Conjugated cyclothiophenes

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... 

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. 

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. 

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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