SYNTHESIS AND SOLVATOCHROMIC PROPERTIES OF DONOR-ACCEPTOR-SUBSTITUTED OLIGOTHIOPHENES

The Pd-catalyzed cross coupling reaction of electron donor-substituted thiophenes 1 with electron acceptor-substituted halothiophenes 2,10 via organozinc intermediates or organotin compounds 5 to give bi-, ter-, and quaterthiophenes 3, 4, 6, 8, 12, 13, and 14 is described. (Dimethylamino)-bithiophenes 8 with acceptor groups of varying reactivity were prepared via 5d in 60-80% yield, whereas bithiophenes 6d,e were obtained as a mixture with phenylthiophenes 7d,e. Symmetrical substituted byproducts 11 were formed in the conversion of 1b with bromothiophenes 10 via organozinc compounds yielding oligothiophenes 6b and 12. The ter- and quaterthiophenes 13 and 14 were isolated in about 50-70% yield. The electronic interactions between donor and acceptor end groups in the conjugated bithiophenes 3, 4, and 8 are expressed in the intensive and markedly solvatochromic CT transitions. The solvatochromic behavior of 3, 4, and 8 was determined by linear regression analyses of absorption maxima in 11 solvents, whereby bithiophene 3d was found to be a very appropriate indicator dye whose absorption wavenumbers ($) over bar nu(max)(3d) in aliphatic and dipolar aprotic and-on consideration of the polarizability correction term d delta-in aromatic and chlorinated solvents excellently correlate with the pi* values defined by Kamlet and Taft.