Poly(triacetylenes) are rodlike molecules with electrons delocalized over a one-dimensional path. We show that they exhibit a power-law dependence of the second-order hyperpolarizability γ on the monomer unit <i>n</i> for short molecules and a smooth saturation toward a linear increase in longer molecules. The power law of γ∝n<sup>a</sup> with a≈2.5 from dengenerate four-wave mixing and third-harmonic generation measurements is in good agreement with quantum-chemical calculations. The critical conjugation length for saturation in the three cases is shown to be approximately 60 carbon–carbon bonds, which indicates the upper boundary for the electron delocalization in such a one-dimensional molecular wire.
© 1999 Optical Society of America
(160.4330) Materials : Nonlinear optical materials
(160.4890) Materials : Organic materials
(190.4160) Nonlinear optics : Multiharmonic generation
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing
(190.4710) Nonlinear optics : Optical nonlinearities in organic materials
U. Gubler, Ch. Bosshard, P. Günter, M. Y. Balakina, J. Cornil, J. L. Brédas, R. E. Martin, and F. Diederich, "Scaling law for second-order hyperpolarizability in poly(triacetylene) molecular wires," Opt. Lett. 24, 1599-1601 (1999)