The Preparation of Highly Conjugated Organic Materials for Device Applications

1. The Preparation of Highly Conjugated Organic Materials for Device Applications Alex R. Ketchum, Ji-Min Jeon, Eric P. Lauzon, Kaleb L. Topp and Elizabeth Sanford* Department of Chemistry, Hope College, Holland, MI 49423 Abstract Synthetic Strategy Synthetic Route to Molecule 3 Conclusions In conclusion, the Wittig synthesis has proven a successful method in the preparation of extended conjugated organic molecules with potential for light-emitting applications. The McMurry reaction as well as formylation have both demonstrated their synthetic effectiveness. Isomerization using iodine has proven successful for converting Molecules 2 and 3 to the desired trans/trans stereoisomers. Future utilization of the technique will allow substantially larger yields. Although isomerization did not appear to be effective with Molecule 1, it was determined that using the dialdehyde central molecule produces solely the trans/trans product. Williamson ether synthesis has shown to allow flexibility in product solubility by attaching variable length alkyl and polyether chains via ether linkages, and future work will continue to explore its use. It has found to be considerably more efficient to make halogenated endcaps using a chlorination method, as opposed to the originally employed radical bromination technique. By starting with a benzyl alcohol, a chlorine substitution has shown to not only produce a higher yield initially, but also to not decompose (a problem experienced with brominated compounds) as readily. Past organic light emitting applications have employed conjugated polymers, and future work may also include polymer preparation via bifunctional core and “capping” molecules. Current work is being made in preparing an extended conjugated system using a porphyrin molecule. Porphyrins with an single aldehyde functionality (from formylation of one thiophene ring) could potentially be used to produce the following molecule: Eventual collaboration with industrial chemists and engineers will soon determine the molecules’ effectiveness for device applications. Analysis will provide direction for molecular adaptations, or characteristics key to successful semiconducting conjugated organic molecules and guide our future synthetic work. Our work in the synthesis of new materials for device applications focuses on substituted poly(arylenevinylenes).  Poly(arylenevinylenes) are a class of semiconducting polymers that can emit light through electroluminescence, a form of luminescence where light is emitted from a substance when it is excited by an electric field. Poly(arylenevinylenes) are now used as components of organic light emitting diodes (OLED's) and light emitting electrochemical cells (LEC's).  These technologies are used for molecular electronics including cutting edge displays and light sources.  This poster describes the preparation of conjugated thiophene derivatives. These small molecules can be used to study the properties of poly(arylenevinylenes). The synthetic plan includes making core and end cap molecules that can be connected via carbon-carbon double bonds formed using the Wittig reaction.  This methodology allows us to synthesize a family of materials ranging from small to polymeric for device testing. Ti(IV)Cl4 ZnTHF 77% yield 1) n-BuLi 2) anhydrous DMF THF Na+O-Me 73% yield THF, MeOH Introduction Target Molecules A highly conjugated molecule has alternating multiple and single bonds like the Target Molecules shown to the right. From the mid- to late 20th century many scientists thought that highly conjugated organic compounds would provide synthetic, light-weight versions of traditional conducting metals because electrons could potentially zip along the metal-like conjugated bond frame work. Progress has indeed been made in this area, but independence from the use of “real” metals has not been achieved. A side product of this research, however, was the discovery of a class of semiconducting polymers, poly(arylenevinylenes) that can emit light through electroluminescence. If you apply voltage to the materials, they give off light. They are intimately related to solar cells in which the opposite occurs—light is absorbed and can be transformed into electricity. Similar materials can be used for both light and energy generation. In the area of light generation, applications include those for energy efficient, high quality lighting and display technology. For energy generation, these compounds are providing better materials for solar cells or photovoltaics. The advantage these compounds have for applications is that as plastic-like organic materials, they are flexible, moldable and thin. Prototype TV displays and computer screens made from these materials look like overhead transparency sheets. They are currently used in small screen devices but stability problems are preventing their use in large screen devices that need longer lifetimes. That’s where our work comes in. Previous work in our group focused on polymeric materials. Because polymers are mixtures, attributing differences in the properties of the materials to their chemical structures is very difficult. Getting to the bottom of differences and solving problems requires better characterization. Therefore it is our intent to synthesize highly conjugated materials that are small molecules not polymers to see how they function in device applications. A versatile synthetic plan will allow a large number of compounds to be made to complete a structure function study that hopefully will lead to advances in these materials. Solubility Effects Alex Ketchum: Molecule 1: 2,5-bis(E-2-phenylethenyl) thiophene Figure 1: Synthesis of 2,5-bis (p-styrylstyrl) thiophene. Na+O-Me + MeOH / THF Kaleb Topp: Molecule 2: 2,2’-bis(E-2-phenylethenyl)-5,5’-bithiophene Not soluble Figure 2: Williamson ether method for solubility variations. Light energy Electrical energy Photovoltaics: K2CO3 acetone 95% yield + Eric Lauzon: Molecule 3: 1,2-Di-(2-(E-2-phenylethenyl)thienyl) ethene Electroluminescent Devices: Electrical energy Light energy SOCl2 Pyridine CH2Cl2 Synthetic Routes to Molecule 1 Synthesis of 2,5-bis(p-styrylstyryl) thiophene was performed using the dialdehyde central molecule with trans-4-stilbene-carboxaldehyde endcaps (see Fig. 1). Difficulties arose for product characterization, as well as potential application use, when solubility difficulties were encountered. This instigated the use of ether linkages to attach alkyl and polyether groups which improves solubility. By using Williamson ether syntheses, ether substituents were added to endcaps such as 3,5-dihydroxybenzyl alcohol and 3-hydroxybenzaldehyde. See Figure 2 for an example. Several products have successfully been synthesized since then, although 2,5-bis(p-styrylstyryl) thiophene has not been attempted to be recreated with ether linkages yet. The following compounds have been used to prepare ether linkages thus far: 1-bromohexane, 1-bromo-2-ethylhexane, and a tosyl-ether chain (see Figure 2). Initially we intended to add these compounds to p-cresol via an ether linkage followed by radical bromination to prepare a handle for phosphonium salt preparation. Problems arose, however, because the resulting benzyl bromides decomposed quickly and were difficult to purify without further decomposition. In addition, because the 2-ethylhexyloxy substituted p-cresol has both a tertiary C center as well as a benzylic site, bromination was achieved at both positions in roughly equal amounts. Therefore the route to these compounds via radical bromination of p-cresol derivatives was abandoned in favor of chlorination of benzyl alcohols. NBS PPh3 DMF AIBNbenzene 1) Na+O-Me 73% yield 57% yield 2) Benzaldehyde methanol 63% Stereoisomer mixture yield PPh3 1) Na+O-Me DMF 77% yield 2) 62% Pure trans/trans stereoisomer yield THF Synthetic Route to Molecule 2 Sony’s Flexible 11-Inch OLED Screen ` Pd(OAc)2,EDIPA Four Generations of Lighting Toluene www.hitech-projects.com/.../olla/downloads.html 49% yield Special thanks to: Brookstra Student Faculty Development Fund, The Neckers Summer Research Fund, HHMI, S-Stem (NSF), Gentex Corporation and Hope College. Na+O-Me THF 26% yield http://www.gizmodo.com.au/2008/10/sonys_flexible_11inch_oled_screen