Cyclothiophenes

To hnish this section and chapter we will briefly discuss the synthetic routes pursued in order to obtain cyclothiophenes, fully a-conjugated macrocyclic oligothiophenes, i.e., joining boths ends of linear a-thiophenes (Kromer et al., 2000). One advantage of these macromolecules is that the mean diameter of the cavity can be tuned by selecting the initial building blocks. In Section 4.2 we will explore how such molecules, in particular cyclotrimeric terthiophenediacetylene, self-assemble on a surface. 

Monolayers of cyclothiophene macrocycles have been observed to form a complex with Cgo molecules, when studied with STM at a 1,2,4-trichlorobenzene solution/ graphite interface [13]. 

Cyclobutadithiophene 4.1 can formally be regarded as the smallest member of the cyclothiophenes. Also known as cyclo [2]thiophene or cyclodi(3,4-thienylene), it was prepared by Shepherd as the most stable of the four possible isomeric dithia analogues of biphenylene [356]. In the NMR spectrum of 4.1, a paramagnetic shift of the a-thiophene protons has been found, indicating a cyclobutadienoid or [4]annulenoid structure. In the UV spectrum, absorptions are red shifted (334, 318 nm) compared with that of thiophene (230 nm), providing evidence of electronic interaction and conjugation between the two thiophene units. 

Linkages including a-positions of thiophene units leading to conjugated cyclothiophenes... 

The feasibility and importance of fully a-conjugated cyclothiophenes was supported by theoretical calculations which were carried out in 1995 by Tol on cyclo [12]thiophene and on its corresponding charged... 

Figure 1.5 STM images of submolecularly resolved cycio [12]thiophene 4.26 (40 x 40 A) (left), Ceo fullerene epitaxially adsorbed in a second layer [long-range order, (2x2 fj.m)] (middle) and the structural model of the 1 1 D-A complex (right, top) and charge distribution in the FIOMO. Courtesy of Elena Mena-Osteritz, University of Ulm. Middle image from E. Mena-Osteritz and P. Bauerle, Complexation of Ceo on a cyclothiophene monolayer template, Adv. Mater., 18, 447-451 (2006), Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

In a recent theoretical paper on cyclothiophenes with ring sizes from 6 to 30 repeating units, Zade and Bendikov studied syn and anti conformations using density functional theory (DFT) [407]. The properties... 

A mixed cyclothiophene including phenylene units was synthesized by Tobe et al. [423], Macrocycle 4.55 (Chart 1.56) was made, which includes four alternating 2,5-thienylene and 1,3-phenylene units and is similar in shape to cyclo [8]thiophene 4.31. The meto-branched phenylene units helped to form the macrocycle and reduced strain energy, but interrupted the overall conjugation. The synthesis of a corresponding pyridine-containing macrocycle 4.56 was attempted, but the ring-closure reactions of penta- and heptacyclic precursors failed [424]. 

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