Self-assembled monolayers (SAMs) form highly ordered, stable dielectrics on conductive surfaces. Being able to attach larger-area contacts in a MIM (metal-insulator-metal) diode, their electrical properties can be determined. In this paper, the electrical conduction through thiolate SAMs of different alkyl chain lengths formed on gold surfaces were studied and discussed. The influence of the headgroup with respect to the surface quality and prevention of short circuits is investigated. Phenoxy terminated alkanethiols were found to form high quality SAMs with perfect insulating properties. Synthesis of the required terminally substituted long chain thiols have been developed. The I(V) characteristics of MIM structures formed with these SAMs are measured and simulated according to theoretical tunneling models for electrical conductivity through thin organic layers. SAM based electronic devices will become especially important for future nanoscale applications, where they can serve as insulators, gate dielectric of FETs, resistors, and capacitor structures.
The terminally 2-thienyl-and phenyl-substituted eicosane derivatives 4 and 5 were prepared by reacting 1-[(20-bromoeicosyl)oxy]-4-methoxybenzene (3) with 2-lithiothiophene and lithium diphenylcuprate, respectively. Reaction of 4 and 5 with BBr 3 leads to the corresponding bromides 6, 7 which afforded the aryl-substituted eicosanethiols 8 and 9 by nucleophilic substitution with thiourea. 4-Eicosylthiophenol (13) was prepared from eicosylbenzene (11) by chlorosulfonation and subsequent reduction of the sulfonyl chloride. Synthesis of the thiol 22 with phenylenedioxy groups incorporated in the alkyl chain involves formation of 19 from allyloxy(bromobutoxy)benzene (18) and (4-(2-thienyl)butoxy)phenol (17). Hydroboration of 19 yields the corresponding alcohol 20, which under Mitsunobu conditions and subsequent reduction with LiAlH 4 can be transformed to 22.Key words: functionalized long-chain alkanethiols, 4-alkylthiophenol, self-assembled monolayers on gold Long-chain alkanethiols are known to form well ordered monolayers on gold. 3,4 These self-assembled monolayers (SAMs) are of interest due to their numerous applications. 4 The properties of SAMs (conductivity, wettability etc.) depend strongly on packing and orientation of the molecules in the layer. The two-dimensional order results from intermolecular interactions, e.g. van der Waals interactions. SAMs with aromatic systems incorporated may act as electron donors or acceptors, and furthermore, the p-systems could influence the layer structure sterically and by p-p interaction of the aromatic moiety. Compounds, suitable for formation of SAMs, can be structured in three parts: an anchor group for linkage to the substrate, a carbon chain of 2-3 nm length, and a terminal group with variable chemical functionalities.In the present publication we report on the synthesis of terminally thienyl-and phenyl-substituted alkanethiols, a 4-alkylthiophenol as well as a long-chain thiol with phenylenedioxy groups incorporated in the alkyl chain. The formation and characterization of SAMs formed from these adsorbates will be published elsewhere.Long-chain alkanethiols can be prepared simply by S N reactions from the corresponding alkyl bromides. 5 In analogy to a literature-known procedure 6 bromide 1 was converted to the Grignard reagent which was coupled with 1,10-dibromodecane (2) in the presence of a catalytic amount of Li 2 CuCl 4 to give the a,w-substituted long-chain alkane 3 (Scheme 1). In bromide 1 7 hydroquinonemonomethyl ether is used as a protecting group which can be transformed easily in a bromide function. 7,8
Scheme 1Whereas 3 reacted with 2-lithiothiophene 9 to give the w-protected a-(2-thienyl)alkane 4 in 77% yield, the conversion of 3 with phenyllithium was accompanied by numerous side-reactions. 10 The yield of the phenyl derivative 5 was only 14%. Replacing phenyllithium by lithium diphenylcuprate 10 afforded the coupling product 5 in 80% yield.BBr 3 in boiling dichloromethane 8 was found to be more effective for simultaneous deprotection and bromination of...
The acylation of alkylbenzenes 1 with benzoyl chloride 2 and catalytic amounts of perfluorobutanesulfonic acid affords the corresponding 2-, 3-, and 4-alkylbenzophenones 3 with unusually high amounts of ortho products. Surprisingly, even tert-butylbenzene reacts under these reaction conditions without any acid catalyzed dealkylation. The yield of benzoylation of p-xylene 4 with benzoic acid in the presence of 5 mol% C 4 F 9 SO 3 H to give 2,5-dimethylbenzophenone 5 could significantly be improved from 14% to 90% by continuous removal of water formed during the acylation. Also in the preparation of alkylanthraquinones 7 by reaction of alkylbenzenes with phthalic anhydride, water removal is the decisive factor to obtain satisfactory yields for the second acylation, allowing to cyclize 6 with catalytic amounts (10 mol%) of CF 3 SO 3 H in organic solvents to the corresponding alkylanthraquinones 7.
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