Fabrication Of Keplerate-Type Polyoxometalates-Porphyrins-Zn₂Al Layered Double Hydroxide Composite Films And Their Third-Order Nonlinear Optical Properties

Polyoxometalates (POMs) are a class of transition metal-oxygen cluster compounds. Among all kinds of POMs, Keplerate-type POMs have wide applications in nonlinear optical field because of their unique structure. As an important organic nonlinear optical material, porphyrins molecules have large ?-conjugated macrocycles so that they have a high nonlinear optical coefficients and short response time. As a class of intercalation functional materials, layered double hydroxides(LDHs) have a wide applications in photofunctional fields because of their specificity in two-dimensional structure. In this study, we chose (NH4)42{Mo132-Ac}? (NH4)60{Mo,32-Ac-SO4}?(NH4)72{Mo132-SO4}(denoted as {Mo132-L} (L=Ac,Ac-SO4,SO4)) series POMs, tetra (4-sulfophenyl) zinc porphyrin (ZnTPPS), tetraphenylporphyrin perchlorate ([H2TPP](ClO4)2) and zinc-aluminum layered double hydroxide (Zn2Al-LDH) for parent materials, a series of composite films have been fabricated by using layer-by-layer(LBL) method. The third-order nonlinear optical properties of the composite films were characterized by Z- scan technique at 532 nm with a pulse duration of ?=6 ns. The main research results are as follows:1. Novel composite films have been fabricated by the exfoliated Zn2Al-LDH and Keplerate-type POMs {Mo132-Ac} through layer-by-layer method and characterized by UV-vis absorption spectroscopy, SEM, AFM and XPS. The investigation on third-order nonlinear optical properties of composite films with different layers indicates all of films have saturated absorption and self-defocusing properties by Z-scan technique. Comparing to the{Mo132-Ac} aqueous solution, (Zn2Al-LDH/{Mo132-Ac})n composite film shows higher saturable absorption and self-defocusing properties (x(3) value is up to 1.99×10-11esu when n=24).2. The water-soluble tetra (4-sulfophenyl) zinc porphyrin (ZnTPPS) and exfoliated Zn2Al-LDH have been assembled to composite films by using layer-by-layer method. The process monitored by UV-vis absorption spectroscopy, which shows films grow uniformly and orderly. SEM and AFM characterizations show films have smooth surface and ZnTPPS macrocycles maybe in a perpendicular orientation to the LDH nanosheets. Z-scan curves shows (Zn2Al-LDH/ZnTPPS)n (n=10,20,30,40) composite films with different layers have saturated absorption and self-defocusing properties. Composite films compared to the ZnTPPS aqueous solution having a higher third-order nonlinear susceptibility x(3) values (up to 7.86×110-11esu) with the increase layers of composite films, which is due to excite electrons were captured by the surface states.3. Composite films (H2TPP/{Mo132-L})n(L=Ac,Ac-SO4,SO4) (n=6,12,18,24) derived from{Mo132-L} (L=Ac,Ac-SO4,SO4) series POMs and tetraphenylporphyrin perchlorate ([H2TPP](ClO4)2) have been fabricated and charactered by UV-vis absorption spectroscopy, SEM and AFM. We found different species of anions in{Mo132-L} molecules may have an influence on the third-order nonlinear optical properties of the composite films and{Mo132-L} aqueous solution after being characterized by Z-scan technique. After calculation by cyclic voltammetry curves, we found that the order of x(3) value is x(3)(L=Ac)<x(3)(L=Ac-SO4)<x(3)(L=SO4), and it is consistent with the films with same layers, proportional to the discrepancy between LUMO levels of[H2TPP](C104)2 and that of {Mo132-L}.