Third-order Optical Nonlinearities of a Series of Compounds Derived from a Keplerate Type Molybdenum-oxide-based Polyoxometalate

By reacting the unique Keplerate type molybdenum-oxide based polyoxometalate (NH₄)₄₂·Mo₁₃₂O₃₇₂·(CH₃COO)₃₀(H₂O)₇₂]·ca.300H₂O·ca.10CH₃COONH₄(1) with tetramethylammonium bromide, a new derivative (NH₄)₂₆TMA]₁₆{Mo₁₃₂O₃₇₂(H₂O)₇₂(CH₃COO)₃₀}·ca.7NH₄CH₃COO·ca.189H₂O(2, TMA=tetramethylammonium)was prepared. Compound 2 was characterized by Fourier transform infrared spectroscopy(FTIR), UV-Vis, elemental and thermogravimetric analyses. By the well-established Z-scan technique, investigations on the nonlinear optical(NLO) properties of the series of compounds derived from the Keplerate type molybdenum-oxide-based poly-oxometalate, namely, the newly prepared compound 2, the three previously reported compounds, included compound 1, (NH₄)₁₈(TBA)₂₄{Mo₁₃₂O₃₇₂(H₂O)₇₂(CH₃COO)₃₀}·ca.7NH₄CH₃COO·ca.173H₂O(3,TBA=tetrabutylammonium) and (DODA)₄₀(NH₄)₂(H₂O)_nMo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂](4, DODA=dimethyldioctadecylammonium), reveal that the third-order nonlinearityχ⁽³⁾] values of compounds 1, 2 and 3 in the DMF/H₂O solution and compound 4 in chloroform are almost the same, which indicates that the counter cations with different length of alkyl chains show ignorable impacts on the NLO susceptibility. In other words, the remarkable third-order nonlinearitiesχ⁽³⁾≈10^-19 m²/V²] mainly come from theMo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]^42- anions. This fact reveals that the applications of the NLO active polyoxometalates in various environments(such as hydrophilic, hydrophobic, polar, apolar, etc.) can be achieved by simply varying cations to meet the demands in the design of diverse devices.