Gas-phase ionization potentials
Ferrocene is a good electron donor, and as such has been used to test asymmetric conduction (rectification) in molecules that contain ferrocene. Of the five ferrocene-containing molecules that rectify (structures 11, 15, 19, 20, and 22), the last (22) exhibits a record rectification ratio, which should be a dramatic incentive for searching for more high-efficiency rectifiers.
- unimolecular electronics
- rectification ratio
- highest occupied molecular orbital
- Aviram-Ratner proposal of 1974
“Unimolecular electronics” (UME)  was born in 1974 with a theoretical proposal by Arieh Aviram and Mark Ratner (AR) for a one-molecule rectifier (or diode) of electrical current donor-bridge-acceptor (D-σ-A)  (Figure 1, structures
The first rectifier (
As of 2015, 53 unimolecular rectifiers had been measured worldwide , 15 of which at the UA (Figure 1, structures
The present article focuses on the use of one particular powerful one-electron donor in rectifiers: ferrocene.
In the 1980s, UME had hoped to develop useful molecular-scale (~2 nm3) devices for ultrahigh-density and high-speed industrial electronics. To interrogate such molecules (or monolayers of molecules), metal electrodes or nanoelectrodes (Al, Ag, Au, etc.) are used: this is sketched below and explained in detail in many review articles [8, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]. UME learned how inorganic metals couple (associate with or bond to) single organic molecules and how one can reliably exchange electrons and photons with these molecules.
In the meantime the much wealthier and commercially driven electronic industry has made gigantic strides and has developed high-speed almost-nanoscale electronic circuits using inorganic semiconductors (Si, Ge, and GaAs). Therefore the original hope and promise of UME have been defeated. Nevertheless, UME has learned to interrogate and control individual molecules, and its present challenge is how to combine and exploit electronic, photonic, and spintronic functions in new ways.
The present review looks at how ferrocene-containing molecules have contributed valid and promising and most recently also very dramatic UME rectifiers: in particular molecules
Table 1 shows some relevant gas-phase ionization potentials
Asymmetric placement of the electrophore in the electrode gap (“A” rectifier) .
Unimolecular processes depending on molecular energy levels (“U” rectifier) .
A recent fourth mechanism for rectification is asymmetric polarization (“AP” rectifier), when highly polar solvents can induce an asymmetric conductance of a symmetrical molecule between very asymmetric electrodes in a scanning break junction (SBJ) .
Purists would prefer pure-“U” rectifiers, requiring “S” = 0 and “A” = 0. For many molecules, for reasons of assembly, “U” and “A” effects are combined  (e.g., Figure 1 for structures
We next discuss rectifier
The Whitesides group (including Nijhuis) studied the rectification of self-assembled monolayers of thiol-containing molecules
The key improvements in  were (i) using Pt as the “bottom” electrode, because PtTS tolerates a larger bias range than AuTS or AgTS, (ii) a presumed efficient van der Waals contact between Fc-C ≡ C-Fc and EGaIn, and (iii) a “long enough alkyl tail” to get a very small reverse-bias current .
Also, light emission was measured (with blinking) for
However, the measured
There has also been a recent brief review on this exact topic ; for the sake of brevity, we refer the reader to the papers cited for other significant rectifiers containing the donor ferrocene [43, 44, 45, 46, 47, 48, 49].
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