Red- and Blue-Shifts in Oligo(1,4-phenyleneethynylene)s Having Terminal Donor−Acceptor Substitutions

Four series of oligo(1,4‐phenyleneethynylene)s (OPEs), 1−4 (a−d), each having a terminal dialkylamino group as their electron donor, were prepared by applying Sonogashira−Hagihara reactions and a protecting group strategy. To study the influence that the push−pull effect has on the long‐wavelength absorption, three of the four series of OPEs contain terminal acceptor groups (CN, CHO, NO2). Extending the conjugation (increasing the number of repeat units, n) lowers the energy E(n) of the electron transition in the purely donor‐substituted series 1a−4a (bathochromic shift). This effect is superimposed in the push−pull series 1−4 (b−d) by the effect of the intramolecular charge transfer (ICT), which decreases with increasing the distance between the donor and acceptor groups. In the case of the relatively weak acceptor CN, both effects annihilate one another, whereas the reduction of the ICT predominates in the CHO and NO2 series. Therefore, E(n) is virtually independent of n in the CN series 1b−4b and shows a hypsochromic effect in the CHO and NO2 series (1c−4c and 1d−4d, respectively). We rationalize these results based on AM1−INDO/S calculations. The results of the commonly used intuitive VB model having electroneutral and zwitterionic resonance structures are compared with those of a more appropriate MO model having dipole segments at both chain ends. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)