Non-linear optical properties of group 10 metal alkynyls and their polymers

Conjugated diacetylene and phenylacetylene systems incorporating Group 10 transition-metal complexes have been obtained via lithium acetylide intermediates. In the UV-VIS spectra, the positioning of the longest-wavelength absorption shows a strong dependence on the incorporated metal. Polymers of both the transnickel and the frans-platinum bisphosphine diacetylide [M(PR₃)₂(C≡C - C≡CH)2] (M = Ni, or Pt; R = alkyl group) were synthesised. UV absorption spectra show a significant shift of the absorption maxima from those of the monomers, towards longer wavelengths, consistent with an increase in the degree of electron delocalisation per monomer unit. The third-order non-linear optical properties of these materials in solution were examined using the method of self-diffraction from laser-induced gratings with 70 ps pulses of wavelength 1.064 μm. In the case of the monomer solutions, intensity dependences of the diffraction efficiencies show the influence of the proximity of a three-photon resonance at low concentrations. At higher concentration the influence of this resonance is not apparent. Among the materials synthesised, a clear trend, down the group, is seen, the hyperpolarisabilities decreasing with increasing atomic number of the metal. Measured hyperpolarisabilities are compared with those of enyne oligomer solutions and are found to be substantially higher. Metal-containing polymeric systems show an intensity dependence that is characteristic of a pure third-order process. Molecular hyperpolarisabilities show trends similar to those of the monomeric species. Both polymers possess a non-linearity higher than that of equivalent polydiacetylene solutions.