Notable Third-Order Optical Nonlinearities Realized in Layer-by-Layer Assembled Composite Films by Intercalation of Porphyrin/Polyoxometalate into Layered Double Hydroxide

A series of uniform and smooth (TCPP/LDH)_<i>n</i> films and (TCPP/P₅W₃₀/LDH)_<i>n</i> films were fabricated with <i>meso</i>-tetrakis­(<i>p</i>-carboxyphenyl)­porphyrin (TCPP), Preyssler-type polyoxometalate K_12.5Na_1.5[NaP₅W₃₀O₁₁₀]·15H₂O (P₅W₃₀), and exfoliated Mg₂Al–NO₃ layered double hydroxide (LDH) monolayer nanaosheets by layer-by-layer assembly technique. The resulting films were characterized by UV–visible spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and fluorescence spectroscopy. Their third-order nonlinear optical properties were studied by <i>Z</i>-scan measurement with laser pulse duration of 6–7 ns at a wavelength of 532 nm. The (TCPP/LDH)_<i>n</i> films exhibit notable self-defocusing effect and saturated absorption effect which is different from TCPP solution. The optical nonlinearity of films becomes larger with the increase of the number of bilayers. The third-order nonlinear optical coefficient χ⁽³⁾ of (LDH/TCPP)₅₀/LDH is calculated to be (6.4 ± 0.18) × 10^–11 esu. Remarkably, experimental results showed that the (LDH/P₅W₃₀/LDH/TCPP)_<i>n</i>/LDH films exhibit much larger nonlinearity than that of the (LDH/TCPP)_<i>n</i>/LDH films when <i>n</i> is the same, which is thought to result from the interlayered charge/energy transfer between porphyrin and polyoxometalate although the LDH sheet is electron-inert material.