Integrating polyoxometalates into porphyrin moieties via covalent bond is expected to be a new approach to tune and enhance the nonlinear optical responses of porphyrins. The effectiveness and efficiency of this proposal has been examined and witnessed by studying two new hybrid compounds as a prototype, namely, (, in which porphyrins acting as electron donor and Dawson type polyoxometalate acting as electron acceptor are connected via short tether through covalent bond. The new compounds are systematically characterized by means of elemental analyses, FT-IR, 1 H NMR, ESI-MS, TG/DTA, UV−vis, fluorescence emission spectra, and cyclic voltammetry measurement. Remarkably, great enhancement in reverse saturation absorption is achieved in 1 and 2 which is ca. 1 order of magnitude greater than their individual reactants. Additionally, optical-limiting thresholds are obtained being 0.484 J/cm 2 for 1 and 0.501 J/cm 2 for 2, respectively, implying their high potential as low-power optical-limiting materials.

The investigation on interactions between tetraphenylporphyrin (TPP) and α-Keggin-type heteropolyoxometalates (POMs) (POMs sequentially refer to H 5 PMo 10 V 2 O 40 •36H 2 O (PMoV), H 3 PMo 12 O 40 •14H 2 O (PMo), H 4 SiMo 12 O 40 •xH 2 O (SiMo), and H 4 SiW 12 O 40 •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 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.

A family of covalently bonded hybrid compounds composed of Anderson type polyoxometalate (POM) moiety and porphyrin moiety have been synthesized and thoroughly characterized. The compounds all show remarkable nonlinear reverse saturable absorption and self-defocusing effect at 532 nm with a pulse duration τ = 6 ns, rendering them promising candidate materials for device applications in photonics and optoelectronics. More importantly, it is found that the hybrid wherein POM is coupled covalently to porphyrin through shorter bridge has an NLO response superior to the hybrid wherein POM is bonded via longer bridge to porphyrin, and the hybrid having two porphyrins connected to POM shows more enhancement than the hybrid having single porphyrin fused to POM. Disclosure of the inherent structure–property relationship is expected to be instructive for exploration of new porphyrin-POM based NLO materials. Meantime, the hybrid compounds have optical-limiting thresholds lower than 1.0 J/cm2, implying their high potential as lower power OL materials.

Third-order optical nonlinearities of ultrathin films derived from cationic phthalocyanine and Keggin polyoxometalates can be well tuned and enhanced.

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