A family of solution-processable macrocyclic and open-chain oligothiophenes with atropoisomeric scaffolds : structural and electronic features for potential energy applications

FeCl3oxidation of the racemate of C2symmetric, inherently chiral, sexithiophene monomer 1 (2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene) affords a mixture of cyclic oligomers, from the prevailing dimer to traces of the pentamer. The oligomers are constituted by mixtures of stereoisomers which are two for dimer 2, four for trimer 3 and six for tetramer 4. Cyclooligomers 2 and 3 could be separated by chromatography, while 4 was synthesized by ring closure of open chain dimer 2a, prepared in turn by controlled coupling of the anion of racemic 1. The optical properties of open-chain stereoisomer 2a and tetramer 4 have been compared with those of 2 and 3 respectively. The macrocyclic oligomers have been tested as donor materials in bulk heterojunction solar cell prototypes both as a crude mixture resulting from oxidation of 1 and as a single oligomer. Theoretical calculations support the photophysical properties of these new materials.