Push‐Pull Oligomers with 2,2‐Dicyanovinyl Groups as Electron Acceptors

Three conjugated oligomer series with terminal donor–acceptor substitution were studied: oligo(1,4‐phenylenevinylene)s DAOPVs 4a – d ( n = 1–4), oligo(1,4‐phenyleneethynylene)s DAOPEs 6a – d ( n = 1–4) and oligo(2,5‐thienyleneethynylene)s DAOTEs 8a – e ( n = 1–5). Dialkylamino or methoxy groups served as electron donors and 2,2‐dicyanovinyl groups as strong electron acceptors. The push‐pull effect polarizes the molecular chains – an effect which is documented by the splitting of the 13 C chemical shifts of two carbon atoms of the double or triple bonds in the chain. For higher oligomers ( n ≥ 3), the effect is mainly localized at the chain ends. The long‐wavelength absorption results in a charge‐transfer band which loses gradually its CT character since the intramolecular charge transfer (ICT) declines with increasing numbers n of repeat units (increasing chromophore extension, increasing distance D–A). As a consequence, theDAOPVs 4a – d and the DAOPEs 6a – d represent hypsochromic series, for which the λ max values decrease steadily from n = 1 to n = 4. This is also true for the DAOTE series 8a – e , but contrary to 4a – d and 6a – d the decrease is very small in the beginning [ λ max (1) – λ max (2) = 1 nm] and grows with increasing n to λ max (4) – λ max (5) = 29 nm. Thus, series 8a – e represents an until now unknown type of conjugated push‐pull oligomers. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)