Polyhydroxyoligothiophenes .2. Hydrogen-bonding-oriented solid state conformation of 3,3'-bis(2-hydroxyethyl)-2,2'-bithiophene and regioselective synthesis of the corresponding head-to-head/tail-to-tail quater- and sexithiophene

This paper reports the X-ray structure of 3,3'-bis(2-hydroxyethyl)-2,2'-bithiophene (1), which is the building block for the synthesis of head-to-head/tail-to-tail 2-hydroxyethyl-substituted oligothiophenes. Contrary to all the bithiophenes reported so far, 1 exhibits a noncoplanar anti conformation and an inter-ring twist angle (67.5 degrees) which is the largest ever measured for adjacent rings of a-conjugated oligothiophenes. This unusual conformation appears to be dictated by intermolecular hydrogen-bonding interactions involving the OH groups, which bind the molecule in close packed layers. The paper also describes the regioselective synthesis of the dimer and the trimer of 1, namely of 3,3',4 '',3 triple prime-tetrakis(2-hydroxyethyl)-2,2':5',2 '':5 '',2 triple prime-quaterthiophene (3c) and of 3,3',4 '',3 triple prime,4'''':3''''-hexakis(2-hydroxyethyl)-2':5',2 '': 5 '',2'''':5'''',2'''': 5'''',2''''-sexithiophene (4b). 3c And 4b were obtained through palladium(0)-catalyzed coupling of the mono- and distannanes of the tetrahydropyranyl derivative of 1 with the appropriate monobromo compound (Stille's reaction). Finally, the paper reports force-field calculations which suggest that the low lambda(max) values measured for 1, 3c, and 4b are also the result of intramolecular hydrogen-bonding interactions which favor highly twisted conformations in solution.