Rapid Syntheses of Oligo(2,5-thiophene ethynylene)s with Thioester Termini:  Potential Molecular Scale Wires with Alligator Clips

Abstract

The syntheses of soluble oligo(3-ethyl-2,5-thiophene ethynylene)s via an iterative divergent/convergent approach starting from 3-ethyl-2-(trimethylsilylethynyl)thiophene are described. The monomer, dimer, tetramer, octamer, and 16-mer were synthesized. The 16-mer is 100 Å long in its minimized extended zigzag 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. 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. Attachment of thiol end groups, protected as the thioacetyl moieties, was achieved. These serve as binding sites for adhesion to gold surfaces. In some cases, one end of the oligomeric chains were capped with a thiol group so that the surface attachments to gold could be studied. In other cases, thiol groups were 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.

Room-temperature negative differential resistance in nanoscale molecular junctions

“…Acetate hydrolysis and coupling with 4-ethynyl͑thioacetyl͒benzene 10 afforded the desired compound 1a. As was previously established, 11 thioacetyl groups can be selectively hydrolyzed with ammonium hydroxide in tetrahydrofuran during the self-assembly step to afford the free thiol, 2Ј-amino-4,4Ј-di͑ethynylphenyl͒-5Ј-nitro-1-benzenethiol ͑1b͒, in this case.…”

Giant meso–meso‐Linked Porphyrin Arrays of Micrometer Molecular Length and Their Fabrication

“…[42] However, usual thiol protective groups, such as acetyl and benzoyl groups, were found not to able to tolerate the basic conditions of the coupling reaction. [12,43] To avoid this, a diphenylmethyl group was selected as a protective group. [44] The synthesis of meso-thiophenylated porphyrin arrays is shown in Scheme 4.…”

Synthesis and properties of oligo(2,5-thienylene-ethynylene)s

“…Pd(0) is inserted into the C−Hal bond of R The relatively rigid conjugated OTE chains 3 (X=S) soon become (n≥4) insoluble unless solubilizing side chains are attached. Therefore, alkyl, 11,13,14,18,19,[21][22][23][25][26][27][28][31][32][33] aryl, 17 alkoxy 34 or alkylsulfanyl 34 groups are normally attached to one or both free positions of the thiophene rings.…”

“…Apart from OTEs with (substituted)-thienyl and/or free or protected ethynyl end-groups, some systems with special end-groups have been investigated: benzene, 21,31,32 fluorene, 13 fullerene, 23 aldehyde, 23,32 ester, 21 thioester, 21 1,3,4-oxadiazole, 29a 2,2′: 6′,2"-terpyridine. 27,28 We were mainly interested in the synthesis of push-pull substituted OTEs, which bear an electron-donor group, D, on one end and an electron-acceptor group, A, on the other end of the conjugated chain.…”