Fabrication and optical nonlinearities of composite films derived from the water-soluble Keplerate-type polyoxometalate and chloroform-soluble porphyrin

Abstract

Composite films derived from the water-soluble Keplerate-type polyoxometalate (NH₄)₄₂[Mo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·ca. 300H₂O·ca. 10CH₃COONH₄ (denoted (NH₄)₄₂{Mo₁₃₂}) and chloroform-soluble tetraphenylporphyrin perchlorate [H₂TPP](ClO₄)₂ are successfully fabricated by a layer-by-layer self-assembly method and characterized by UV-vis spectroscopy and X-ray photoelectron spectroscopy (XPS). The structure of the {Mo₁₃₂} and [H₂TPP]²⁺ in the films remain intact in light of the results of UV-vis spectroscopy and XPS. UV-vis spectra measurements reveal that the amounts of deposition of {Mo₁₃₂} and [H₂TPP]²⁺ remain constant in every adsorption cycle in the composite films assembly process. Nonlinear optical properties of the composite films have been investigated by using the Z-scan technique at a wavelength of 532 nm and pulse width of 7 ns. The results show that the composite films have notable nonlinear saturated absorption and self-defocusing effects. The combination of {Mo₁₃₂} with [H₂TPP]²⁺ can result in composite films with remarkably enhanced optical nonlinearities. The interfacial charge transfer induced by laser from porphyrin to POM in the films is thought to play a key role in the enhancement of NLO response. The third-order NLO susceptibility χ⁽³⁾ of the composite films increases with the increase of film thickness.