COPLANARITY OF PRECURSOR MOLECULES FOR CONDUCTING POLYMERS - 3'-ARYL-SUBSTITUTED AND HETEROARYL-SUBSTITUTED 2,2' 5',2''-TERTHIOPHENES AND THEIR DIMERS/

The coplanarity of 2,2':5',2''-terthiophene and its six 3'-substituted derivatives, namely 3-phenyl- ,3'-(2-thienyl)-(16),3'-(5-methyl-2-thi enyl)-(17), and 3'-(4-pyridyl)-2,2':5',2''-terthiophene (18), has been studied by UV spectroscopy and compounds 13 and 17 have also been stu died by X-ray diffractometry. For comparison, also an isomer of compou nd 13, 2'-phenyl-2,3':5',2''-terthiophene (20), was synthesized and it s structure determined by XRD. Compounds 13-15, 17, 18, and 20 have no t been previously reported. The UV spectra reveal that the aromatic an d heteroaromatic substituents do not appreciably conjugate with the te rthiophene backbone. Single-crystal X-ray diffraction analysis of comp ounds 13 and 17 shows that in the solid phase these compounds are enti rely nonplanar, in contrast with parent terthiophene which is known to be strictly planar. The complete analysis of diffractograms is diffic ult because of the number of rotational conformers present in the soli d phase. The conformational equilibrium of compound 13 is frozen at 18 3 K, as shown by variable temperature H-1-NMR. Oxidation of terthiophe ne and compounds 13-16 and 18 with Cu(II) in acetonitrile gives soluti ons which are shown to contain dimers on the basis of visible-near-inf rared (vis-NIR) spectra reported previously. Comparison with the spect ra of parent terthiophene shows that in the case of cationic dimers th e conjugation of the pendent aromatic side group with the main chain i s negligible, but the oligothiophene chain has maintained its coplanar ity.