Student Project Abstracts 2005 - Pluto - University of Washington

Determination of Molecular Orientation of Self-OrganizedAggregates of New Liquid-Crystal Perylene DyeNatalie ThompsonGeorgia Institute of TechnologyNeal R. Armstrong, P. Alex Veneman,Armstrong Lab, Department of ChemistryThe University of ArizonaINTRODUCTIONOrganic compounds with π-conjugated systems which oftenexhibit high charge mobilities have a growing interest within thescientific community because of the possibility of creating organicfilms that can act as conducting materials. These compoundsare expected to be used to create cheaper electronics through allplasticcircuits (1) . Also, these materials could be used in otherorganic electronic applications, such as organic light emittingdiodes, organic photovoltaics, and organic field effect transistors.Liquid-crystalline organic materials are of interest due totheir ability to self-aggregate into columns. It is thought thatthe charge mobilities of the aggregates are related to the cofacialoverlap of the conjugated system. Brédas, et al. reported that thelargest splitting between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO)correlates with the cofacial overlap and then influences the hole(electron) transport of the aggregate. (1) Donley, et al. reportedthat cofacial overlap results in low activation energies for chargemigration. However, this migration, or hopping, prefers to movealong a certain axis of the aggregate. (2) Flora, et al. reporteda much higher charge mobility along the axis that is parallel tothe columnar axis, as opposed to perpendicular to it. (3) It hasbeen reported that discotic mesophase materials, such as phthalocyanines,triphenylenes, and hexabenzocoronenes, organize intocolumnar aggregates within films which are then stiff enough totransfer via horizontal transfer on a Langmuir-Blodgett trough.(4)Often these π-conjugated cores have surrounding side chainswhich are used to help control the self-organization process. Itis also thought that the side chains may help prevent electricalcross-talk between columnar aggregates due to their insulatingnature. (5)OBJECTIVES / THESISThis summer’s research focused on the characterization of arelatively new perylene-dianhydride-bisimide. Scheme 1The Derivatives of perylene-dianhydride-bisimide are of interestbecause of their high electron mobility, which would aidin the efficient transport of electrons in a thin film comprised ofthis molecule. Also, these derivatives have an electron affinitythat is high enough that the reaction with oxygen in the electrontransport is not significant, as compared with hole transportmaterials, indicating an ability to use organic electronics in air.The perylene-dianhydride-bisimide was made into thin films usingLangmuir-Blodgett methods and was then characterized byatomic force microscopy (AFM), UV-Vis transmission, FourierTranform Infrared (FTIR), reflective-absorptive infrared spectroscopy(RAIRS), and X-ray reflectivity (XRR).Scheme 1EXPERIMENTALLangmuir-Blodgett film preparationThe substrates for Langmuir-Blodgett thin films were siliconwafers, highly ordered pyrolytic graphite (HOPG), glass, andcommercial gold. The silicon wafers and glass slides were modifiedvia sonication for thirty minutes in a solution of 1,1,1,3,3,3-hexamethyldisilazane (HMDS) and 1,3-diphenyl-1,1,3,3-tetramethyldisilazane(DPTMDS) in chloroform (5 : 5 : 90 :: HMDS: DPTMDS : CHCl 3). After sonication, the substrates remainedin solution until LB deposition. The preparation of the HOPGinvolved cleaving the substrate to obtain a flat clean surface. Thegold surface was rendered hydrophobic by sitting in a solution ofbenzyloxyethanethiol in ethanol overnight.Monolayer and multilayer films of N,N’-1,2,3-tridodecoxyphenyl-3,4,9,10-perylenetetracarboxylicdiimide(PTCDI) wereprepared on a Nima model 611D LB trough using chloroform asCMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005 119