Extension of Conjugation Leading to Bathochromic or Hypsochromic Effects in OPV Series

Abstract

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 1 a–4 a and the series 1 b–4 b with CN as weak acceptor. The two effects annihilate each other in the series 1 c–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 1 d–4 d, 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). The HOMO–LUMO transition, which is mainly responsible for the ICT, becomes less important in the electron transitions S₀→S₁ 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 ≥ 2.