Synthesis and Characterization of Conjugated Mono- and Dithiol Oligomers and Characterization of Their Self-Assembled Monolayers

Abstract

The characterization of self-assembled monolayer molecular arrays, which serve as active layers in electronic devices, is an important step toward understanding molecular-scale electronics. To correlate the properties of self-assembled monolayers with their molecular structures, a series of π-conjugated monoand dithiols and aromatic dithiols with an oxygen or sulfur atom between two aromatic units have been designed and synthesized. Their optical properties were determined by UV-vis spectroscopy. Their selfassembled monolayer films on gold surfaces were characterized by cyclic voltammetry, grazing incidence Fourier transform infrared spectroscopy (GI-FTIR), and contact angle and ellipsometry measurements. Increasing the chain length from two to four phenyl rings showed a more than linear increase of the intensity of the aromatic CdC ring stretch modes in GI-FTIR, indicating that the longer p-phenylene system is oriented toward the surface normal. Similar to oligophenylenes, when the number of repeat units for oligothiophene is increased, a more than linear increase of the intensity of the CdC stretch and C-H bend modes implies that the longest oligothiophenedithiol molecule is oriented close to the surface normal. Ellipsometry showed a smaller deviation from the calculated monolayer thickness with increasing number of thiophene or phenylene rings, corroborating the GI-FTIR data. We conclude that the conjugated phenyleneand thiophene-based dithiols demonstrate a less tilted molecular orientation with respect to the surface normal with increasing chain length.

Electrical characteristics of conjugated self-assembled monolayers in large-area molecular junctions

“…The molecular end-to-end distance is calculated with ACD LABORA-TORIES 11.0 software and 2.3 Å is added for the Au-S bond. For both series, the layer thicknesses are consistently below Tables I and II are in agreement with previously published results 23,24 and establish the formation of close-packed monolayers.…”

“…The chemical structures are presented in Tables I and II. Synthesis of the molecules was performed via prescribed procedures. 23 Monolayers of P1DT-P4DT and P1MT-P3MT were grown from 300 M SureSeal THF solution in a N 2 glovebox for 36 h. For P3DT and P4DT the acetyl-protected compounds were used which were deprotected during assembly by the addition of two drops of ammonia to 90 ml of SAM solution. SAM formation was checked by variable angle spectroscopic ellipsometry in the spectral range 300-500 nm at angles of incidence 65°, 70°, and 75°.…”

Electron tunneling through alkanedithiol self-assembled monolayers in large-area molecular junctions

“…Adsorption of HS-C x H 2x -SH (Cx-dithiol) from ethanol onto a gold surface results in a monolayer with one thiol bonded to gold, the so-called standing-up phase (28,(35)(36)(37)(38)(39). X-ray photoelectron spectroscopy (XPS) is a versatile surface technique that discriminates between chemisorbed S-Au (S2p: 161.9 Ϯ 0.1 eV) versus unbound S-C (S2p: 163.5 Ϯ 0.1 eV), and is used to validate the orientation of the monolayers.…”

Asymmetric transfer hydrogenation of ketones in aqueous solution catalyzed by rhodium(III) complexes with C2-symmetric fluorene-ligands containing chiral (1R,2R)-cyclohexane-1,2-diamine

“…Ligand 3 was prepared following reported methodology 13 from commercially available fluorene, chlorosulfonic acid, phosphorus pentachloride and (1R,2R)-cyclohexane-1,2-diamine. Spectra and analytical data of all compounds are given below.…”