Covalent Synthesis of Two Hybrids Composed of Dawson-Type Polyoxometalate and Porphyrin with Remarkable Third-Order Optical Nonlinearities Reflecting the Effect of Polyoxometalates.

Two new porphyrin-polyoxometalate hybrids, namely, [(C4H9)4N]10H2[{COHNC(CH2O)3P2V3W15O59}2C44N4H28]·CH3CN 1, bearing two covalent-bonded Wells-Dawson-type polyoxometalates (POMs), and [(C4H9)4N)]5H[COHNC(CH2O)3P2V3W15O59{C44H29N4}]·CH3CN 2, bearing one covalent-bonded POM, have been synthesized and thoroughly characterized by means of elemental analysis, powder XRD, FT-IR, 1H (31P, 51V) NMR, MALDI-TOF-MS, UV-vis spectra, and cyclic voltammetry measurement. Experimental results demonstrate that while all the compounds show remarkable third-order optical nonlinearities, the hybrids 1 and 2 are superior to their corresponding porphyrin precursors (molecular second hyperpolarizability γ = 8.0 × 10-28 esu for 54-N-N'(1,3-tetrahydroxy-2-(dihydroxymethyl)propan-4-diyl)benz-diamide,10,15,20-triphenyl porphyrin that is the precursor for the hybrid 1, γ = 2.6 × 10-28 esu for 54-N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)benz-amide,10,15,20-triphenyl porphyrin that is the precursor for the hybrid 2), and the hybrid 1 (γ = 12.9 × 10-28 esu) is superior to the hybrid 2 (γ = 12.2 × 10-28 esu) reflecting more POM moieties covalently bonded to the porphyrin moiety exerting more significant influence on the third-order optical nonlinearities. Meanwhile, attachment of POMs on the porphyrin results in significant fluorescence quenching (fluorescence intensity is decreased 97% for the hybrid 1 and 80% for the hybrid 2 with respect to that of their corresponding porphyrin precursors) indicating strong electron transfer from porphyrin moiety to the polyoxometalate moiety. Lower transition energy, small energy difference between singlet and triplet excited states, and faster intersystem crossing (ISC) process of the hybrids are favorable to enhance the NLO responses of hybrids 1 and 2 resulting from the facile electron transfer from the porphyrin moiety to the Dawson POM moiety when the hybrids are subjected to laser irradiation, which is thought to be responsible to the superior of the hybrid 1 to hybrid 2 and the superior of the hybrids to their corresponding porphyrin precursors as well.