Mechanisms of molecular electronic rectification through electronic levels with strong vibrational coupling

Abstract

We present a new view and an analytical formalism of electron flow through a donor-acceptor molecule inserted between a pair of metal electrodes. The donor and acceptor levels are strongly coupled to an environmental nuclear continuum. The formalism applies to molecular donoracceptor systems both in vacuum or air, and in aqueous solution under electrochemical potential control. Multifarious patterns of rectified electron flow from the negatively to the positively biased electrode arise. The electronic interaction between the donor and acceptor fragments, mutually and with the electrodes, can be weak, corresponding to the fully diabatic limit. The rectification process then reduces to a sequence of vibrationally relaxed single-electron transfer steps. In the limits where the interactions are strong, denoted as the partially and fully adiabatic limits, the character of the rectification process is different, and electron flow proceeds coherently, without vibrational relaxation. In still another class of mechanisms the electronic level broadening of either donor or acceptor from the adjacent electrode is so strong that it is comparable to the vibrational broadening. The process then reduces to a three-level transition similar to STM of large redox molecules. Recent data for rectification in hexadecyl-quinolinium tricyanodimethanide monolayers by Metzger and co-workers ͓J. Am. Chem. Soc. 119, 10455 ͑1997͒; Acc. Chem. Res. 32, 950 ͑1999͔͒, are discussed in terms of the reported views and formalism.

Coulomb repulsion effect in two-electron nonadiabatic tunneling through a one-level redox molecule

“…Rectification is usually associated with two-quantum dot systems [46][47][48][49] or a single-level bridge molecule in asymmetric tunneling contacts. 49,50 Equation ͑30͒ shows that rectification is impossible for symmetric vacuum tunneling contacts even when electron-phonon interactions are included.…”

Kinetic control of the current through a single molecule

“…Our main objective for using such a simplified description is to derive analytic expressions for the current which can be directly compared with those following from the scattering approach. Corrections related to important electron-vibration interactions result in some fine structures in the I-V-curves [16,19,20,22,26,[37][38][39][40] and are not considered here further. Therefore, the derived analytic results describe the coarsened I-V characteristics of the LMR-device with a single active MO.…”

“…During the (elastic or inelastic) tunneling processes the molecular orbitals (MOs) are not populated by the transferred electrons. Their population is only caused by sequential transfer [4,[23][24][25][26], where the transferred electron jumps between the conduction band states of the lead and the molecular levels. In Ref.…”

Principles of a Single-Molecule Rectifier in Electrolytic Environment

“…First this has been achieved in the presence of redox molecules in the bridge, which was first predicted 5 and then experimentally confirmed, 6 giving a bell shape dependence of the current voltage plot with a maximum at the bias voltage corresponding to the energy of the lowest lying unoccupied level of the redox center. That area has been later widely developed 7,8,9,10,11,12 with an intention to, potentially, exploit the putative or real diode-and transistor-like behavior of such junctions, central to molecular electronics. 13,14 However, later another direction has evolved in this research area, boosted by creation of temporary nanoscale junctions, 15,16 which appeared to be a good and to some extent complementary method for studying single-molecule charge transport in electrochemical environment.…”