Donor-acceptor thieno[2,3-b]thiophene systems: synthesis and structural study of 3-anisyl-4-pyridyl(pyridinium) thieno[2,3-b]thiophenes

Synthesis of a new class of cofacially oriented neutral donor-acceptor thienothiophene, namely 3-anisyl-4-pyridyl thieno[2,3-b]thiophenes 7 and its ionic analog 8 has been described to probe the presence of through-space charge transfer interaction. However, structural studies by H-1 NMR and UV-visible data indicate at best rather weak charge transfer interaction. The single crystal X-ray analysis of the ionic donor-acceptor thienothiophene 8 reveals many interesting features, i.e. (i) the donor and acceptor rings are not parallel to each other (ii) the interchromophoric distance is much larger (4.08 Angstrom) than the van der Waals distance and (iii) the torsional angle between the pyridinium ring and the thienothiophene is unusually small, being only 38degrees. These structural features preclude effective through-space charge transfer interaction in 8. Conformational analysis of a model system, 3,4-diphenyl thienothiophene indicated that conformations with reduced torsion between the phenyl rings and the thienothiophene plane are possible to maintain a balance between conjugation and steric congestion and the calculated low energy barrier between the high and low energy conformations of the model system suggest the possibility of aryl ring rotations in peri disubstituted thienothiophenes. (C) 2003 Published by Elsevier B.V.