Solution Properties and Effect of Anions on Third-Order Optical Nonlinearity of Porphyrin-Heteropolyoxometalate Hybrid System

The investigation on interactions between tetraphenylporphyrin (TPP) and alpha-Keggin-type heteropolyoxometalates (POMs) (POMs sequentially refer to H5PMo10V2O40 center dot 36H(2)O (PMoV), H3PMo12O40 center dot 14H(2)O (PMo), H4SiMo12O40 center dot xH(2)O (SiMo), and H4SiW12O40 center dot xH(2)O (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, and SiW are formed in the THF solution. The reaction equilibrium constants calculated for the 1:1 complexes are quite large, indicative of strong interaction between TPP and POMs. When the concentration of the reactants were simply increased, four hybrids [H2TPP]-[H4PMo10V2O40](2)center dot 6C(4)H(4)O center dot H2O ( 1), [H2TPP][HPMo12O40]center dot 5C(4)H(8)O center dot 5H(2)O (2), [H2TPP][H2SiMo12O40]center dot 3C(4)H(8)O center dot 3H(2)O (3), and [H2TPP] [H2SiW12O40]center dot 4C(4)H(8)O-8H(2)O (4) were isolated and characterized. Z-scan measurement showed that POMs themselves had negligible nonlinear optical response, whereas 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 highest occupied molecular orbital-lowest unoccupied molecular orbital (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.