1. The field of molecular electronics centers around the
synthesis of single-molecule electronic components. These
components have the potential to one day replace their
silicon-based counterparts, resulting in both smaller and
more efficient devices which can be assembled from the
ground up rather than with the inefficient top-down
approach taken today. As part of a project involving
molecular scale electronic devices, we are exploring the
synthesis of potential rectifiers. A rectifier functions to
convert alternating current (AC) to direct current (DC) by
allowing unidirectional electron flow. Our target molecules
contain electron-rich dimethoxybenzene donor rings and
electron-poor quinone acceptor rings separated by single
bonds.
Figure 1. Br2-HBQ
Here is shown the design of Dibromohemibiquinone (Br2-
HBQ), the precursor molecule which exhibits the acceptor
and donor features (orange and blue, respectively) as well as
bromine substitution sites (white). It was posited by Aviram
and Ratner in 1974 that such a molecule, if synthesized,
could rectify. Their hypothesis was that an electron-rich ring
and an electron-poor ring separated by a long enough
insulator would allow a molecule to conduct electricity. While
their theoretical molecule possessed a two-carbon bridge as
an example of such an insulator, our molecules of interest
possess only a single bond (here shown in green).
Our goals were to:
The proof-of-concept reaction suggests that our substitutions
may in fact be possible. By finding an optimized synthesis
reaction, we were able to improve yield, reduce waste, and
more efficiently synthesize materials for our further
experiments. In the future, we will attempt further
substitutions and test how well the derivatives bond to a gold
surface. Each of these derivatives’ HOMO/LUMO calculations
will be compared to spectral and electrochemical data
gathered by J. Meany. These calculations will be compiled in
order to find the optimum HOMO/LUMO levels for use with a
gold electrode (work function 5.1 eV).
Reaction Name Addition ACN/H2O Ratio Separate Dissolution % Yield
KSW 1-08 quick 1:1 (100 mL ACN/g) No 26%
KSW 1-09 slow 1:1 (50 mL ACN/g) No 21%
KSW 1-10 quick 2:1 (100 mL ACN/g) No 26%
KSW 1-12 slow 1:1 (50 mL ACN/g) No 37%
KSW 1-22 quick 1:3 (25 mL ACN/g) Yes 22%
KSW 1-24 quick 1:3 (50 mL ACN/g) Yes 31%
KSW 1-36* quick 1:3 (50 mL ACN/g) Yes 66%
KED 1-05 slow 1:1 (100 mL ACN/g) No 25%
KED 1-06 quick 1:1 (100 mL ACN/g) Yes 29%
KED 1-07 quick 1:2 (100 mL ACN/g) Yes 34%
KED 1-24 quick 2:9 (40 mL ACN/g) Yes 26%
KED 1-30 quick 1:3 (25 mL ACN/g) Yes 11%
KED 1-32 quick 1:3 (50 mL ACN/g) Yes 20%
KED 1-50* quick 1:3 (50 mL ACN/g) Yes 50%
*Replication of JEM 3-63, a confirmation of OCR 1-13
Figure 2. Alkylation using 1-bromohexane
For proof-of-concept, alkyl substitution was carried out
using 1-bromohexane. This showed the feasibility of future
substitutions for assemblage.
Figure 3. Substitution using cyanobenzoyl
Cyanobenzoyl presents a potential substituent capable of
bonding to a gold surface. The cyano-group nitrogen has a lone
pair which may serve this purpose. Characterization is ongoing.
Figure 4. Br2-HBQ HOMO Figure 5. Br2-HBQ LUMO
Figure 6. Cyanobenzoyl-
substituted HOMO
Figure 7. Cyanobenzoyl-
substituted LUMO
Compound
Br2-HBQ NH2,Br-HBQ Alkyl Sub Cyanobenzoyl Sub
HOMO (eV) -8.96 -8.85 -8.83 -8.92
LUMO (eV) -1.94 -1.55 -1.54 -1.83
• Optimize the synthesis of Br2-HBQ from 2,5-
dimethoxy-1-bromobenzene
• Perform proof-of-concept substitution reactions of
NH2,Br-HBQ
• Add a substituent capable of bonding to a gold
electrode surface
• Model highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO)
characteristics
• Calculate HOMO and LUMO energies
Table 1. Optimization of Br2-HBQ synthesis
Table 2. PM3 calculations of HOMO and LUMO energies
This data is in reference to the optimization of reaction two of the synthesis scheme shown above. Prior to these attempts, yield
was between 20 and 30%. Multiple iterations were performed, varying the rate of addition, type of dissolution (2,5-dimethoxy-1-
bromobenzene separately from vs. together with cerium ammonium nitrate), and ratios of the solvents (acetonitrile and water)
each time. Highlighted reactions were the most successful – these were replications of a promising reaction attempted by O. Roe,
a lab colleague. The optimum addition method appears to be the quick addition of 2,5-dimethoxy-1-bromobenzene and cerium
ammonium nitrate, each separately dissolved in acetonitrile (overall 50 mL per gram of 2,5-dimethoxy-1-bromobenzene) and
water (overall 150 mL per gram of 2,5-dimethoxy-1-bromobenzene). While these yields include some minor impurities, they are
overall more efficient than the previous iterations.
• “Does molecular electronics compute?” Editorial. Nature
Nanotechnology. 8, 377 (2013).
• Aradhya, S. & Venkatamaran, L. Single-molecule junctions beyond
electronic transport. Nature Nanotechnology. 8, 399- 410 (2013).
• Aviram, A. & Ratner, M. A. Molecular Rectifiers. Chemical Physics
Letters. 29, 277-283 (1974).
• Ratner, M. A brief history of molecular electronics. Nature
Nanotechnology. 8, 378-381 (2013).
Special thanks to The University of Alabama, the College of Arts and
Sciences, the Department of Chemistry, and the Computer-Based
Honors Program.