Extension of conjugation leading to bathochromic or hypsochromic effects in OPV series

Four OPV series 1-4 (a-d) with a terminal dialkylamino group as electron donor were prepared by Wittig-Horner reactions. To study the influence of the push-pull effect on the long-wavelength absorption, three of the four series contained terminal acceptor groups (CN, CHO, NO2). The length of the chromophores strongly affects the intramolecular charge transfer (ICT)-an effect which superimposes upon the extension of the conjugation. Increasing numbers n of repeat units cause an overall bathochromic shift for the purely donor-substituted series 1a-4a and the series 1b-4b with CN as weak acceptor. The two effects annihilate each other in the series 1c-4 c with terminal CHO groups, so that the absorption maxima are almost independent of the length of the chromophore. A hypsochromic shift is observed for the series 1d-4d, which contains the strong acceptor group NO2. This anomaly disappears on protonation of the dialkylamino group because the push-pull effect disappears in the ammonium salts. The results can be explained by semiempirical quantum mechanics (AM1, INDO/S). ne HOMO-LUMO transition, which is mainly responsible for the ICT, becomes less important in the electron transitions S-0 --> S-1 when the distance between donor and acceptor is increased. The commonly used VB model, which contains an electroneutral and a zwitterionic resonance structure, is contrasted with a MO model with dipole segments at both ends of the OPV chains. The latter model turned out to be more appropriate-at least for donor-acceptor-substituted OPVs with n greater than or equal to 2.