Molecular Electronics: Synthesizing the Technology of the FutureKindle S. Williams, Katherine E. Donaldson, Joseph E. Meany, and Stephen A. WoskiDepartment of ChemistryThe University of Alabama, Tuscaloosa, AL 35487
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-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).
Our goals were to:
Synthesis and Methods
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 NameAdditionACN/H2O RatioSeparate Dissolution% YieldKSW 1-08quick1:1 (100 mL ACN/g)No26%KSW 1-09slow1:1 (50 mL ACN/g)No21%KSW 1-10quick2:1 (100 mL ACN/g)No26%KSW 1-12slow1:1 (50 mL ACN/g)No37%KSW 1-22quick1:3 (25 mL ACN/g)Yes22%KSW 1-24quick1:3 (50 mL ACN/g)Yes31%KSW 1-36*quick1:3 (50 mL ACN/g)Yes66%KED 1-05slow1:1 (100 mL ACN/g)No25%KED 1-06quick1:1 (100 mL ACN/g)Yes29%KED 1-07quick1:2 (100 mL ACN/g)Yes34%KED 1-24quick2:9 (40 mL ACN/g)Yes26%KED 1-30quick1:3 (25 mL ACN/g)Yes11%KED 1-32quick1:3 (50 mL ACN/g)Yes20%KED 1-50*quick1:3 (50 mL ACN/g)Yes50%*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 LUMOFigure 6. Cyanobenzoyl-substituted HOMOFigure 7. Cyanobenzoyl-substituted LUMO
CompoundBr2-HBQNH2,Br-HBQAlkyl SubCyanobenzoyl SubHOMO (eV)-8.96-8.85-8.83-8.92LUMO (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-HBQAdd a substituent capable of bonding to a gold electrode surfaceModel highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) characteristicsCalculate HOMO and LUMO energies
Table 1. Optimization of Br2-HBQ synthesisTable 2. PM3 calculations of HOMO and LUMO energiesThis 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). Acknowledgements
Special thanks to The University of Alabama, the College of Arts and Sciences, the Department of Chemistry, and the Computer-Based Honors Program.