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

Abstract

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.