Design, Preparation And Optical Nonlinearity Of Polyoxoetalate-Tetraphenlyporphyrin Systems

In this study, the porphyrin-POM hybrids or composite films composed of tetraphenylporphyrin (selected as protype) and different type Keggin POMs are prepared and characterized. The nonlinear optical (NLO) properties of these hybrids or composite films are studied by Z-scan technique for the purpose of studying the effect of POM anions on the NLO properties of porphyrin. The mechanism of the enhancement of the NLO properties of porphyrin-POM hybrids or composites films is also discussed. The relationships between NLO properties of porphyrin-POM hybrids or composite films and their structure are preliminarily established.1. The investigation on interactions between tetraphenylporphyrin (TPP) and α-Keggin-type heteropolyoxometalates (POMs) (POMs sequentially refer to H5PMo10V2O40·36H2O (PMoV), H3PMo12O40·14H2O (PMo), H4SiMo12O40xH2O (SiMo) and H4SiW12O40·xH2O (SiW)) in THF reveals that TPP evolves into a monosalt and successively into a disalt by accepting protons from POMs and that the 2:1 hybrid between PMoV and TPP and the 1:1 hybrids between TPP and PMo, SiMo, SiW are formed in the THF solution, respectively. The reaction equilibrium constants calculated for the 1:1 complexes are quite large indicative of strong interaction between TPP and POMs. By simply increasing the concentration of the reactants four hybrids [H2TPP][H4PMo10V2O40]2·6C4H4O·H2O 1, [H2TPP][HPMo12O40]·5C4H8O·5H2O 2, [H2TPP][H2SiMo12O40]·3C4H8O·3H2O 3 and [H2TPP][H2SiW12O40]·4C4H8O·8H2O 4 were then isolated and characterized. Z-scan measurement showed that POMs themselves had negligible nonlinear optical response, while they imposed remarkable effect on the third-order nonlinear optical properties of resulting TPP-POM hybrid systems. It is found that the second hyperpolarizability values of compounds 1-4 are inversely proportional to the HOMO-LUMO gaps of these compounds and directly proportional to the discrepancy between the LUMO levels of TPP and POMs, i.e., the low-lying LUMO level of the POMs is important for the improved second hyperpolarizability value of the compounds.2. Three new supramolecular compounds [H2TPP]1.5[SVW11O40]·5CH3CN·4H2O 1, [H2TPP]2[SV2W10O40]4CH3CN·3H2O2, [H2TPP][SW12O4O]4H2O 3 composed of [H2TPP]2+ cation and vanadium substituted Keggin-type polyoxometalate (POM) anion [SW12-nVnO40](2+n)-(n=0-2) are prepared and characterized. The third-order nonlinear optical (NLO) and optical limiting (OL) properties of resulting hybrids were studied by using an Nd:YAG laser at 532 nm with a pulse duration of τ=7 ns. The results demonstrate that all of these supramolecular compounds have significant nonlinear reverse saturated absorption, self-defocusing behavior, and good OL performance, especially for compound 1, which has a second hyperpolarizability γ value of 7.05×10-29 (esu) and exhibits a comparable OL performance with the most well-known OL materials such as In(Pc*)Cl indicating that they are potentially excellent NLO materials. Remarkably, it was also observed that the degree of vanadium substitution in POM anions corresponding to different onset of reduction potential influences the NLO and OL properties of the resulting compounds:third-order NLO and OL properties of compound 2 is inferior to that of compound 1, and is better than that of compound 3. Both their second hyperpolarizability γ values and the OL performance were observed inversely proportional to the HOMO-LUMO gaps of the resulting compounds. The excited charge transfer from porphyrin to POM anions in the supramolecular compounds when exposed in laser is thought to play key role in the enhancement of NLO and OL response.3. 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 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 keep intact in light of the results of UV-vis. spectroscopy and XPS. UV-vis. spectra measurements reveal that the amounts of deposition of{Mo 132} 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 the 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 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.4. [H2TPP](ClO4)2, [H2TPP](BF4)2, [H2TPP]C12, [H2TPP]Br2, [H2TPP](NO3)2, [H2TPP]SO4 and two porphyrin-POM hybrids [H2TPP]1.5[PMo12O40]2DMF·THF·2H2O and [H2TPP]1.5[PW12O40]·DMF·3H2O are prepared. [H2TPP]1.5[PMo12O40]·2DMF·THF·2H2O and [H2TPP]1.5[PW12O40]·DMF·3H2O are characterized by elemental analysis, IR, UV-vis., TG, XRD and PXRD. The Single crystal structures of the hybrids [H2TPP]1.5[PMo12O40]·2DMF·THF·2H2O and [H2TPP]1.5[PW12O40]·DMF·3H2O display that they are formed via hydrogen bonds (H-bonds) between the [H2TPP]2+ and PM12O4O3- (M=W or Mo). The [H2TPP]2+ cations are formed in the hybrids by protonation of N atoms from porphyrin macrocycle. The 2-Dimension plane of porphyrin is distorted due to the H-bond between O and N from its axial direction. Z-scan measurement shows that the inorganic anions ClO4-、 BF4-、Cl-、Br-、NO3- and SO4- have negligible effect on the nonlinearity of porphyrin while PM12O4O3- (M=W or Mo) have notable effect. The introduction of POM forming porphyrin-POM hybrids can enhance the NLO properties of TPP, while the axial H-bond in the hybrids is adverse for its NLO properties due to the distortion of the porphyrin macrocycle caused by the H-bond5. The nonlinearity of Keplerate type POMs{Mo132} in poly(methylmethacrylate) (PMMA) matrix and DMF solution are studied by Z-scan technique. The{Mo132}/PMMA composite films are prepared by spinning the mixed DMF solution of PMMA and Keplerate type POMs{Mo132} and characterized by UV-vis. spectroscopy and XPS. The stability of the composite films are studied in HC1 (pH=0.5) and NaOH (pH=13) solution by UV-vis. spectroscopy and XPS, and show highly stability in both solutions. The Z-scan results show that the NLO absorption coefficient β of{Mo132} is improved compared with that of {Mo132} in DMF solution, embedding Bu4N-{Mo132} into PMMA film can improve the NLO absorption coefficient β.6. The porphyrin-polyoxometalate hybrid (TPP)Zr(PW11O39)[TBA]5} are successfully prepared and characterized by UV-vis. spectroscopy, elemental analyses, TG, NMR, ESI-MS and IR spectroscopy. The nonlinear optical properties of the hybrid are studied by Z-scan method. The results display that comparing with that of Zr(tpp)(OAc)2, the nonlinear absorption coefficient β are improved about 3 times indicating porphyrin-polyoxometalate can improve the nonlinear absorption coefficient of porphyrin.7. Powders compounds [H2TPP][H2SiMo12O40]·3C4H8O·3H2O and [H2TPP][H2SiW12O40]·4C4H8O·8H2O in the DMF/THF solution are kept in ether vapor for 5 days at room temperature to give dark purple flat crystals of [H2TPP]2[SiW12O40] 4DMF (1) and [H2TPP]2[SiMo12O40]·9DMF (2), respectively. The new hybrids 1 and 2 are characterized by UV-vis. spectroscopy, elemental analyses, TG, IR spectroscopy, PXRD and XRD. Single crystal structures of the new compounds 1 and 2 clarify that the [H2TPP]2+ cations are formed in the hybrids by protonation of N atoms from porphyrin macrocycle. The 2-Dimension plane of porphyrin is distorted due to the H-bond between O and N from its axial direction.