Molecular Electronics: Synthesizing the Technology of the FutureKindle S. Williams, Katherine E. Donaldson, Joseph E. Meany, and Stephen A. Woski
Department of ChemistryThe University of Alabama, Tuscaloosa, AL 35487
IntroductionThe 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-HBQHere 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).
ObjectivesOur goals were to:
Synthesis and Methods
Optimization Data
HOMO/LUMO Calculations
Conclusions/Moving ForwardThe 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 % YieldKSW 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
O HN
O
OMe
OMeBr
ONH2
O
OMe
OMeBr
NaH, 1-bromohexane
THF
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.
ONH2
O
OMe
OMeBr
NaH, cyanobenzoyl chloride
DMF
O HN
O
OMe
OMeBr
N
O
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.
References• “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).
AcknowledgementsSpecial thanks to The University of Alabama, the College of Arts and Sciences, the Department of Chemistry, and the Computer-Based Honors Program.
OMe
OMe
OMe
OMe
BrNBS
OBr
O
OMe
OMeBr
1.75 Ce(NH4)2(NO3)6
CH3CN, H2O, RTCH3CN
NaN3
iPrOH, H2O
ON3
O
OMe
OMeBr
NaBH4
iPrOH, H2O
ONH2
O
OMe
OMeBr
OBr
O
OMe
OMeBr