Cyclic and helical fused oligothiophenes

The structure of dithieno[2,3-fc 3, 2 - f]thiophene, where the three thiophene rings are in a syn arrangement, is curved and the extension of this unit with further s yn-fused rings has provided some fascinating 

Wynberg and co-workers have prepared helicenes composed of fused thiophene and benzene units from the photocyclization of alkene precursors [134], It was shown that the effect of substituting thiophenes for benzene rings in these structures led to a blue shift of the absorption maxima. Compound 61 can be converted to the circulene structure 62 in two steps and the latter compound represents another interesting class of fused thiophene stmctures [135]. Trithiophenes with a benzene core (63) can be prepared from diacetylene-fimctionalized bithiophenes via a five-coordinate rhodium(I) intermediate by the addition of elemental sulfur [136]. A related structure (64) was reported by Pei et al. [137], along with the elaborate helicene 65. The compounds were constracted via oxidative (FeCb) cyclization reactions of 1,2-dithienyl benzene derivatives. 

Subramanian, P. v. R. Schleyer and H. Jiao, Are the most stable fused heterobicycles the most aromatic , Angew. Chem. Int. Ed. Engl., 35, 2638-2641 (1996). 

Gleiter, R. Bartetzko, G. Brahler and H. Bock, Electronic-structure of sulfur-compounds. 28. Electronic-structure of tetraphenylthieno[3,4-c]thiophene - photoelectron, electron-spin resonance and electronic absorption-spectra, J. Org. Chem., 43, 3893-3897 (1978). 

Schweig, M. P. Cava and M. V. Lakshmikantham, Theory and application of photoelectron-spectroscopy. 69. Thieno[3,4-c]thiophenes - electronic structure, /. Am. Chem. Soc., 98, 7187-7189 (1976). 

---