Monodisperse poly(triacetylene) oligomers extending from monomer to hexadecamer: Joint experimental and theoretical investigation of physical properties

Rainer E. Martin*, Ulrich Gubler, Jérôme Cornil, Marina Balakina, Corinne Boudon, Christian Bosshard, Jean Paul Gisselbrecht, François Diederich, Peter Günter, Maurice Gross, Jean-Luc Bredas

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

67 Scopus citations

Abstract

A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligomers ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotectionoxidative Hay oligomerization protocol. The PTA oligomers exhibit an increasingly deep-yellow color with lengthening of the π-conjugated backbone, feature excellent solubility in aprotic solvents, and exhibit melting points up to >220°C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to investigate the evolution of the physico-chemical effects in PTAs beyond the linear 1/n versus property regime into the higher oligomer region where saturation becomes apparent. We report the results of joint experimental and theoretical studies, including analysis of the 13C NMR spectra, evaluation of the linear (UV/ Vis) and nonlinear [third-harmonic generation (THG) and degenerate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the hexadecameric rod, an increasingly facile one-electron reduction step is observed, showing at the stage of the dodecamer, a leveling off tendency from the linear correlation between the inverse number of monomer units and the first reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a π-electron-delocalization length of about n = 10 monomeric units, which corroborates well with the oligomeric length for which in the 13C NMR spectrum C(sp2) and C(sp) resonances start to overlap. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) measurements revealed for the second-order hyperpolarizability y a power law increase γ·α·na for oligomers up to the octamer with exponential factors a = 2.46 ± 0.10 and a = 2.64 ± 0.20, respectively, followed by a smooth saturation around n = 10 repeating units. The power law coefficient a calculated with the help of the valence effective Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalism corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap)-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units displaying a strong electron-hole symmetry. The onset of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.

Original languageEnglish (US)
Pages (from-to)3622-3635
Number of pages14
JournalChemistry - A European Journal
Volume6
Issue number19
DOIs
StatePublished - Oct 2 2000

Keywords

  • Alkynes
  • Conjugation
  • Molecular wires
  • Nonlinear optics
  • Oligomers

ASJC Scopus subject areas

  • Chemistry(all)

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