Third-order optical nonlinearities of zinc porphyrins accommodated in the cavity of a doughnut-like molybdenum crown cluster.

In this paper, three zinc porphyrins bearing carboxyl, amino or hydroxyl groups on the porphyrin ring, namely, 5,10,15,20-tetra(4-carboxyphenyl) zinc porphyrin (ZnTCPP), 5,10,15,20-tetra(4-hydroxyphenyl) zinc porphyrin (ZnTHPP) and 5,10,15,20-tetra(4-aminophenyl) zinc porphyrin (ZnTAPP), reacted with the doughnut-like molybdenum crown cluster [(MoO3)176(H2O)63(CH3OH)17H16]16- (denoted as {Mo176}), resulting in the formation of three porphyrin-polyoxometalate hybrids (denoted as hybrids 1-3), respectively, with the ratio of {Mo176} to the porphyrin molecules in each compound being 1 : 3. Compared with the corresponding parent porphyrins, the fluorescence quenching efficiency of hybrid 1, hybrid 2 and hybrid 3 was found to be 51%, 58% and 66%, respectively, indicating electron/energy transfer occurring from the porphyrin moiety to the POM moiety. Z-Scan experimental results showed that the three porphyrins and hybrids 1-3 exhibited nonlinear reverse saturated absorption and self-defocusing properties. The molecular second hyperpolarizability (γ) values of hybrids 1-3 (7.38 × 10-27, 7.26 × 10-27 and 4.26 × 10-27 (esu)) are higher than those of the corresponding porphyrins (1.18 × 10-27, 1.34 × 10-27 and 1.36 × 10-27 (esu)), indicating that the doughnut-like molybdenum crown cluster in the hybrids 1-3 improved the third-order optical nonlinearities of the accommodated H-aggregate of the zinc porphyrin molecules, wherein the optical nonlinearities of the porphyrin with the electron-withdrawing functional group of -COOH (γ = 7.38 × 10-27 for hybrid 1vs. γ = 1.18 × 10-27 for ZnTCPP) were enhanced the most. In addition, studies on the optical limiting properties of hybrid 1 and hybrid 2 showed that the nonlinear attenuation factor (NAF) of both hybrid 1 and hybrid 2 was larger than that of their corresponding porphyrins (NAF = 5.65 for hybrid 1vs. 1.82 for ZnTCPP) and the limiting threshold of hybrid 1 and hybrid 2 can reach 0.3 J cm-2 and 0.32 J cm-2, which proved that hybrids 1 and 2 are potential optical limiting materials.