respectively (20). I11 the case of two-photon absorption, the line-shape f~~n c t i o r l(21 ), where n is the transition frequency, v is the laser frequency, and r is the molec-SCIENCE VOL. 271 22 MARCH 1996 22. C. SchGnenberger, J. Jonitsma, J. A M. Sondag-Huethorst. L G. J. Fokkink, J. Phys. CChem. 99.3259(1 995). 23. We thank W. Emst for the loan of a microwave source.

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Alkanethiolate self-assembled monolayers on Au{111} are used for the two-dimensional matrix isolation of conjugated organic thiolates, studied for their potential to act as molecular wires. The conjugated organic molecules are inserted from dilute solution into boundaries between structural domains and at substrate step edges of preassembled n-alkanethiolate monolayers so as to preserve the order of the initial alkanethiolate lattice. In contrast, when both molecules are codeposited from a single solution, the structure of the assembled monolayer shows no ordered molecular lattice of the alkanethiol component. Scanning tunneling microscopy measurements on the isolated conjugated component molecules show them to have enhanced conductivity compared to neighboring alkanethiolates.

We present the investigation of novel metal/organic monolayer/metal heterostructure diodes. Our technique provides well-defined, stable, and reproducible metallic contacts to a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measurements show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal that thermal emission of electrons over a barrier of 0.22 eV dominates for electron injection from Ti into the organic layer while the transport for electron injection from Au into the organic layer satisfies the formula for hopping conduction.

The syntheses of soluble oligo(2-alkyl-1,4-phenylene ethynylene)s via an iterative divergent/convergent approach starting from 1-(diethyltriazyl)-3-alkyl-4-[(trimethylsilyl)ethynyl]benzenes are described. When the solublizing alkyl group is an ethyl substituent, the monomer, dimer, tetramer, and octamer can be synthesized. The octamer, however, is only minimally soluble. When the alkyl substituent is 3-ethylheptyl or dodecyl, the compounds are easily dissolved even at the 16-mer stage. The 16-mer is 128 Å long in its near-linear extended conformation. At each stage in the iteration, the length of the framework doubles. Only three sets of reaction conditions are needed for the entire iterative synthetic sequence:  an iodination, a protodesilylation, and a Pd/Cu-catalyzed cross coupling. Synthesis of the dodecyl-containing 16-mer was also achieved on Merrifield's resin using the iterative divergent/convergent approach. The oligomers were characterized spectroscopically and by mass spectrometry. The optical properties are presented which show that at the octamer stage, the optical absorbance maximum is nearly saturated. The size exclusion chromatography values for the number average weights, relative to polystyrene, illustrate the tremendous differences in the hydrodynamic volume of these rigid rod oligomers verses the random coils of polystyrene. These differences become quite apparent at the octamer stage. Equations were derived for assessing the efficiency of the polymer-supported reactions based on resin weight differences, molar concentration differences, and elemental analysis data. Each of these methods' limitations are discussed. Attachment of thiol end groups, protected as thioacetyl moieties, was achieved. These serve as binding sites for adhesion to gold surfaces. In some cases, one end of the oligomeric chains is capped with a thiol group so that the surface attachments to gold could be studied. In other cases, thiol groups are affixed to both ends of the molecular chains so that future conduction studies could be done between proximal metallic probes. The rigid rod conjugated oligomers may act as molecular wires in molecular scale electronic devices, and they also serve as useful models for understanding analogous bulk polymers.

No abstract

Molecular resolution scanning tunneling microscopy (STM) imaging and high sensitivity infrared reflection spectroscopy (IRS) measurements have been combined to characterize the structures of mixed self-assembled monolayers (SAMs) of fully conjugated, linear thiolate-terminated molecules and short chain (8-12) n-alkanethiolates on Au{111}. Immersion of preformed, ordered alkanethiolate SAMs into dilute solutions of the conjugated molecules results in two-dimensional matrix isolation of conjugated adsorbates in the host SAM. The post-immersion host SAM matrix shows retained alkanethiolate ordering with the guest molecules inserted both singly into boundaries between SAM structural domains and in bundles at substrate step edges. Inserted molecules of lengths in the ∼15 Å range adopt surface orientations similar to those of alkanethiolate molecules at all compositions, including the pure conjugated SAMs. In contrast, the configuration of an inserted, long conjugated molecule (∼40 Å), varies monotonically with the final SAM composition. When inserted at decreasing fractions into an n-octanethiolate matrix, the average tilt of the long molecular axis decreases and approaches alignment with the host SAM. Combined quantum chemical calculations and IRS data for the short guest-host SAMs support a picture of a dense local environment of the host SAM around the guest molecules, despite their insertion at host defects. These results have important implications for designing electronic devices based on the addressing of individual, fully conjugated molecules self-assembled at gold electrodes.

Strategies for self‐assembling molecule‐based devices are considered in terms of current chemical issues whose resolution appears critical to efficient connection and addressing of electronically active molecules between electrodes. We discuss issues related to the type and shape of the molecules, chemical bonding at junctions, molecular lengths and electrode gap matching, molecular alignment at electrodes, chemistry of deposited metal contacts, and the doping of molecular conductors. Examples of each of these aspects is given using fully conjugated molecules, constituted from rigid rod phenylene‐ethynylene units, self‐assembled onto metal and semiconductor surfaces.