Mechanism of optical limiting in metalloporphyrins under visible continuous radiation

Here, we present an experimental investigation on the nonlinear optical (NLO) and optical limiting properties of metalloporphyrin compounds (Cu-1-OH, Zn-1-OH, Cu-1-E and Zn-1-E) using spatial self-phase modulation (SSPM) method in the visible range. It is found that all of the samples show a large self-defocusing effect at 532 nm, which is attributed to the thermal nonlinear optical effects with negative nonlinear refractive index coefficient n2 due to the relatively high absorption at 532 nm. In contrast, at 780 nm where absorption is weak for both Zn- and Cu-porphyrins, Zn-porphyrins still exhibit visible self-defocusing while Cu-porphyrins do not show any nonlinear diffraction pattern. Such a phenomenon can be explained by the Kerr effect of Cu-porphyrins at 780 nm. As the thermal nonlinear optical effects (of negative n2) at 780 nm are reduced due to the low absorption, the Kerr effect with positive n2 becomes comparable and the overall nonlinearity is reduced. The Kerr effect of Cu-porphyrins is stronger than that of Zn-porphyrins because of the enhanced π-electron delocalization effect as Cu(II) has a variable number of valence electrons and incompletely filled d atomic orbitals. Finally, the optical limiting performance of Zn-porphyrins is demonstrated as a representative and its dependence on sample position is examined. This work not only enriches the understanding of the physical mechanism of optical limiting in porphyrin materials, but also provides a significant reference to improve the third-order NLO coefficient by adjusting the structure of compounds.

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.

Theoretical screening of promising donor and π-linker groups for POM-based Zn-porphyrin dyes in dye-sensitized solar cells

A series of Zn-porphyrin dyes with different donor and π-linker groups based on the dye SM315 were systematically investigated to screen highly efficient candidates based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Compared with SM315, dye 1 not only has stronger and broader absorption spectra and larger intramolecular charge transition (ICT) parameters, but also has a larger maximum short circuit density (JmaxSC) and an open-circuit photovoltage (VOC) due to the insertion of polyoxometalate (POM). In addition, dye 3 with an electron donor 3-ethynyl-7-(4-methoxyphenyl)-10-methyl-10,10a-dihydro-4aH-phenothiazine (D3) exhibits better photovoltaic performance among dyes 1-3 due to its broadest absorption spectra and largest JmaxSC, hence, D3 can act as a promising donor candidate for the construction of efficient dyes. Furthermore, the performance can be further improved by replacing the π-linker 2,1,3-benzothiadiazole (dye 3) with benzo[1,2-b:4,5-b']difuran-2,6-dione (dye 4) and pyrrolo[2,3-f]indole-2,6(1H,5H)-dione (dye 5). Compared with dye 3, dyes 4 and 5 possess broader absorption spectra and light harvesting efficiency curves, larger ICT parameters, JmaxSC and conduction band energy shift. In particular, dye 4 might obtain a larger short-circuit photocurrent density, and dye 5 might obtain a larger VOC.

New supramolecular compounds based on porphyrin and polyoxometalate: synthesis, characterization and nonlinear optical and optical limiting properties

Compounds composed of [SW_12−nV_nO₄₀]⁽²⁺ⁿ⁾⁻ and [H₂TPP]²⁺ show significant nonlinear absorption, self-defocusing behavior and excellent optical limiting performance.