Thienothiophenes structural isomers

Thieno[3,4- )]thiophene Thieno[3,4-c]thiophene Chart 3.1 The structural Isomers of thienothiophene... 

The major products resulting from the acetylation of thienothiophenes (7) and (3) in the presence of Lewis acids are the C-2 substituted isomers (67 and 69 Schemes 16 and 17). GC-MS analysis, however, indicated the presence of j3-substituted isomers (68) and (70) as well (76AHC(19)123). Chemical proof of structures (67) and (69) was provided by standard transformations. For example, ketone (69) upon desulfurization with Raney nickel furnished 2-octanone (Scheme 17). 

Compounds characterized by three modes of fusion of the thiophene rings, viz., [2,3-Z ] (structure A), [3,2-Z ] (B) and [3,4-Z ] (C), have attracted the most attention. These isomers are refered to as conventional or classical thienothiophenes because structures of the type D ([3,4-c]-fusion) are much less accessible. 

We have recently shown that 3,6-dimethoxythieno[3,2-h]thiophene 21 leads to a polymer presenting low oxidation potential and moderate bandgap (1.7 eV) [74]. The advantage of thienothiophene unit compared to bithiophene one resides in the planar structure and absence of positional isomers. Furthermore, the crystallographic structure of the dimer 22 (Figure 13.1), shows a hilly planar conjugated system stabilized by noncovalent intramolecular sulhir-oxygen interactions. 

Extending the structure of thienothiophene by an additional fused ring affords the family of dithienothiophenes, of which are there are six isomers (Chart 3.3). An excellent review on the synthesis and chemistry of dithienothiophenes was presented by Ozturk et al. in 2005 [45] and therefore the many different synthetic routes towards these materials will not be reproduced here. In their papers, Mastragostino and Sariciftci have termed four of the isomers DTT0-DTT3 for ease of reference (see Chart 3.3). 

Thienothiophenes (TTs) are the simplest fused thiophenes, with four isomers, namely thieno[3,2-Z>]thiophene (1), thieno[3,4-6]thiophene (2), thieno[2,3-Z>]thio-phene (3), and thieno[3,4-c]thiophene (4) [4, 6, 7,12,13]. Syntheses and polymerizations of aU the TTs, except unstable thieno[3,4-c]thiophene (4), have been studied extensively. The three TTs, 1-3 possess stable and electron-rich structures, which make them useful building blocks for conjugated and low band gap organic semiconductors [14]. 

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