Three series of alternating donor-acceptor-substituted co-oligomers (with different chain lengths) have been prepd. by application of the Pd-catalyzed Stille coupling methodol. They contain pyrrole or thiophene as the electron-rich unit and quinoxaline or 2,1,3-benzothiadiazole as the electron-deficient unit. The trimethylstannyl group is always located on the electron-rich unit, whereas the bromo substituent is always located on the electron-deficient one. The tBoc-protecting group is used in the synthesis of the pyrrole-contg. oligomers. The incremental bathochromic shift of lmax upon chain elongation of the three series of oligomers is less than that of the homo-oligomers of thiophene and pyrrole; this decrease is caused by a diminished dispersion of the LUMO level upon chain elongation. This conclusion was drawn after comparing the oxidn. and redn. behavior of the thiophene/benzothiadiazole co-oligomers with that of thiophene oligomers. The incremental bathochromic shift is similar for all three series of oligomers and is used as a tool in the band-gap engineering of donor-acceptor-substituted p-conjugated polymer
