better rectification with simultaneous hole and electron transport in the D and A blocks, respectively.