Third-order microscopic nonlinearities of very long chain polyenes: saturation phenomena and conformational effects

Using the living cyclopolymerization of diethyldipropargylmalonate, very long polyenes (up to 1100 carbon-carbon double bonds) are now available, Molecular masses of these polyenes are determined using the MALDI-TOF technique. Third-order polarizabilities gamma of polyene-like molecules with very long conjugation length (N up to 1100 carbon-carbon double bonds) are measured using third-harmonic generation at 1.9 mu m in THF solutions. gamma values saturate around N = 60 double bonds. Modelization of the dependence of gamma with respect to N is based on a representation of a conjugated chain as a collection of planar segments separated by large angular breaks. A very good quantitive agreement between calculated and experimental data, both in terms of saturation length and gamma/N values is clearly demonstrated. (C) 1999 Elsevier Science B.V. All rights reserved.