**3. Conclusion**

The frustrating issue of historically low measured *RR*s [8] has thus been resolved experimentally [36]: organic monolayer rectifiers may finally challenge the *RR* of inorganic *pn* junction rectifiers.

However, the measured *RR*s for alkanethiols are hundreds of times smaller than expected from careful theoretical simulations [42]: this puzzle must be solved, so that measurements are not victims of unforeseen inefficiencies in the "metal | molecule" interface. The number of measured unimolecular rectifiers has grown dramatically, but their preselection as candidate rectifiers has been somewhat haphazard. Once the "metal | molecule" interface is brought under experimental control, better measurements may provide valid physical organic criteria to guide the design of the better unimolecular rectifiers of tomorrow.

There has also been a recent brief review on this exact topic [43]; for the sake of brevity, we refer the reader to the papers cited for other significant rectifiers containing the donor ferrocene [43–49].
